GAMMA-RAYBURSTS: CURRENTOBSERVATIONALPICTURE

Jay P. Norris

Laboratory for High Energy Astrophysics

NASA/Goddard Space Flight Center

Greenbelt. MD 20706

ABSTRACT

We now know that long (duration > 2 s) gamma-ray burst (GRB) sources lie at distances

comparable to the ‘Yadius” of the Universe and that they are momentarily the most

luminous known objects: several GRBs have now been observed to have associated

X-ray, optical, and radio counterparts following power-law decays that fit theoretical

expectations for relativistic blastwaves. Three GRBs (and/or their host galaxies) have

determined redshifts larger than z = 0.8, and virtually all GRBs with associated optical

transients appear to lie in distant galaxies. Spectroscopic analysis of afterglows and

modeling of the GRB brightness distribution indicate that GRE3 sources lie in star-

forming regions. The population statistics of GRE3s as well as their energetic, spectral,

and temporal aspects-also consistent with the cosmological distance scaleare difficult

but not impossible to explain via neutron star or neutron star-black hole mergers. The

suggested association of smooth, single-pulse GRE3s with supernovae in nearby redshift

space can only involve an insignificant fraction of the total GRE3 population; moreover

the association is contraindicated by the random sky distribution of single-pulse GRBs.

Future observations of GRBs with rapid temporal variations at supra-GeV energies by

GLAST, the Gamma-ray Large Area Space Telescope, may provide a unique cosmic

experiment for constraining the scale of quantum gravity. Detection of a gravitationally

lensed GRB would be evidence of a non-zero value for A, the cosmological constant.

0 1998 by Jay P. Norris.

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1 Introduction: GammaRay Bursts Become Highly Visible

Gamma-Ray Bursts (GRBs) are currently enjoying the best public relations since their

discovery 30 years ago. Even Sky and Telescope (February 1998), which reaches a large

popular scientific audience, featured a fantastic looking cover picture of the suggested

progenitor object moments before the GRB explosion. The object which looks like a

crackling bowling pin, frozen in time, represents the merger of a neutron -star (NS) binary.

Of course the ends of the bowling pin would be rotating at a sizable fraction of the speed of

light during the final moments before coalescence. The feature article,’ appearing one year

after the discovery of the first X-ray and optical counterparts to a GRB, concerned the

gigantic energy output of GRBs and the theoretical burst of cosmic rays which would follow

the electromagnetic (EM) radiation of the GRB in a matter of days. Since the sources of

GRBs (at least those of the ‘¶ong”duration variety) lie at cosmological distances-billions of

light-years from our Galaxy-GRBs must be luminous in the extreme. The rate of detected

GRBs in the Universe (- 10” galaxies) -a few per day-translates into - one GRB per

galaxy per - 10’ years. The total rate would be much higher if the radiation pattern were

strongly beamed into a small solid angle: only a small fraction of GRBs would then be

detected by Earth observers. Thus, every two rotations of the Sun about the Galactic Center

(a half billion years), a GRB would be close enough (- 1 kiloparsec) for the accompanying

cosmic ray burst to annihilate all life on Earth. This is just one of several recent cosmological

theories which might provide a sufficiently large energy release to power a GRB! We shall

also examine the recent putative connection between type Ic supernovae (SNe) and GRBs,

and find that only a very small fraction of GRBs can be connected with SNe.

Observations of NS binaries have already provided interesting tests of physical theory.

Table 1 lists several confirmed and suspected NS systems, along with their distances Erom

Earth, rotational periods, and orbital decay times. Included is the famous Hulse-Taylor

binary pulsar, which provided the first quantitative evidence for gravitational radiation-

energy derived from orbital decay. The distance column is relevant for the GRB/cosmic;ray

burst extinction theory, but the decay, or coalescence, timescale provides an astrophysical

constraint: When the components of the binary each go supernova, a large velocity kick, -

IO3 km s-‘, can be imparted to the system. Assuming coalescence timescales of 0.1 - IO Gyr

(still comparable to the age of the Universe), we would look to find at least some of these

systems - 100 kpc - 10 Mpc outside their parent galaxies. Note however, the systems listed

in Table 1 are those we have detected; systems with much shorter decay timescales are much

less likely to be discovered before coalescence.

The central questions of source progenitor population and their distribution in redshift, as

well as the mechanisms for generation of y-radiation and the presumed beaming into a narrow

solid angle (probably completely distinct from processes governing the counterpart

afterglows), all will be con&mine&some in ways we do not yet apprehend-not only by

evidence from afterglows, but also from the extrinsic characteristics of GRBs and their

spectral/temporal behavior.

Table 1: Pairs of Neutron Stars in Our Galaxy

Name Distance Period

(kiloparsecs) (hours)

Decay Time

(10’ years)

PSRB1534+12 0.5 10.1 2.73

PSR B1913+16 ’ 7.3 7.75 0.30

PSR B2127+11C 2 10.6 8.05 0.22

PSR B2303+46 3 2.3 296 4000

PSR J 15 IS+4904 3 0.7 207 2400

’ Hulse-Taylor pulsar (first direct evidence for gravitational radiation)

’ In Globular Cluster Ml3

’ Suspected binary neutron stars - masses not firmly established

The picture of GRBs at gamma-ray energies as revealed by current instruments on the

Compton Gamma Ray Observatory is reviewed in Section 2. We then proceed to examine

some more definitively derived constraints imposed by the recent detections at longer

wavelengths in Section 3. The apparent association of at least one GRB with an unusual SN

is critiqued in Section 4. Section 5 describes two future measurements: a constraint on the

energy scale of quantum gravity, using GRBs detected by GLAST; and a constraint on A via

detection of gravitationally lensed GRBs.

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brightness distribution would follow a -3/2 power law. The two dashed lines are -3/2 power

laws. Shown separately are distributions for long (Tgo > 2 s) and short bursts (TN < 2 s); see

section 2.4 for a discussion of GRB durations. T,,, is the interval between the times to

accumulate 5% and 95% of the total counts in a GRB. Clearly the long bursts depart more

markedly on the dim end from the -312 power law, suggesting that we see through more

nearly to the edge of their spatial distribution than we do for the short bursts. Said

differently, short bursts would appear to be dimmer and not sampled as deeply in redshift

space. This is significant, since we have yet to see an X-my, optical, or radio counterpart for

a short burst (the GRB instrument on the BeppoSAX satellite-which has signaled the alert

for almost all GRB counterparts so farAoes not trigger on short bursts). So short bursts

could still be galactic. But, short and long bursts separately have isotropic sky distributions.

The third curve labeled ‘YJHE” ( No High Energy) is the brightness distribution for long

bursts that have no significant emission above - 300 keV. Because of its steeper, nearly -3/2

slope, this subsample has been claimed to be in nearer redshifi space, and less luminous

because they are supposedly not beamed directlytowards Earth.4 This would constitute the

main observational evidence for beaming. However, the apparent NHE classthat is, the

approximate slope and normalization of the NHE curve in Figure 1 -an be synthetically

realized by equalizing the signal-to-noise levels (s/n) of bright bursts (practically all of

which have HE emission) to successively dimmer steps in the log N - log PF, and redshifting

their spectra (dimmer bursts, presumably at larger redshifts, have softer energy spectra). In

this explanation, the steepness of the NHE curve then is due to the combined rapid onset of

dimmer and ‘kedder”bursts at lower PF essentially a brightness bias.’

Careers have been spent modeling the GRB brightness distribution, including such

extrinsic effects as cosmological time dilation and rate-density evolution with cosmic time;

and effects like a GRB luminosity distribution, which may also depend on cosmic time; while

also attempting to account for such instrumental effects as incomplete sampling on the dim

end and dependence on spectral response. Clearly, we do not presently know enough about

GRBs’intrinsic attributes and extrinsic distributions to hope for these efforts to be entirely

successful, but we will know enough soon, after accumulation of a sufficient number of

redshifts (and therefore luminosities) from counterparts that will calibrate the brightness

distribution. The current wisdom in one camp is that the GRB rate -density follows that of

star formation,6.7 which apparently peaked near redshift epoch z - 1.25 - 1.5, and then

declined to the present time. But other modelers’ claim that a constant GRB rate-density is

equally acceptable. Models including luminosity distributions are more realistic (but a broad

luminosity range may not be needed) and are consistent with the peak fluxes and redshifts of

GRBs 970508, 971214, and 980703 (Ref. 9). It may also be judicious to consider the log N-

log PF for long and short bursts separately.”

For the cosmological distance scale, it is easy to understand qualitatively why the dim end

of the GRB brightness distribution should depart from a -312 power law: assume a constant

number of GRBs per year, per Mpc3 everywhere in the Universe. Then two effects diminish

the observed number of dim bursts on Earth. First, time dilation makes the “per year”longer

for the observer, and second, gravitational lensing of distant volumes by the nearby matter

distribution makes the ‘per Mpc ‘“appear larger.

2.2 Cosmological Time Dilation and Spectral Redshift

Before the modem era of counterparts, two additional pieces of evidence supported the

cosmology camps position. (The incredible energetics required of nature was the primary

obstacle for the cosmological distance scale, hut that is manifestly no longer a problem for

nature.) Cosmological time dilation (TD) comes into play not only in log N - log PF as part

of the rate-density consideration, but also for the innards of bursts: pulse widths, intervals

between pulses, and overall durations. Since GRBs are immensely varied in temporal appearance, there is ample opportunity to measure these timescales. However, measurement

difficulties ensue since the dynamic ranges of GRB durations and pulse widths are large.

Also, brightness bias and instrument triggering effects enter the game (GRB monitors usually

are half asleep, waiting in low temporal resolution mode for a GRB to occur; then the

instrument switches to higher time resolution modes to better record the fast time structure).

We have measured the relative, average widths of dimmer bursts (presumably more

distant) compared to those of the brightest bursts (presumably closer) by a variety of

methods. All our TD measures include a preparatory step which approximately equalizes the

s/n levels of all burst time profiles to that of the dimmest bursts in the sample. “J’ One

particularly ‘tommon -sense” method is to group bursts by PF, then within a group, bring the

time profiles into temporal registration with the highest peak in each burst positioned at ta.

Estimating the peak time in an unbiased manner is not necessarily straightforward for the

artificially dimmed bursts, but is facilitated by a wavelet “denoising” approach. One then

measures, e.g., the FWHM of the average profile for each peak-flux grouping and compares

these widths with that of the brightest group. This is the observed time-dilation factor (TDF),

and would be equal to (I + z)/(l + z bright&, where z is the average redshift for a given peak-

flux group, except that some corrections need to be estimated and applied.

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nebula) power-law spectrum, and have been mapped to EGRET energies, - 20 GeV. The

spectral peak in v-f(v) provides a break in the otherwise featureless spectra, with which to

measure relative redshift of the sources. Redshift is time dilation of the EM radiation itself.

Mallozzi and colleagues performed the analysis:’ utilizing a sample of 1 ong and short bursts,

down to a PF - 1 photon cn? s-‘, where BATSE detection is nearly complete. From their

Figure 1 b, a plot of Speak vs. PF, we see that dimmer bursts have progressively lower average

v*f(v) measures, consistent with time-dilation results.

100 lo1 102 10s 104 105 10s Energy (keV)

Fig. 3 - Canonical spectra (v*f[v]) of high-energy transients: Gamma-Ray Bursts, X-Ray

Binaries, Black Hole Binaries, and the Crab.

2.3 Summary, GRB Extrinsic Characteristics

This much was clear for long bursts, before the detection of counterparts: isotropy of the

GRB celestial distribution, the deficit of dim bursts in the log N - log PF, time-dilation and

v*f(v) trends with PF were mutually consistent with the cosmological distance scale

hypothesis. None of these indicators individually, nor all combined, proved anything, given

that subclasses of GRBs with different distributions could be invoked in models. There are

still reasons to entertain possible subclasses.

For example, the distance scale for short bursts has not been established definitively.

Short bursts’ localizations by themselves are isotropically distributed, as are those of long

bursts. But the log N -log PF relation for short bursts does not indicate a definitive deficit at

low peak fluxes, since the leveling off on the dim end is more attributable to BATSE

detection efftciency: some short bursts are so short that they do not till the shortest trigger

accumulation bin (64 ms). To my knowledge, no one has troubled to model their log N - log

PF, probably because of the more severe detection completeness problem. Moreover, short

bursts may not be sampled sufficiently deeply in redshift space to manifest curvature, thus

modeling may not provide robust constraints given measurement uncertainties.

Some work on time dilation of short burst profiles has been reported. ” Our work on short

bursts does not reveal acceptably stable results as the sample enlarges, or as adjustable

parameters are varied. The whole treatment is more difficult: the profiles are so short,

containing little information; the definition of peak flux on short timescales is more arbitrary

than for long bursts; and unbiased time-dilation measures are difficult to construct and

calibrate. The optimum BATSE data type for short bursts, time-tagged two us photon data, is

of limited use due to its 32K maximum event buffer size, which effectively windows the

burst record, such that the start and stop times depend on burst brightness. The v*f(v) vs. PF

relation for short bursts shows a barely significant trend (R. Mallozzi, private comm.). Worst

for short bursts, BeppoSAX has not detected any, and so mere are no X-ray or optical

counterparts to go chase, so far.

2.4 GRB Duration Distribution

The brightness distributions for short and long bursts might suggest that they are truly

different groups. In fact, their spectral distributions are similar and overlapping, with short

bursts tending to be slightly harder. Of course the defining difference is temporal: the

logarithmic duration distribution is clearly bimodal, with maxima near 25 s and 300 - 500 ms

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300 GRB 930131 - Superbowl Burst

1 I I I I I I

- BATSE profile

A EGRET photons

-iii -4

0 1 2 3 4 5 6 7 6 Time (seconds)

Fig. 5 - GRB 930131 (whose wavefront passed by the Earth during Superbowl XXXII) as

detected by BATSE and EGRET. Vertical axis is truncated: maximum BATSE count rate

was z 7 x 10’ s-t , and limited by deadtime effects. EGRET spark chamber deadtime per

event is - 100 ms. Thus neither instrument recorded actual fluxes during the initial intense

spike.

Short and long bursts are similar in two other respects. Both tend to exhibit hard -to-soft

spectral evolution, within a pulse and across the entire duration of an event: Pulses. tend to be

longer and their onsets later at lower energies, and bursts tend to be longer at lower energies.

However, both groups exhibit the ‘Cdsymmetry pulse paradigm’! the more asymmetric a

pulse, the wider it is, with lower energies peaking at later times. Conversely, in nearly

symmetric pulses, the peaks are aligned across energy bands.“829

Since the radiation mechanisms of GRBs may be completely separate from processes

governing their counterpart afterglows, the temporal/spectral development at gamma-ray

energies will remain central to eventual understanding. Recent temporal analysis work

employing objective algorithms for the determination of the number of pulses in a burst, as

well as pulse rise and decay times, has been done by Scargle”, using a newly formulated

“Bayesian Block”approach to estimate positions of change-points -where the mean changes

significantly. Constraints that GRB time profiles impose on internal vs external shock

models, via considerations of kinematics of the expanding emission front, have been

expounded by Fenimore and co-workers.3’

3. GRB Counterparts at Longer Wavelengths

3.1 Discoveries of GRB Counterparts and High Redshifts

On February 28, 1997, the gamma-ray burst community was surprised by the breakthrough

discovery of an X-ray counterpart to GRB 970228, detected by the BeFpoSAX GRB monitor

and the narrower field X-ray instruments. The evidence from two observations, separated by

three days, revealed the appearance and decay of a previously unknown X-ray source

consistent with the position of the GRB.3Z Subsequent X-ray detections of GRBs,

outnumbering optical detections by about two to one, have established a clear pattern of

power-law temporal decays, predicted by Meszaros and Rees as the signature of cosmological

tireballs/blastwaves in the aftermath of a GRB.33

The X-ray localization accuracy (- 3 arc minutes) from BeppoSAXs wide field camera

was sufficiently refined to allow deep pointed observations with large optical telescopes. The

path to The Holy Grail of GRBs was then established by van Paradijs, Sahu, and (3a,ma,34.35.36 upon observing the X-ray error circle on Mar 1 and Mar 8 with the William

Herschel (4.2 m) and Isaac Newton (2.5 m) telescopes, and finding a decaying optical source

at visual magnitude - 214xtremely faint compared to previous claims of possible archival

optical associations with GRBs. Two weeks later the field was observed by HST. The

closeup revealed the still-bright GRB counterpart, compared to an extended nebulosity-the

host galaxy. Comparison with a second HST observation made on Apr 2 led Caraveo and

colleague? to declare that the object had manifested a proper motion of 18 milliarcseconds,

and was probably a compact star moving at - 250 km se’ at a distance of - 100 pc; - 0.1 pixel

centroiding was claimed, and possible to achieve.

-531-

-ZES-

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average flux would be - 16* ergs 5.‘. The energy in the optical afterglow is estimated as - 2

x 10” ergs, largely independent of beaming considerations, since the fireball expansion is

presumed to be subrelativistic and nearly symmetric. And since the afterglow is best

understood as an adiabatic phenomenon, a very small fraction of the energy available in the

fireball is radiated. This is one energetics basis for the claim” that NS-NS or NS-BH

mergers come up short, given the likely conversion efficiency to yS (< few percent) and the

available rest mass of the binary, - 5 x 10” gm.

3.2 Summary: GRB Counterparts

Table 2 summarizes the nine optical counterparts to GRBs identified through July 1998.

Some assumptions about conversions between energy bands and crossinstrument calibrations

were necessary. Host galaxy magnitudes for the more recent GRBs are in the process of

revision as the optical transients continue to decay. So not all entries are definitive.

Variations in the r/X peak flux ratio and X-ray spectra are taken as evidence of varying

amounts of obscuration near the GRB source, indicative of star-forming regions.51 In each case we have imaging, spectroscopic, and/or light-curve evidence of a host galaxy. The

images are very faint extended/elongated smudges, but coincident in position with the optical

counterpart. Given precise optical positions, radio counterparts are now routinely detected

for a good fraction of GRBs. A third redshift (z = 0.966) for the optical transient associated

with GRB 980703 has been obtained. The absorption lines in its spectra also indicate the

presence of a star-forming galaxy.52 Note that GRBs 970508 and 980703 have comparable

redshifts, and their gamma-ray peak fluxes differ by a factor of only - 2, an indication that

the GRB luminosity distribution may not be too broad. However, Fruchter” argues that the

IR-optical colors for the GRB 980329 transient are best interpreted as absorption due to the

Lyman a forest extending to a redshift z - 5, implying a luminosity distribution in peak flux

greater than 100.

The combined astrometric and spectroscopic evidences indicate that those GRS sources

with associated optical counterparts are inside star-forming galaxies. Bloom and colleague?

derive a median merger timescale of - 10’ years, and thus the sites of NS-NS mergers should

be in close association with star formation regions, rather than outside host galaxies. Van den Heuvel maintains that many NS-NS binaries may merge on even shorter timescales. His

calculation extrapolates from the observed properties of B-emission X-ray binaries, which

comprise one neutron star and a massive companion, to arrive at the expected distribution

of orbital periods after the second supernova. The result is that - 50% of NS -NS

Table 2: Gamma-Ray Bursts with Optical Counterparts.

GRB Peak Fluxes ’ Host Galaxy

y-ray ’ x-my * y/X Ratio Optical 3 Radio 4 Brightness ’ z 6

970228 3.5 2.3 80 20.5 - 24.6 970508 1.2 3.0 25 19.8 1.2 25.8 971214 2.3 2.5 56 21.7 - 25.5 980326 1.3 4.7 17 21.0 - 25.3 980329 13.3 70.0 12 23.6 0.25 -29? 980425 1.1 2.6 26 13.7 49 14.3 980519 4.7 2.9 100 20.4 0.1 24.7 980613 0.63 0.7 57 22.9 24.4 980703 2.6 4.0 40 20.1 1.0 23.0

r photon cm-’ se’ (50-300 keV); conversion factor, photons to ergs, = 6.15 x lo-‘.

’ x IO-* ergs cm-’ s-r (2-l 0 keV).

3 R band magnitude.

4 milli Jansky, at 8.4 GHz (10 GHz for GRB 980425).

’ R band magnitude, corrected for galactic extinction.

6 Redshift.

’ See these two web pages for many current references on GRB afterglows:

GRB Coordinates Network notices, used to distribute GRB news rapidly,

gcn.gsfc.nasa.gov/gcn/ ; and www.obs.aip.de/-jc&rbgen.html.

- 0.835

3.412 -

-5??

0.0085 -

-

0.966

binaries may form with sufficiently short orbital periods such that they merge within 10’

years, leaving GRB sources well inside star-forming regions in host galaxies.” However,

optical counterparts have been detected only for the twothirds of GRBs in which rapid,

longer wavelength observation strategies were possible; the other third may be those where

the progenitor long ago escaped the host. We would hypothesize that optical emission

requires a dense environment and so is not generated in those cases.

-533-

-PES-

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+90

We visually searched our collection of background-subtracted, noiseequalized (to the

peak intensity of GRB 980425) time profiles of long bursts, liberally including in a first-cut

sample anything remotely resembling a single-pulse event. In automated passes, we fitted a

lognormal pulse model, excluding from further consideration those bursts with poor x2 or

correlated residuals in channels 1, 2, or 3-considering such to be evidence of more than one

pulse. The automated pass largely confirmed our expectations from the visual inspection.

We also restricted consideration to those bursts with FWHM within a factor of three of GRB

980425’s (4 - 35 s). Thirty-two bursts passed these cuts, and about five of these have “no

high-energy emission.” Most are dim bursts, and they have a clear resemblance to GRB

980425. Most but not all exhibited hard-tosoft spectral evolution.

For the two sets, with and without high-energy emission, we searched an exhaustive list

of 900 SN (most are the more luminous SN type II or type Ia) from the BATSE era. The list

is more complete by - factor of two during the last two to three years due to heightened

interest in high z objects. We found one or two possible SN associations for the NHE + HE

set, given the time constraint that the GRB must precede the first SN observation in any case.

Correcting for SN detection incompleteness in previous years, we estimated that - 5 GRBs

could be associated with SN type Ib/Ic. We were able to find redshifts for two-thirds of the type IbiIc SNe, and they are indeed

nearby: the redshifts are z <- 0.0175, i.e., up to roughly twice the distance of SN 1998bw (z

= 0.0085). Both the nearby SNe and the singlepulse GRBs might be expected to show a

preference for the Supergalactic Plane. In Figure 7 the SNe are plotted in the supergalactic

coordinate system. The continuous line is the Galactic Plane. Note that 16 of these 21 SNe

fall within 30” of the Supergalactic Plane and that they tend to avoid the region near the

Galactic Plane (due to obscuration). As expected, the quadrupole moment, <sin* b- l/3>, for

the SN distribution with respect to the Supergalactic Plane (b = supergalactic latitude) is

significantly oblate at the 2-o level, - 0.138’?,~~, where errors are estimated via a bootstrap

approach. Figure 8 shows the 32 single-pulse GRBs in the same coordinate system. There

appears to be no tendency for any subset of single-pulse GRBs to fall near the Supergalactic

Plane. The quadrupole moment for the entire GRB single-pulse set is - 0.003~0,~~ - not

significantly different from zero.

For economy of hypotheses, we suggest that some source population other than nearby

SNe generally accounts for the smooth, single-pulse GRBs. We conclude that either the

SNl998bw - GRB980425 association is coincidental, or events of this type comprise a very

rare and distinct phenomenon6’

+180

+180

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-90

Fig. 7 - The 21 SNe of type Ib or Ic found in the IAUC supernovae catalog, plotted in the

supergalactic coordinate system. The continuous line is the Galactic Plane. Sixteen SNelie

within 30” of the Supergalactic Plane. The symbol inside the square indicates SN1998bw.

The quadrupole moment for the SN distribution with respect to the Supergalactic Plane is

significantly oblate at the 2-o level, - 0.138’~~~.

I80

Fig. 8 - The 32 single-pulse GRBs in the supergalactic coordinate system. Circles indicate

GRBs with peak > 720 counts se’; triangles indicate bursts below this threshold-too dim to

determine reliably the number of pulses present. Filled (open) symbols are NHE (HE). No

subset of single-pulse GRBs shows a tendency to cluster near the Supergalactic Plane.

-535-

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