ÆGISAn Astrophysics Experiment for Grating and Imaging Spectroscopy

A high-throughput, high-resolution, moderate cost, soft X-ray spectrometerA mission concept to be presented to NASA in response to the Request for Information on

“Concepts for the Next NASA X-ray Astronomy Mission”(Solicitation Number NNH11ZDA018L)

M. W. Bautz (mwb@space.mit.edu, 617-253-7502)Senior Research Scientist & Associate Director

Kavli Institute for Astrophysics and Space ResearchMassachusetts Institute of Technology

G. E. Allen J. Bookbinder D. Bower J. Bregman N. BrickhouseD. N. Burrows C. R. Canizares S. Casement D. Chakrabarty K.-W. ChanB. Costello J. E. Davis D. Dewey J. J. Drake M. S. ElvisD. Evans A. Falcone K. Flanagan R. K. Heilmann J. C. HouckD. P. Huenemoerder S. Jordan A. Klavins J. C. Lee J. LempkeC. Lillie M. Loewenstein H. L. Marshall S. Mathur R. S. McClellandJ. M. Miller R. Mushotzky T. Nguyen F. Nicastro M. A. NowakS. L. O’Dell F. Paerels R. Petre A. Ptak T. T. SahaM. L. Schattenburg N. Schulz R. K. Smith D. Tenerelli R. VanbezooijenG. Vasudevan D. Wang S. Wolk W. W. Zhang

28 October 2011

NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

1 Summary

A compelling portion of the science achievable by theInternational X-ray Observatory (IXO), as judged by theAstro2010 Decadal Survey, was made possible by IXO’sX-ray Grating Spectrometer (XGS). With an effectivearea exceeding 1000 cm2 and a spectral resolving powerR = λ/∆λ > 3000, the IXO XGS was designed to ad-dress a broad range of fundamental questions, including“How does cosmic feedback connect the growth of super-massive black holes with that of large-scale structure?”;“How does large scale structure evolve?”; “What hap-pens close to a black hole?”; and “How does matter be-have at very high density?” The IXO XGS offered morethan an order-of-magnitude increase in effective area overcurrent X-ray spectrometers, and unprecedented spectralresolution. Had IXO been developed, its XGS would un-doubtedly have driven enormous progress on these ques-tions and on many others as well.

Here we respond to NASA’s Request for Information onConcepts for the Next NASA X-ray Astronomy Mission(RFI) by describing ÆGIS, an Astrophysics Experimentfor Grating and Imaging Spectroscopy. At a small frac-tion of the cost of IXO, ÆGIS provides capabilities thatsurpass IXO XGS requirements, and are far superior tothose of any existing soft X-ray spectrometer. Just one ofthe many dramatic advances ÆGIS enables is illustratedin Figure 1, which compares the capability of ÆGIS tomeasure line centroids, and thus line-of-sight velocities,to that of previous instruments. At the crucial K lines ofOVII and OVIII, for example, ÆGIS’s statistical preci-sion is 40 times better than that of any previous spectro-meter. Moreover, no planned micro-calorimeter is com-petitive with ÆGIS in its 0.2 - 1 keV band, where manyof the most astrophysically important lines are located.As we discuss in detail below, ÆGIS provides compara-ble breakthroughs in line-detection and equivalent-widthmeasurement capability.

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Figure 1: ÆGIS is 30 - 50 times more sensitive than any existingsoft X-ray spectrometer, and will bring the power of X-ray linespectroscopy to a vast range of objects and phenomena beyondthe reach of current instruments. The figure of merit indicatesaccuracy of line centroid (and thus velocity) measurement.

High-resolution line spectroscopy has provided muchof our astrophysical knowledge. ÆGIS will make thistechnique as powerful in X-ray astronomy as it has be-come in other wavebands. Atomic physics renders al-most all of the ionization states of all of the abundantelements accessible in its passband, so ÆGIS can pro-vide a comprehensive physical picture often unavailablein other wavebands, where diagnostics from only a smallnumber of ionization states and/or elements exist. At thesame time, ÆGIS will enable powerful multi-wavelengthinvestigations, for example with Hubble/COS in the UV tocharacterize the intergalactic medium. ÆGIS will be thefirst observatory with sufficient resolution at E < 1 keVto resolve thermally-broadened lines in hot (T & 107 K)plasmas, and with its superb sensitivity it will do so in abroad range of objects. This sensitivity will also undoubt-edly produce unanticipated new discoveries, as profound,perhaps, as the finding by the grating spectrometers onXMM and Chandra that something must be heating coolgalaxy cluster cores. The search for that heat source hasrevolutionized our understanding of cosmic feedback.

ÆGIS incorporates innovative technology in X-ray op-tics, diffraction gratings and detectors. The flight mirroruses the high area-to-mass ratio segmented glass archi-tecture developed for IXO, but with smaller aperture andwith larger graze angles optimized for high-throughputgrating spectroscopy at minimum mass and cost. Theunique Critical Angle Transmission (CAT) gratings, alsoconsidered for IXO, combine the low mass and relaxedfigure and alignment tolerances of the transmission grat-ings on Chandra with the high diffraction efficiency andresolving power of blazed reflection gratings. For max-imum spectral resolution, each of six parallel CAT grat-ing spectrometers is illuminated by a sub-aperture of theflight mirror. Technology development is advancing suc-cessfully in accordance with detailed development plans,consisent with an ÆGIS new start at the midpoint of thecurrent decade.

As befits the broad range of astrophysics it will address,ÆGIS will be available to the entire astronomical com-munity. A cost-effective Falcon-9 launcher is baselined tobring the observatory to its L2 orbit, where observing ef-ficiency is maximized. In the baseline three-year missionÆGIS will record more than four times as much on-sourceexposure as was planned for the IXO XGS, with greatersensitivity. Using an efficient operations model, and witha substantial general observer program, the ÆGIS ROMmission cost, at ∼ $758M (FY2012), lies near the lowend of the RFI’s medium cost category.

Among national space agencies engaged in high-energyastrophysics, only NASA is prepared to furnish the high-resolution grating spectroscopy capability X-ray astron-omy must have. ESA’s ATHENA mission, in particular,would have no grating spectrometer. Here we show thatNASA can provide this crucial capability at moderate costby developing ÆGIS.

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

2 ÆGIS ScienceÆGIS outperforms current X-ray grating spectrometerson Chandra and XMM by more than an order of magni-tude, as we show in Figure 2. It took 10 days to get a sin-

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Figure 2: ÆGIS has a greater effective area (top) and higherresolving power (bottom) than any other instrument. ÆGIS’ val-ues are based on end-to-end Monte-Carlo ray-trace simulations.

gle high quality Chandra HETGS spectrum of a bright Ac-tive Galactic Nucleus (AGN). ÆGIS will do as well in just4 hours, about the light crossing time for a 108 M� blackhole, making spectra of such quality routine. The superbresolving power will allow detailed kinematic studies ofgalactic outflows, hot gas in galactic halos, and stellar ac-cretion. ÆGIS will address many fundamental astrophys-ical problems highlighted in the 2010 Decadal Survey“New Worlds, New Horizons in Astronomy and Astro-physics,”11 NWNH, and especially those to be addressedby IXO3,4 as listed in Table 1.

ÆGIS X-ray absorption-line spectroscopy of objectsthroughout the Universe will advance our knowledge oflarge scale structure, cosmic feedback, and the growth ofblack holes. Chandra and XMM have been used on thefew brightest sources in the sky to study the interstellarmedium (ISM). ÆGIS can observe thousands of cosmicX-ray sources to systematically study neutral and ionizedmatter to great distances. These investigations will revealthe composition, distribution, and kinematics of matter inthe Galaxy, the local Universe, and beyond.

ÆGIS will reveal the physics that drives disk accretion,and its radiative and mechanical feedback for both super-

ScienceQuestions

Measurements PerformanceRequirements

IXO ScienceWhat is therelationbetweenSMBHformation &evolution ofLarge ScaleStructure?(“cosmicfeedback”)(§§2.1.1, 2.1.2)

Line absorptionpositions,strengths,profiles in AGN;resolve flowvelocities.

R ≥ 3000;Aeff ≥ 1000 cm2

∆v ≥ 100 km s−1

Bandpass:0.2–1.0 keV;∆θ ≤ 10 arcsec

How doesLarge ScaleStructureevolve?(§§2.1.1, 2.1.2)

Measure WHIMabsorption (in C,N, O) againstbackgroundAGN; measuremass andcomposition ofgalaxy clusters atredshifts < 2.

Equivalent width:Wλ = 2 mABandpass:0.2–1.0 keVR ≥ 3000Aeff ≥ 1000 cm2

What happensclose to a blackhole? (§2.2.2)

Line absorption& variability inXRB soft state.

R ≥ 3000;Aeff ≥ 1000 cm2

Timing; f ≤ 15 Hz;How doesmatter behaveat very highdensity?(§2.2.2)

Search for linesin long outburstsof slowlyrotating NS.

R ≥ 3000;Aeff ≥ 1000 cm2

Timing; f ≤ 15 Hz;Bandpass:0.2–1.0 keV

Additional NWNH ScienceHow rapidlydid high-energyradiation ofyoung starsdisperse theirdisks? (§2.3)

Emission lineflux, velocity,width of C, N, O,Ne, Fe; flux &velocityvariability;

v ≤ 20 km s−1,∆v ≤ 100 km s−1,Bandpass:0.2–1.0 keV,Aeff ≥ 1000 cm2,∆t ∼ 0.2 day

How dorotation &magnetic fieldsaffect stars?(§2.3)

Emission lineflux, velocity,width of C, N, O,Ne, Fe; flux &velocityvariability;resolve thermalwidth at 107 K

v ≤ 20 km s−1,∆v ≤ 100 km s−1,Bandpass:0.2–1.0 keV,∆t ≥ 200 s (flares)

Table 1: ÆGIS scientific questions, measurements and perfor-mance requirements. References to pertinent sections of this pa-per are given in the first column.

massive and stellar-mass black holes as well as other com-pact objects. Spectra of large samples of pre-main se-quence stars will show how the accretion process createsstars, studies that can only be done on a few sources to-day. Observations of main sequence stars will resolvemany puzzles concerning the interplay of rotation, mag-netic fields and stellar type in generating coronal emis-sion, and at the same time address questions involving X-ray effects on planets throughout their lifetimes.

In Table 1, we map IXO science questions to ÆGIS

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

measurements and requirements. We also list other highpriorities of NWNH that ÆGIS will address.

2.1 Matter in and between Galaxies2.1.1 Galactic Structure and Feedback

What are the detailed abundances, kinematics, and mor-phologies of the cold, warm, and hot phases of the ISMin our Galaxy? What is the composition and state of mat-ter in other galaxies and in the IGM? ÆGIS can addressthese issues of cosmic feedback affecting the growth oflarge scale structure and interactions with supermassiveblack holes (SMBH).

The soft X-ray band is sensitive to K-shell absorptionfrom the high abundance elements C, N, O, Ne as well asL-shell absorption by Si, S, Ca, and Fe. ÆGIS will mea-sure these elements in all phases and forms in the localuniverse. Detection of several transitions in one or moreions of different elements lets us fully determine ther-mal state, turbulent width, ionization balance, and abun-dances. This capability is not limited to gas-phase mate-rial. Cold ISM phases also have dust signatures in thisX-ray band directly measurable34 through near-edge ab-sorption and scattering.

Metals in the Milky Way: The high ÆGIS through-put and spectral resolution will allow us to improve uponChandra by determining to high precision the quantity andcomposition of gas and dust on an element-by-elementbasis, as well as their charge states33,35 (see Figure 3).This is not possible in any other waveband. Such preci-sion measurements of the composition, distribution, anddepletion levels in space environments (local and extra-galactic) are crucial for improving our understanding ofa wide range of astrophysical processes from nucleosyn-thesis to planet formation, with potential additional con-sequences for cosmology49.

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Figure 3: ÆGIS can determine the molecular Fe content anddistribution of interstellar dust. and measure the metallic Fe L-edge (from Lee et al. 34 )

ÆGIS absorption line studies of C, N, O, and Ne fea-tures will map the structure of the the ISM and hot haloof the Milky Way. From the few objects bright enough for

such studies with Chandra and XMM, we know that theselines are strong diagnostics of the states of neutral andionized matter. ÆGIS can observe about 5000 Galacticsources (fx > 10−13 to 10−9 ergs cm−2 s−1) for MilkyWay ISM and halo studies.

Metals in Other Galaxies: Accurate determinations ofabundances, structure, and kinematic properties of theISM and halos of other galaxies (including the LMC,SMC, and M31) will greatly improve our understand-ing of galactic interfaces with the IGM and the impor-tance of cosmic feedback. ÆGIS can easily observe anumber of bright (fx ∼ 10−12 ergs cm−2 s−1) sourcesin M31 with ∼ 10 ks exposures. In the local universethere are at least 150 nearby galaxies with sufficient flux(fx > 10−13 ergs cm−2 s−1) for ÆGIS spectroscopy.XMM data have revealed a new and potentially power-ful diagnostic of hot-cold gas interfaces in other galax-ies: unusual line ratios in He-like ions suggest that chargeexchange is an important process.37,38 Confirmation byÆGIS would show that interactions between hot and coolgas are common, with strong implications for ISM mod-eling.

While designed for point sources, the ÆGIS spectro-meter will retain R ≥ 300 for sources up to ∼ 30 arcsecin size, adequate to resolve cluster cores and the inner,high surface brightness, portions of elliptical galaxies. Byproviding an order of magnitude more resolving powerthan Astro-H for C and N emission lines, ÆGIS can re-liably constrain the abundances of these elements thatare produced via nucleosynthetic channels distinct fromhigher-Z elements. Extending the measured abundancepattern to C and N places strong new constraints on thestar formation history and initial mass function in thesesystems. XMM RGS observations of elliptical galaxieshave shown subsolar Fe, O, and Ne abundances, whilethe available data suggests N is solar or even superso-lar.56 The subsolar results conflict with theoretical mod-els based on optical measurements of stellar metallicitiesand SNIa rates in ellipticals, implying the need for a fun-damental re-evaluation of elliptical galaxy metal enrich-ment.39,50 Since C and N are hydrostatically producedwith distinctive yield dependencies on progenitor massand expelled in stellar winds, while other elements ariseexplosively from supernovae, measurements of these linesare key to distinguishing models for the origin of metalsin ellipticals and the timescale of their injection. ÆGISobservations of ellipticals in a range of environments willresolve this issue.

Missing Baryons in Galaxies: Optical observationshave shown that most galaxies are missing more than2/3 of their baryons, relative to their cosmological ratiowith dark matter.29 Non-detections in IR, optical, and UVbands suggest that these missing baryons are hot, but wedo not know if they are present in the Galactic halo or inthe Local Group,6 either as expelled in a Galactic wind orpreheated so that they simply never coalesced during theformation of the Galaxy.46,57

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

A diagnostic for distinguishing between Galactic andLocal Group components is the velocity of the gas, whichshould vary by ∼ 100 km s−1.47 The difference betweenthe heliocentric velocity and the Galactocentric velocitycan be up to ∼ 200 km s−1, while M31 and the LocalGroup differ by ∼ 300 km s−1. ÆGIS can thus distin-guish a hot Galactic halo rotating with the Milky Wayfrom the Local Group IGM by measuring velocity as afunction of longitude.

2.1.2 Intergalactic Medium

As less than 10% of the baryons in the local Universelie in galaxies as stars or cold gas19, the remainder arepredicted to exist as a dilute gaseous filamentary net-work – the cosmic web.10 Half of this cosmic web is de-tected in narrow Lyα (∼ 30%) or O VI (∼ 10%)13, andin broad Lyα absorption lines (∼ 10%)14, but half re-mains undetected despite vigorous searches. Growth-of-structure simulations predict that these “missing” baryonsare shock-heated up to temperatures of 106−7 K in unviri-alized cosmic filaments and may (or may not) be chem-ically enriched by galactic superwinds – the Warm-HotIntergalactic Medium (WHIM; see Figure 4). This distri-

Figure 4: ÆGIS will distinguish between two possible modelsof IGM enrichment, those with superwinds (solid red curve), orthose without (dashed black curve). Horizontal bars show thetemperature ranges of emission for different ions whose reso-nance lines (except for O VI) are in the 0.2 – 1.0 keV band.(from Cen & Ostriker 10 )

bution of mass as a function of temperature can be deter-mined from X-ray absorption line spectroscopy of highlyionized C, N, O, and Ne detected against backgroundAGNs. Surprisingly, even stacked searches of sightlinesthat show O VI in absorption do not reveal X-ray absorp-tion lines,58 suggesting that either the filaments seen bythe HST COS are a distinct warm population separatefrom the hot fraction, or that our models of structure for-mation and enrichment must be revised. ÆGIS can detectfar smaller filaments than any current or planned instru-ment (see Figure 5). Based on the Cen & Fang 9 modeland using sources selected from the ROSAT blazar cata-log55, we expect to detect ∼ 100 systems with both O VIIand O VIII features with equivalent widths ranging from

2–5 mA.5 Furthermore, the high ÆGIS resolving powerwill allow us to discriminate thermal from Hubble-flow,and to resolve internal turbulence widths in ions such asC and Ne, effects which determine line depths and arecrucial to derivation of elemental abundances.

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O VIII O VII C VINe IX N VII N VI

Figure 5: ÆGIS has a line detection sensitivity which is an or-der of magnitude better than any other instrument.

As the baryons are yet unseen, the optimum distribu-tion of sight lines and exposure times cannot be predeter-mined. ÆGIS can detect filaments with O abundance aslow as 0.03 of solar in ∼ 500 ks. These observations willalso reveal the role of galactic superwinds in enriching theweb both from the proximity of the absorbers to galaxiesand from the kinematics of the hot gas (see Figure 6 andSection 2.1.1).

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Figure 6: ÆGIS can detect WHIM absorption at theoreticallyexpected levels. These 3 differently-redshifted components of theWHIM in O VII Kα are near minimum detectability. The leftpanel shows the spectrum, and the right indicates the geometry,with a bright background source (top) shining through filamentsto an observer at the bottom.

2.2 Matter in and around Compact Objects2.2.1 Active Galactic Nuclei

NWNH identified two related key questions: “What arethe flows of matter and energy in the circumgalacticmedium?” and “How do black holes grow, radiate, andinfluence their surroundings?”, tied together by a com-mon theme of feedback.

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

At pc scales, there is a competition between the fu-eling of the hole and outflowing winds, seen as “warmabsorbers” commonly found in the soft X-ray spectra ofAGN.51 The HETGS and RGS are now used to mea-sure ionization parameters, columns, and velocities of thisgas.30 Mass outflow rates may exceed the mass accre-tion rate onto the hole2 but the mass and energy outflowrates depend critically on the distances of the wind originsfrom their ionizing sources and on wind densities, whichare poorly known. Ionization variability studies haveyielded density estimates for the brightest sources32,45,52

and ÆGIS observations will extend this technique toshorter time scales (for gas closest to the cores) and fainterAGN such as radio loud galaxies that appear to heat intra-cluster gas. Furthermore, the ÆGIS spectral resolution isneeded to clarify the connection between the UV and X-ray absorbing gas; some X-ray features match two or moreUV/optical components, while other absorption lines haveno counterparts.12 Another technique to measure densitiescould be pioneered with ÆGIS due to its improved sensi-tivity. Ratios of metastable lines such as those of Si X,and S XII (near 50.7 A and 36.5 A) to the correspondingground states would give density estimates in the 106−11

cm−3 range.36

At kpc scales, active nuclei ionize outflowing gas.HETGS observations of the canonical Seyfert 2 galaxyNGC 1068 clearly reveal separate components: blue-shifted and Doppler broadened emission associated withthe nucleus, and red-shifted emission associated with anoutflowing photo-ionized gas16 (see Figure 7). The role ofsuch outflows in galaxy feedback processes is still an issueof debate.32 The large effective area of ÆGIS will extendthese studies to fainter off-axis outflows. Furthermore, ina single observation ÆGIS will provide six different ob-serving angles, which will allow for spatial-spectral mod-eling of AGN nuclei and ionization cones.

Figure 7: ÆGIS will provide spatially resolved spectra ofAGN, as in this HETGS “Doppler image” of Ne X emissionin NGC 1068; overlain by contours from the 0th order im-age (inset) at the systemic velocity. Ne X; the arrow showsa 1000 km s−1 redshift along the HETGS dispersion direc-tion. The AGN central source emission is blue-shifted by -700km s−1, while off-nuclear photoionized gas to the NE of the coreis offset +150 km s−1.

ÆGIS may even be able to test physics in strong grav-ity. Three AGN show evidence of relativistically broad-ened Fe-L emission lines arising from the inner accretiondisk18,44,48, although the spectra may be complicated byline blends from ionized absorbers along the line of sight.If present, ÆGIS will easily resolve broad features fromany interloping narrow lines, and potentially could time-resolve emission from hot spots on the disk, testing Gen-eral Relativity as proposed using Fe-K lines54. Interpre-tations are still controversial, but the scientific potential isgreat.

2.2.2 Galactic Compact Objects

Galactic black holes also can exhibit rich line spectra thatprobe their local environment and also provide evidenceof feedback in their accretion processes. HETGS obser-vations show that high mass X-ray binary systems suchas Cyg X-1 are fed by “focused accretion winds” ex-hibiting a wide range of ionization parameters and den-sities24. Variations in the properties of the focused windoccur faster than can be studied with the small effectiveareas of Chandra-HETG and XMM-RGS. Transient lowmass X-ray binary systems show a fascinating evolutionof accretion properties as they move from a corona- andjet-dominated “hard state” to a wind- and disk-dominatedspectrally “soft state”42,43. GRO J1655−40 first exhibitsFe XXV and Fe XXVI absorption as it begins the transitionout of a jet-dominated state, which evolves into extremelystrong, lower ionization absorption as it fully enters a statedominated by a disk and wind, which is best diagnosed inthe 0.2-1 keV band. Models indicate that the wind is notdriven by radiation, but rather must be driven by magneticprocesses in the disk. Observations of such systems havebeen rare, and the transition from jet, to wind, to “quiet”(albeit bright) accretion disk is not yet understood.

Neutron stars also have exhibited a rich variety of ab-sorption features; however, most have been associatedwith absorption in the outer disk in near edge-on systems.One exception has been IGR J17480−2446, a neutron starbinary in the Terzan 5 globular cluster. HETGS observa-tions have shown Fe XXV and Fe XXVI absorption consis-tent with a radiatively driven wind41. More interestingly,this NS has only an 11 Hz spin period, meaning that itis a strong candidate for spectroscopic searches for atmo-spheric lines in the NS atmosphere, which are otherwiseexpected to be smeared out in rapidly rotating systems.This source was a transient, but it does demonstrate thata population of slowly rotating NS exists. Detection ofphotospheric absorption lines and edges will allow a di-rect, distance- and luminosity-independent measurementof mass and radius, through measurement of the grav-itational redshift and the effect of pressure broadeningon line equivalent widths and blurring of photoionizationedges. The prime spectral series for these diagnostics arethe Balmer and Paschen series or their analogs, in Fe XXVand Fe XXVI, in the 7-13 and 16-36 A bands (assuming agravitational redshift of 0.3).

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

2.3 From Disks to Stars and Planets

A priority of NWNH is the understanding the effect ofhigh-energy radiation on protoplanetary disks (see Ta-ble 1). X-rays can strongly affect the dynamical structureof a protoplanetary accretion disk.20 Coupling betweenthe disk and stellar coronal magnetic fields can funnel ac-cretion streams leading to shocks, and can also generateenergetic, hard X-ray flares from reconnection of largemagnetic loops. Whether disk evolution leads to planetformation depends on the detailed influence of ionizingradiation on the disk.1 Even after an accretion disk dissi-pates, stellar coronal X-rays can be powerful enough to al-ter or even erode atmospheres of close-in planets.53 High-resolution X-ray spectroscopy can reveal the dynamicaleffects at scales suggested by modeling of the few high-quality Chandra or XMM grating spectra. Greater sensi-tivity and resolving power will provide such data for morethan the few dozen22 stars accessible to today’s observa-tories at high resolution. While the ÆGIS effective areawill provide access to many more stellar systems, the newdoor opened will be on the kinematics of the high-energyemission at unprecedented levels. In fact, ÆGIS will re-solve the thermal line widths of coronal plasmas.

TW Hya is the best-studied accreting Classical T Taurisystem to date;7,31 it serves as a prototype of a much largerclass observable with ÆGIS, being the nearest, brightestof its class. TW Hya has very soft and metal-depletedX-ray emission which presumably comes from an accre-tion shock, funneled by magnetic fields (see Figure 8). Amodel-predicted post-shock downflow of 100 km s−1 hasnot yet been observed. With ÆGIS, we will centroid linesto ∼ 20 km s−1. Turbulent broadening may have beenmarginally detected by a deep Chandra grating spectrum7,and would be clearly resolved by ÆGIS at ∼ 100 km s−1;the origin of UV and optical line broadening seen on thisscale is not understood23 and X-ray spectra would showwhether the dynamics extend to higher temperature ma-terial. There are only about a dozen other accreting T

Figure 8: ÆGIS spectra can resolve the inflow/outflow veloc-ities of accretion streams and shocks as they are channeled bymagnetic fields coupling the photosphere and accretion disk.(From Long, Romanova & Lovelace 40 ).

Tauri systems bright enough for XMM to have detectedexcesses in O VII relative to O VIII emission (a proxyfor accretion)21, and only about 5 of these have definitivedensity measurements. ÆGIS can revolutionize stellar ac-

cretion disk studies by providing density and kinematicdiagnostics for all these systems and many more.

Other areas, currently poorly understood, will benefitfrom resolving bulk flows and more completely determinethe kinetic energy budget; this falls under the NWNH cat-egory of rotation and magnetic field effects in stars (seeTable 1). Some young stars have outflows in jets, drivenby accretion, and harboring shocks in the outflows.22 Op-tical studies show flows on the order of 200 km s−1, butthere are no X-ray measurements. Solar flares have up-flows and downflows of up to 200 km s−1, and accretion-driven upflows into coronal loops are expected to be oforder 20 km s−1. These velocities would be easily de-termined by ÆGIS and help to characterize a stellar sys-tem’s angular momentum, disk irradiation, and magneticdynamo.

3 The ÆGIS Payload3.1 ÆGIS Science InstrumentsÆGIS exploits modern mirror and grating technology toprovide enormous gains in effective area and a critical im-provement in resolving power (see Figure 2) at modestcost.

The major payload components are a Flight Mirror As-sembly (FMA), a Grating Array Structure (GAS),and aFocal Plane Assembly (FPA). The FMA holds a Wolter-I optic comprised of concentric segmented slumped-glassmirrors. This optic, with a 4.4 m baseline focal length(much shorter than IXO’s 20m) enables a grating spectro-meter effective area comparable to IXO’s at vastly lowercost. Mirror characteristics will be optimized in future de-sign studies.

The GAS is mounted close to the FMA and holds sixpairs of CAT grating arrays; the novel CAT gratings aredescribed in detail below. The FPA consists of three CCDcameras to record dispersed spectra and a single CCDcamera to provide 0th order imaging. An Optical BenchAssembly maintains the mirror, gratings and detectors inthe proper alignment. Each of the ÆGIS payload compo-nents has been studied in detail in the course of IXO def-inition studies. This gives us confidence in our estimatesof the mass, power and bandwidth required, in the morecompact, and much lower-cost configuration of ÆGIS.

ÆGIS instrumentation builds on strong heritage. Theoptical design is a direct design descendant of the high-resolution spectrographs built by the ÆGIS team and nowflying on Chandra. The mirror technology was inten-sively developed for IXO and used to produce flight mir-rors for NuSTAR, which will launch within the next fewmonths. Grating fabrication builds on techniques devel-oped for Chandra and benefits from substantial NASAfacilities and technology development in the MIT SpaceNanostructures Laboratory over the past decade. The fo-cal plane MIT Lincoln Laboratory CCDs derive directlyfrom devices now flying on Chandra and Suzaku,

3.2 Optical DesignÆGIS has six independent objective CAT grating spec-

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

Parameter Value Units RemarksSpectrometer1

Effective Area 1440 cm2 @O VIII Lyα(0.653 keV)

> 800 cm2 0.25− 0.9 keVE/∆E > 3500 0.25− 2 keV

Mirror1

Focal Length 4.4 mDiameter 1.9 mHPD (on-axis) 10 arcsecMass 255 kgPower 175 W heaters

CAT GratingsBlaze angle 3.5 degPeriods 200, 230 nm arranged in

opposing pairsMass 9.1 kgPower 0 W

Focal Plane Assembly1

Spec. Cameras 3 + 1 0th orderArrays/Camera 2CCDs/Array 4CCD

Type BI MIT/LincolnSize 25x25 mm 24µm pixelFrame Rate 15 Hz

Mass 60.4 kgPower 75 W

Table 2: Payload summary for baseline ÆGIS mission.1 Current best estimates for spectrometer performance, massand power.

trographs28 that operate in parallel, with each servedby a pair of diametrically opposed ∼ 30◦ sectors (sub-apertures) of the FMA (see Fig 9). Dividing the tele-scope into 12 sub-apertures maximizes the spectral re-solving power achievable from the ÆGIS mirror becauseeach sub-aperture produces a Line-Spread Function (LSF)in the dispersion direction which is a factor of severalsmaller than the full-aperture point spread function. ForÆGIS a LSF with FWHM . 3” is sufficient to achieveR > 3000. This is obtained by sub-aperturing a mirrorof 10” half-power diameter in the ÆGIS design. Eachmirror sector is fitted with an array of light-weight blazedCritical-Angle Transmission (CAT) gratings.26 Each grat-ing array produces a separate dispersed spectrum. Tomaximize spectral resolution, the gratings for each sec-tor are oriented to disperse normal to the local mirror ra-dius (see Figure 9). The local dispersion direction is thusnearly normal to the dominant mirror scattering direction.

The gratings behind diametrically opposed sectors areblazed to share a single readout array, and the compactÆGIS payload allows the six spectrographic readout ar-rays to be packaged in only three focal plane cameras.The gratings in diametrically opposed arrays have dif-ferent periods (200 nm or 230 nm, respectively). Thissmoothes the effective area curve and provides additionalwavelength redundancy in case of a single CCD failure.A CCD at the telescope focus provides 0th order images

with half power diameter on-axis of ∼ 10 arcsec. Gratingsand readouts for each spectrometer follow the surface of aRowland torus, similar to the optical designs for the Chan-dra HETG8 and XMM RGS.15 Limiting the band to < 2keV allows the use of larger graze angles, giving a lowermass and a more compact system.

CCD Camera

Optical Axis

Rowland Circles

Zeroth Order CCD

Grating Modules

Mirror Shells

Figure 9: Schematic of the dispersion geometry for a pair ofCAT sectors, comprising one (out of six) CAT grating spectrom-eters. The mirrors are completely covered by 12 grating sectors.

3.3 Flight Mirror AssemblyThe large outer diameter and the need for sub-aperturingare well-matched to a segmented mirror design. The seg-mented glass mirror technology that has been under devel-opment for IXO and is being used on NuSTAR is ideallysuited for this purpose.60–62 The entire mirror assemblyis segmented into two radial rings, the inner one coveringradii from 460 mm to 650 mm and the outer one from 650mm to 930 mm. The inner ring is further divided into 12identical 30◦ modules in azimuth and the outer ring into24 identical 15◦ modules. This makes the optics produc-tion amenable to parallel and mass production, reducingboth schedule and cost. In addition, the small (f/2.3) fo-cal ratio (relatively large graze angles) translates into asmall number of mirror shells: 63, barely 1/6th of IXO’s360. ÆGIS mirror sectors are made in a four-step pro-cess: (1) The 2 × 63 forming mandrels, which give thesegments their figure, are fabricated. The same mandrelsare used for all modules. (2) Mirror segments are madeby slumping thin glass sheets on the mandrels and coatingthem to optimize X-ray reflectance; (3) Mirror segmentsare aligned and bonded into modules; and finally (4) Mir-ror modules are aligned and integrated into mirror sectorsto which the grating arrays will be attached.

3.4 CAT GratingsThe recently developed CAT gratings combine the ad-vantages of traditional transmission gratings (low mass,relaxed alignment and figure tolerances, transparency athigher energies) with those of blazed reflection gratings(high broad-band diffraction efficiency, higher dispersion,with its accompanying higher resolving power, at shorterwavelengths due to blazing into higher diffraction orders).

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

Low-mass, large area grating arrays are fixed in place im-mediately behind the mirrors to intercept and disperse thelight reflected from them. By placing the gratings at max-imum distance from the focus, the dispersion distanceand resolving power are maximized. The CAT gratingsare fabricated from silicon wafers using nano-fabricationtechniques. They are described in more detail in a com-panion RFI response on grating technology.25

3.5 Focal Plane AssemblyThe readout arrays incorporate detectors derived directlyfrom those now flying on Chandra and Suzaku. Eachis a 100 mm-long, linear array of 4 back-illuminated,frame-transfer CCD detectors. The high-quality back-side passivation, as demonstrated on Suzaku, provideshigh quantum efficiency and excellent spectral resolution(∆E = 50 − 80 eV) to separate multiple spectral ordersdispersed by the gratings. In operation the CCDs are pas-sively cooled to 183 K. Two spectroscopic readout arraysare contained within each camera housing, and three suchcameras are mounted in the focal plane. A fourth cameracontains a single CCD to provide an accurate origin forthe wavelength scales along with 10” (HPD) imaging on-axis and a 19’ × 19’ FOV (see Fig. 10). While (by design)the gratings diffract most E < 1 keV photons to the spec-troscopic readouts, higher energy photons are transmittedthrough the thin CAT gratings (only 6µm thick) to the 0th

order image. The collecting area of the 0th order imagerpeaks above 1000 cm2 at 1.5 keV and exceeds 500 cm2

between 1.1 and 1.9 keV. The CCDs are read at a framerate of 15 Hz. Each camera is served by a dedicated de-tector electronics assembly, and the entire focal plane isserved by a single, internally redundant digital processingassembly.

Figure 10: Layout of the ÆGIS FPA, showing the 3 cameraswith two readout arrays in each. Each CCD is (25 mm)2 in sizewith 25µm pixels. The radius of the outer circle is ∼ 370 mm.

3.6 Technology Readiness & DevelopmentÆGIS technology readiness has advanced steadily in ac-cordance with detailed plans. We briefly summarize tech-nology readiness levels (TRL) and development plans forÆGIS components here.

Mirror TRL: As of October 2011, we have success-fully fabricated fused quartz mandrels and mirror seg-

ments that meet ÆGIS requirements. We have suc-cessfully and repeatedly aligned and bonded single pairsof mirror segments into module housing simulators andachieved X-ray images better than 10 arc-second HPD andLSF FWHM < 3”, meeting ÆGIS requirements (Figure11).17 We therefore judge the mirror to be at TRL 4 atpresent and expect TRL 5 by the end of CY2012. (See thecompanion RFI response by Zhang et al.)59

Figure 11: (Left) Mirror segments have been aligned andbonded to a housing simulator. (Right) The image obtainedfrom 4.5 keV Ti-K X-rays shows the effect of sub-aperturing—projection onto the x-axis yields 1.4” FWHM.

L1 Supports

Figure 12: Scanning electron micrograph of a dry-etched, free-standing 25 × 25 mm2 grating membrane with 200 nm periodSi CAT grating bars and an integrated “Level 1” support mesh.Left: Bottom view. Right: Cleaved cross section.

Grating TRL: The required ultra-high aspect ratio grat-ing bar geometry (150:1) has been successfully demon-strated. X-ray performance of small wet-etched proto-types is in good agreement with theory.26. The CAT tech-nology was judged to be TRL 3 in 2009 when these mile-stones were reached. Since then, dry-etched, large-area(∼ 6 cm2), free-standing samples with the full hierarchyand complexity of support structures have been fabricated(see Fig. 12),27 and we expect to achieve TRL 4 within ayear.

FPA TRL: The ÆGIS FPA could be implemented withexisting, flight-proven technology and meet all require-ments except the 15 Hz readout rate. The CCDs now fly-ing on Suzaku meet ÆGIS requirements for pixel-size, ar-ray size, detection efficiency and spectral resolution. Thecamera design is a simplified (and smaller) version of theACIS camera now flying on Chandra. The detectors have

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

been judged TRL 5 (relative to the 15 Hz readout rate re-quirement) in IXO evaluations. The required readout ratehas been demonstrated on several optical CCD devicesand the required design modifications are summarized be-low.

Technology Development Plans: Detailed technologydevelopment plans for ÆGIS technology were definedin IXO studies. Here we summarize steps required toachieve TRL 5. With adequate funding we expect to beable to achieve all of these milestones in the next 2-3years. Mirrors: 1) Implement alignment and bondingprocedure for co-alignment and bonding of multiple pairs.2) Verify long-term alignment stability of epoxy-bondedmirrors. 3) Construct and evaluate proto-type modulescontaining multiple pairs.Gratings: 1) Minimize area of support structures thatblock X-rays. 2) Demonstrate KOH polishing and X-ray performance of dry-etched samples. 3) Integrate/aligngrating membrane and facet frame. 4) Demonstrate bread-board grating array. 5) Environmental tests.FPA: 1) Implement CCD bus strapping and high respon-sivity output nodes on ÆGIS devices. 2) Demonstratelow-power analog processors.

4 The ÆGIS Mission4.1 Mission DescriptionÆGIS achieves its science objectives with a powerful pay-load in a mission of moderate size and cost. A capable butconventional spacecraft, an efficient orbit, and a modernoperations model serve to maximize science return on in-vestment. Key characteristics of the nominal ÆGIS mis-sion adopted for costing purposes are presented in Table 3.We will refine these in future definition studies.

ÆGIS requirements are met by conventional spacecraftarchitectures and components. Figure 13 shows the base-line spacecraft concept in its launch configuration and asdeployed. Aspect information is provided by a pair of startrackers on the bus and a fine guidance sensor mountedon the focal plane. Following the successful Swift ap-proach, the attitude control system supports safe slewingin response to time/target-coordinate pairs in the com-mand load. Onboard command and science data stor-age capacity permits autonomous operation for nearly 2weeks without command or data loss, minimizing opera-tions costs (see below). Note that science data downlinkrates are quite modest thus are not a technology concern.

A number of launchers, including Falcon-9 and At-las V-401, can place ÆGIS in the baseline halo orbit atSun-Earth L2 with ample performance margin. The cost-effective Falcon-9 is baselined. The L2 orbit provides ex-cellent observing efficiency and a stable thermal environ-ment that simplifies payload and spacecraft design. Thelaunch vehicle carries ÆGIS directly to L2; the space-craft’s onboard propulsion system provides orbital injec-tion and subsequent station keeping. The nominal 3-yearprimary mission provides more than 70 Ms of science ex-posure time. This exceeds the time programmed for IXO

Parameter Value Units RemarksPayload1

Mass 325 kgPower 250 WTelemetry 128/1280 kbps avg./peak

Spacecraft1

Mass 385 kgPower 320 WPointingcontrol

30 arcsec max. deviation(200 ks)

1 arcsecs−1

max. drift rate

Pointingknowledge

1.3 arcsec 3σ per axis

Observatory2

Mass 1090 kg at launchPower

Total 740 WCapability 1330 W EOLMargin 590 W 44% EOL

Telemetry 208 kpbs averageRecorder 24 Gbyte 12.7 days

MissionLauncher

Vehicle Falcon-9 std. fairingCapability 2010 kg to L2Margin 920 kg 46%

Orbit L2 Sun-EarthDuration

Prime 3 yrGoal 5 yr

Comm. 1 day−1 DSN

Table 3: Characteristics of a baseline ÆGIS mission which willbe refined in future definition studies.1 Current best estimates shown for payload and spacecraft.2 Observatory parameter values include 30% contingency.

XGS observations by a factor greater than 4. The environ-ment at L2 and the requirements it places on the ÆGISpayload and spacecraft are well-understood from exten-sive IXO definition studies. These included exercises inthe Instrument Design and Mission Design Laboratoriesat Goddard Space Flight Center.

An Earth-trailing drift-away orbit, like that used bySpitzer and Kepler, also appears to be a viable option, asit can be reached with the same launchers and would pro-vide essentially the same observing efficiency. We willevaluate the tradeoffs between these candidate orbits infuture defintion studies.

ÆGIS will be open to all astronomers with peer-reviewed competition for all observing time after al-lowances for a short initial verification period and a small(∼ 5%) allocation for calibration and engineering pur-poses. ÆGIS will minimize mission planning costs by us-ing month-long planning cycles. Daily communicationswith the spacecraft are envisioned, mostly for purposesof monitoring health and safety, although as noted abovethe spacecraft would be capable of autonomous operationfor nearly two weeks. Command load segments could be

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

Figure 13: ÆGIS in Falcon-9 standard fairing (top) and as de-ployed (bottom). Graphics produced by Ball Aerospace.

updated and data downloaded in those communications.Target-of-Opportunity (TOO) observations could be ac-commodated without significant impact to the monthlyplan since the L2 orbit is relatively free from complica-tions such as momentum build-up, radiation belt passages,and eclipse management which drive Chandra and XMM-Newton mission planning.

4.2 Cost and Basis of EstimateThe ÆGIS ROM estimated total mission cost of $758M inFY2012 dollars, including launch services and reserves, isnear the low-end of the medium-cost category. ROM costestimates for major elements of the baseline ÆGIS mis-sion are presented in Table 4. Spacecraft, management,systems engineering, mission assurance, and integrationand test cost estimates are based on advice from experi-

enced contractors and are consistent with commonly ac-cepted scaling rules. Mirror and instrument costs were ob-tained by scaling detailed PRICE-H estimates developedduring IXO definition studies, incorporating grass-rootsestimates for grating element and CCD detector costs de-veloped at MIT. Payload costs include 6-months of mirrorand instrument calibration at Marshall Space Flight Cen-ter’s X-ray and Cryogenic Facility (XRCF). Launch costis estimated from public commercial sources, confirmedby contractors, and includes an allocation for NASA over-sight.

Flight operations costs appropriate for a probe-classmission are based on industry estimates of 15 FTEs in-cluding cross-trained engineers, controllers and plannerswith managerial and administrative support. Cost for anobservatory-class mission would be slightly higher. Pre-launch operations costs include all elements required toprepare for full operations support at launch, includingall modifications to the Chandra data system required toproduce archival quality data products. DSN communi-cations costs include early flight operations followed by asingle 30 minute contact per day. Science operations costscapture calibration, operations, planning and administra-tive functions as well as peer review and grants admin-istration. The observer grants budget would support, forexample, 200 programs per year averaging 125ks at $50k(FY2012), together with a small number of postdoctoralfellows. A theory program during the early mission wouldtransistion to an archival program later in the mission.

MissionElement

ROM Cost(M$, FY2012)

Remarks

Management,Sys. Engineering,& SMA

68

PayloadMirror 130 GSFCGAS & FPA 93 Price-H & MITCalibration 3 MSFC XRCF

Observatory Contractor est.Spacecraft 112I & T 16.4Launch

support7.8 incl. pre-launch

LaunchServices

75 Falcon-9 withoversight

Operations 3 yearsPre-launch 28 Flt. & Sci.Flight & Comm. 16 DSN, 3 yearsScience 25.5

Observers 40 GO programCBE Total 615Reserves1 143MISSION TOTAL 758

Table 4: ÆGIS misson cost is in the medium-cost category.1 Reserves: 30% in Phases A-D excluding launch; 20% in PhaseE excluding GO program.

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

AcknowledgementsThe ÆGIS science team is grateful for technical sup-port and advice from a group at Ball Aerospace andTechnologies Corporation, led by Steve Jordan; from agroup at Lockheed Martin Corporation, led by DomenickTenerelli; and from Chuck Lillie and Suzi Casement atNorthrup Grumman Aerospace Systems.

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

A ÆGIS TeamG. E. Allen MIT gea@space.mit.eduM. W. Bautz MIT mwb@space.mit.eduJ. Bookbinder SAO jbookbinder@cfa.harvard.eduD. Bower Lockheed Martin Co. daniel.r.bower@lmco.comJ. Bregman University of Michigan jbregman@umich.eduN. Brickhouse SAO bhouse@head.cfa.harvard.eduD. N. Burrows Pennsylvania State University burrows@astro.psu.eduC. R. Canizares MIT crc@space.mit.eduS. Casement NGAS suzanne.casement@ngc.comD. Chakrabarty MIT deepto@space.mit.eduK.-W. Chan NASA/GSFC kai-wing.chan-1@nasa.govB. Costello Lockheed Martin Co. bill.costello@lmco.comJ. E. Davis MIT davis@space.mit.eduD. Dewey MIT dd@space.mit.eduJ. J. Drake CfA jdrake@head.cfa.harvard.eduM. S. Elvis SAO elvis@cfa.harvard.eduD. Evans Elon University/CfA devans@cfa.harvard.eduA. Falcone Pennsylvania State University afalcone@astro.psu.eduK. Flanagan STScI flanagan@stsci.eduR. Heilmann MIT ralf@space.mit.eduJ. C. Houck MIT houck@space.mit.eduD. P. Huenemoerder MIT dph@space.mit.eduS. Jordan Ball Aerospace sjordan@ball.comA. Klavins Lockheed Martin Co. aklavins@msn.comJ. C. Lee Harvard University jclee@cfa.harvard.eduJ. Lempke Lockheed Martin Co. jim.lempke@lmco.comC. Lillie NGAS chuck.lillie@ngc.comM. Loewenstein NASA/GSFC michael.loewenstein-1@nasa.govH. L. Marshall MIT hermanm@space.mit.eduS. Mathur Ohio State University smita@astronomy.ohio-state.eduR. S. McClelland SGT ryan.s.mcclelland@nasa.govJ. M. Miller University of Michigan jonmm@umich.eduR. Mushotzky University of Maryland richard@astro.umd.eduT. Nguyen Lockheed Martin Co. thang.nguyen@lmco.comF. Nicastro SAO fabrizio.nicastro@oa-roma.inaf.itM. A. Nowak MIT mnowak@space.mit.eduS. L. O’Dell NASA/MSFC steve.o’dell@nasa.govF. Paerels Columbia University frits@astro.columbia.eduR. Petre NASA/GSFC robert.petre-1@nasa.govA. Ptak NASA/GSFC andrew.f.ptak@nasa.govT. T. Saha NASA/GSFC timo.t.saha@nasa.govM. Schattenburg MIT marks@space.mit.eduN. Schulz MIT nss@space.mit.eduR. K. Smith SAO rsmith@cfa.harvard.eduD. Tenerelli Lockheed Martin Co. domenick.tenerelli@lmco.comR. Vanbezooijen Lockheed Martin Co. roel.vanbezooijen@lmco.comG. Vasudevan Lockheed Martin Co. gopal.vasudevan@lmco.comD. Wang University of Massachusetts, Amherst wqd@astro.umass.eduS. Wolk CfA swolk@cfa.harvard.eduW. W. Zhang NASA/GSFC william.w.zhang@nasa.gov

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NNH11ZDA018LMission Concepts for X-ray Astronomy ÆGIS

B Acronym ListACIS Advanced CCD Imaging SpectrometerAEGIS Astrophysics Experiment for Grating and Imaging SpectroscopyAGN Active Galactic NucleusCAT Critical Angle TransmissionCBE Current Best EstimateCCD Charge Coupled DeviceCOS Cosmic Origins SpectrographDSN Deep Space NetworkEOL End of LifetimeESA European Space AgencyEW Equivalent WidthFMA Flight Mirror AssemblyFOV Field of ViewFPA Focal Plane AssemblyFWHM Full Width Half MaximumGAS Grating Array StructureGO Guest ObserverHEG High Energy GratingHETG High Energy Transmission GratingHETGS High Energy Transmission Grating SpectrometerHPD Half Power DiameterHST Hubble Space TelescopeIGM Intergalactic MediumISM Interstellar MediumIXO International X-ray ObservatoryLETG Low Energy Transmission GratingLMC Large Magellanic CloudLSF Line Spread FunctionMEG Medium Energy GratingNS Neutron StarNWNH “New Worlds, New Horizons” (a.k.a., “Decadal Survey 2010”)OBF Optical Blocking FilterPMS Pre-Main-SequenceRFI Request for InformtionRGS Reflection Grating SpectrometerROM Rough Order of MagnitudeSAT Strategic Astrophysics TechnologySMA Safety and Mission AssuranceSMBH Super-massive Black HoleSMC Small Magellanic CloudSXS Soft X-ray SpectrometerTOO Target of OpportunityTRL Technical Readiness LevelWHIM Warm-Hot Intergalactic MediumXGS X-ray Grating SpectrometerXIS X-ray Imaging SpectrometerXMM X-ray Monitoring MissionXRB X-ray Binary

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