Swift/BAT Hard X-ray Survey
Preliminary results in Markwardt et al 2005
35'
energy coded color
BAT Instrument Parameters
• Energy Range 14 - 195 keV• Area 5200 cm2 (x 50% open fraction)• Field of View 2 Steradian, partially coded• Background 10,000 ct/s (cosmic diffuse
dominated)• Spatial 21’ sky pixel, centroided to <1-3’
Resolution• Spectral 6 keV FWHM @ 60 keV, average
Resolution• Sensitivity 0.2 photons/cm2/sec• Timing Resolution 100 usec• Observing “Random” (piggy-back Swift
Strategy GRB observing plan)
9-month Swift/BAT Survey> 1.5 Ms
1-1.5 Ms
0.5-1 Ms
Eclip
tic P
lane
8.5 mCrab (T/20 ks)-0.5
Exposure Map• Covers whole sky, mostly >1Ms• deficit on Ecliptic Plane due to
Sun avoidance• Sensitivity improves as square
root of time (1.2-2 X statistical) to 0.6 milliCrab in 3 years
Sensitivityvs Time
9 months---------------------• 323 sources• 39 unidentified
sources• 158 galactic• 5 galaxy clusters• 158 AGN
• 16 beamed
|Galactic b| >15 (74%)----------------------• 159 sources• 9 unidentified sources• 29 galactic• 2 galaxy clusters• 121 AGN
• 15 beamed
already the most sensitive all-sky hard x-ray surveyAt 3 years expect 450 AGN.
BAT Source Detections
BAT GALAXY Identifications
• 70% of the galaxies with no Sy classification are absorbed.• Most unknown galaxies are absorbed Sy2's or edge on spiral
galaxies with no lines (3 with no spectroscopy)• Sy2/Sy1 ratio is 1.2 (0.5 Sy1.5/Sy1) and 52% of Seyfert
galaxies are absorbed confirming Markwardt et al 2005 Much lower than 75% predicted by the standard model for the CXB
absorbed Log nH > 22|Gal b| >15
abs unabs total
Sy1 3 31 34
Sy1.5 5 11 16
Sy2 36 3 40
no Sy ID 9 4 13
total 53 49 103
Correlations of BAT Rate with other Bands
• no correlation between BAT and ROSAT count rates
• 44 BAT sources not detected by ROSAT
• no close correlation with total 2MASS J band
• soft x-ray and IR do not measure true AGN luminosity or complete populations
BATXMM peaks at
20 keV
• >25% of BAT sources
have weak soft X-ray emission
• only a survey at >20 keV is unbiased by absorption
• Many have extremely complex spectra with soft and hard components that seem unrelated
• Simultaneous X-ray and hard X-ray spectra are required to understand AGN
Heavily Absorbed and Complex Spectra Abound
LogN/LogS and Luminosity Function• errors 25% in normalization,
~10% in slopes and <1% in break luminosity
• New, much tighter constraints will test CXB models
• Treister, Urry, and Lira: standard unified AGN model 2500 AGN, LBAT>10-11ergs cm-2 s-1
• BAT measures 1100 AGN, LBAT>10-11ergs cm-2 s-1
• Pashak Ghandi800 AGN, LBAT>10-11ergs cm-2 s-1
Average BAT AGN Spectrum
• no evidence for a break below 200 keV
• same with brightest 36 AGN removed
• Same for luminosity <1044 or >1044
• Contrast with strong break at 40 keV in CXB
• spectrum is much flatter than CXB (slope = 2.75) above the break
Crab NormalizedResponse
E-2
Background vs BAT AGN Spectrum• BAT AGN
spectrum is a bad fit to CXB for E>40 keV
• Soft x-ray surveys put source of CXB at Z=0.7
• to fit the CXB the BAT AGN must be at Z > 1 or show strong spectral evolution
150 keVBAT UL
What is the Blazar Contribution to the CXB?
• BAT beamed sum spectrum is 10-16% AGN sum
• In 3 years, BAT will detect >40 beamed sources.
• BAT survey can answer this question.
Beamed Sources
Beamed Fraction
Conclusions• The first complete AGN survey is answering many of
the old questions about AGN and their contribution to the CXB.– What are the numbers of hard x-ray AGN?
• 1100 AGN, LBAT>10-11ergs cm-2 s-1 14-200 keV
– Are the hard x-ray AGN and CXB spectra the same? No– Does the standard unified model explain all? No– How many of the AGN are absorbed? ~0.6 not 3/4– What is the blazar contribution to CXB? 10-15%
• Follow up of the BAT selected AGN offers many exciting new opportunities.
We are just getting started!
BACKUP SLIDES
BAT Survey Characteristics• survey sensitivity is still improving
as the square root of time• a longer survey detects more
sources but it also makes better spectra for the previous sources
• 4 energy bands will be expanded to 8 bands soon
• extragalactic BAT sources are easily to identify at high latitudes– very bright IR galaxy– low redshift (z<0.06)– optical AGN or high absorption
• XRT positions and joint XRT/BAT spectra make ID's unambiguous
8.5 mCrab (T/20 ks)-0.5
XRTUVOTBATjointspectra
BATXRT
Sensitivity vs. Exposure
Tests of the Standard Model
• BAT selected Sy1's have higher luminosity than Sy2's (3.6)
• differences are small, so no selection effect for BAT
With <E>~50 kev BAT measures the true nature of the continuum relatively unaffected by absorption or scattering
• BAT selected Sy1's have softer spectra than Sy2's (5.7)
Absorption's Effect on Luminosity Function• more high luminosity with
NH<22 and more low luminosity NH>22
• more statistics required• the Swift/BAT survey will
get there
Swift/BAT and Integral luminosity functions• the INTEGRAL and
Swift/BAT luminosity functions are in rough agreement
• The Sazanov data seems to be systematically higher
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