J/ApJ/746/54 Chandra X-ray observations of SDSS-DR5 QSOs (Gibson+, 2012)
The X-ray variability of a large, serendipitous sample of spectroscopic quasars.
Gibson R.R., Brandt W.N.
<Astrophys. J., 746, 54 (2012)>
=2012ApJ...746...54G 2012ApJ...746...54G
ADC_Keywords: X-ray sources ; QSOs ; Redshifts
Keywords: galaxies: active - X-rays: general
Abstract:
We analyze the X-ray variability of 264 Sloan Digital Sky Survey
spectroscopic quasars using the Chandra public archive. This data set
consists of quasars with spectroscopic redshifts out to z ~ 5
and covers rest-frame timescales up to Δtsys~ 2000
days, with three or more X-ray observations available for 82 quasars.
It therefore samples longer timescales and higher luminosities than
previous large-scale analyses of active galactic nucleus (AGN)
variability. We find significant ( ≳ 3σ) variation in
~30% of the quasars overall; the fraction of sources with
detected variability increases strongly with the number of available
source counts up to ~70% for sources with ≥ 1000 counts per
epoch. Assuming that the distribution of fractional variation is
Gaussian, its standard deviation is ~16% on ≳ 1 week
timescales, which is not enough to explain the observed scatter in
quasar X-ray-to-optical flux ratios as being due to variability alone.
We find no evidence in our sample that quasars are more variable at
higher redshifts (z > 2), as has been suggested in previous studies.
Quasar X-ray spectra vary similarly to some local Seyfert AGNs in that
they steepen as they brighten, with evidence for a constant, hard
spectral component that is more prominent in fainter stages. We
identify one highly variable Narrow Line Seyfert 1-type spectroscopic
quasar in the Chandra Deep Field-North. We constrain the rate of
kilosecond-timescale flares in the quasar population using ~8
months of total exposure and also constrain the distribution of
variation amplitudes between exposures; extreme changes (>100%) are
quite rare, while variation at the 25% level occurs in <25% of
observations. [O III] λ5007 Å emission may be stronger in
sources with lower levels of X-ray variability; if confirmed, this
would represent an additional link between small-scale (corona) and
large-scale (narrow-line region) AGN properties.
Description:
In order to obtain a list of quasars observed by Chandra, we searched
the Chandra archive to find all ACIS-S or ACIS-I observations of SDSS
Data Release 5 (DR5) quasars (Schneider et al. 2007, Cat. VII/252,
superseded by Cat. VII/269) that used no gratings and were public as
of 2009 January 13. Our final sample of multiply observed sources
includes 763 Chandra observations of 264 SDSS quasars.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 72 264 Properties common to each source
table2.dat 183 763 Observations of each source (count rates and
total counts)
table3.dat 214 763 Observations of each source (counts in each band)
table4.dat 79 87 Spectral model consisting of a Galactic-absorbed
power law absorbed by one absorption edge
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See also:
B/chandra : The Chandra Archive Log (CXC, 1999-2013)
VII/252 : SDSS-DR5 quasar catalog (Schneider+, 2007)
J/ApJS/201/10 : SDSS quasar catalog with Swift observations (Wu+, 2012)
J/ApJ/726/20 : X-ray emission from quasars (Miller+, 2011)
J/ApJ/690/644 : Properties of SDSS QSOs in the ChaMP (Green+, 2009)
J/ApJ/692/758 : BAL QSOs in SDSS-DR5 (Gibson+, 2009)
J/ApJS/183/17 : The SDSS DR5/XMM-Newton quasar survey (Young+, 2009)
J/AJ/133/313 : AGN from RASS and SDSS DR5 (Anderson+, 2007)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 A18 --- SDSS SDSS name (HHMMSS.ss+DDMMSS.s; J2000)
20- 24 F5.3 --- z [0.086/4.976] Redshift
26- 27 I2 --- Nobs [2/24] Number of observations
29 A1 --- RL [*/1] Known radio loud source (1)
31 I1 --- BAL [0/1] Known broad absorption line
(1=BAL quasar)
33- 38 F6.3 10-30W/m2/Hz F2500 [0.107/22.112]? 2500Å flux density (2)
40- 47 F8.3 10-6ct/cm2/s Rate [-1.975/1481.93] Constant count rate
49- 55 F7.3 10-6ct/cm2/s e_Rate [0.352/404.062] Rate uncertainty
57 I1 --- Var? [0/1] Variable source? (3)
59- 64 F6.3 10-6ct/cm2/s RateHQ [-0.004/72.24]? Constant count rate
in HQ sample (G1)
66- 70 F5.3 10-6ct/cm2/s e_RateHQ [0.035/3.077]? RateHQ uncertainty (G1)
72 I1 --- VarHQ? [0/1]? Variable source in HQ sample? (3)(G1)
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Note (1): Flag as follows:
1 = radio loud source: following the method of Gibson et al.
(2008ApJ...675..985G 2008ApJ...675..985G), these sources have a ratio of R*>10 (log(R*)>1).
* = Sources not known to be radio loud, but limits were not sensitive enough
to guarantee that they were radio quiet with high confidence.
Note (2): Blank entries indicate where F2500 could not be reliably measured
due to bad spectral bins.
Note (3): We flag an epoch as "variable" (=1) if the observed count rate is
higher or lower than the number of counts corresponding to a deviation
from the best-fit value at >99% confidence, according to a Poisson
statistic. Any source with at least one variable epoch is considered a
"variable source". For the full sample, 74 of 264 sources are
classified as variable; 54 of 167 sources are variable in Sample HQ.
See section 3.1.
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 18 A18 --- SDSS SDSS Name (HHMMSS.ss+DDMMSS.s; J2000)
20- 23 I4 --- ObsId [304/9880] Observation number
25- 34 F10.3 s Exp [161/170433] Exposure time
36- 41 F6.3 arcmin OA [0.08/14.88] Off-Axis Angle
43 I1 --- HQ? [0/1] In HQ sample flag (1=yes) (G1)
45- 57 F13.3 s TSTART Chandra TSTART parameter (1)
59- 67 F9.4 0.01/m2/s CRate [-5/1600] Source count rate
(in units of 10-6ct/cm2/s)
69- 74 F6.3 0.01/m2/s E_CRate Upper limit uncertainty on CRate
76- 82 F7.3 0.01/m2/s e_CRate Lower limit uncertainty on CRate
84- 91 F8.3 10+52ct/s CtLum [-237/3533] Count luminosity
93- 99 F7.3 10+52ct/s E_CtLum Upper limit uncertainty on CtLum
101-108 F8.3 10+52ct/s e_CtLum Lower limit uncertainty on CtLum
110-118 F9.3 ct Full [0/33089] Full (0.5-8 keV) band total
counts (2)
120-126 F7.3 ct E_Full Upper limit uncertainty on Full
128-134 F7.3 ct e_Full Lower limit uncertainty on Full
136-144 F9.3 ct Soft [0/24936] Soft (0.5-2 keV) band total counts
146-152 F7.3 ct E_Soft Upper limit uncertainty on Soft
154-160 F7.3 ct e_Soft Lower limit uncertainty on Soft
162-169 F8.3 ct Hard [0/8337] Hard (2-8 keV) band total counts
171-176 F6.3 ct E_Hard Upper limit uncertainty on Hard
178-183 F6.3 ct e_Hard Lower limit uncertainty on Hard
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Note (1): Indicates the time of the start of the observation in seconds
since 1 Jan 1998.
Note (2): The number of full-band counts is not exactly equal to the
combination of soft- and hard-band counts due to band-dependent
factors in aperture corrections and background estimation.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 A18 --- SDSS SDSS Name (HHMMSS.ss+DDMMSS.s; J2000)
20- 23 I4 --- ObsId [304/9880] Observation identification number
25- 33 F9.3 ct SFcts [-6.3/33089] Source full (0.5-8keV) band
counts (1)
35- 41 F7.3 ct E_SFcts Upper limit uncertainty in SFcts
43- 49 F7.3 ct e_SFcts Lower limit uncertainty on SFcts
51- 59 F9.3 ct SScts [-3.7/24936] Source soft (0.5-2 keV) band counts
61- 67 F7.3 ct E_SScts Upper limit uncertainty on SScts
69- 75 F7.3 ct e_SScts Lower limit uncertainty on SFcts
77- 84 F8.3 ct SHcts [-8.6/8337] Source hard (2-8 keV) band counts
86- 91 F6.3 ct E_SHcts Upper limit uncertainty on SHcts
93- 99 F7.3 ct e_SHcts Lower limit uncertainty on SHcts
101-107 F7.3 ct bgFcts [-0.4/428] Background full (0.5-8 keV) band
counts (1)
109-115 F7.3 ct E_bgFcts Upper limit uncertainty in bgFcts
117-122 F6.3 ct e_bgFcts Lower limit uncertainty in bgFcts
124-129 F6.3 ct bgScts [-0.4/75] Background soft (0.5-2 keV) band
counts (1)
131-137 F7.3 ct E_bgScts Upper limit uncertainty in bgScts
139-143 F5.3 ct e_bgScts Lower limit uncertainty in bgScts
145-151 F7.3 ct bgHcts [-0.3/356] Background hard (2-8 keV) band
counts (1)
153-159 F7.3 ct E_bgHcts Upper limit uncertainty in bgHcts
161-166 F6.3 ct e_bgHcts Lower limit uncertainty in bgHcts
168-173 F6.3 --- HR [-1/1]? Hardness ratio (2)
175-179 F5.3 --- E_HR ? Upper limit uncertainty on HR
181-185 F5.3 --- e_HR ? Lower limit uncertainty on HR
187-195 F9.3 10+35W Lum [-3900/58166] Approximate 0.5-8keV luminosity (3)
197-204 F8.3 10+35W E_Lum Upper limit uncertainty in Lum
206-214 F9.3 10+35W e_Lum Lower limit uncertainty in Lum
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Note (1): The number of full-band counts is not exactly equal to the
combination of soft- and hard-band counts due to band-dependent
factors in aperture corrections and background estimation.
Note (2): HR=(H-S)/(H+S) where H=2-8keV and S=0.5-2keV.
Note (3): Calculated using a power law model with photon index Γ=2.
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 A18 --- SDSS SDSS name (HHMMSS.ss+DDMMSS.s; J2000)
20- 23 I4 --- ObsId [580/9876] Observation ID number
25- 29 F5.2 --- Norm [1.17/44.1] Normalization at 1keV
in units of 10-5photon/keV/cm2/s
31- 36 F6.4 --- E_Norm [0.0616/2.47] Positive uncertainty on Norm
38- 43 F6.4 --- e_Norm [0.0553/1.71] Negative uncertainty on Norm
45- 49 F5.3 --- Gamma [0.087/2.899] Photon index Γ
51- 55 F5.3 --- E_Gamma [0.024/0.354] Positive uncertainty on Gamma
57- 61 F5.3 --- e_Gamma [0.024/0.222] Negative uncertainty on Gamma
63- 67 F5.3 --- tau [0/3.431] Edge optical depth (τ)
69- 73 F5.3 --- E_tau [0.014/3.163] Positive uncertainty on tau
75- 79 F5.3 --- e_tau [0/1.837] Negative uncertainty on tau
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Global notes:
Note (G1): We define a high-quality sample, which we call "Sample HQ",
consisting of 167 sources. To construct Sample HQ, we culled all
observations of any source that had exposures <2.5ks and off-axis
angles>10arcmin. We also required that sample-HQ observations be
performed at an off-axis angle >1arcmin. A typical source has about
5 more counts per epoch in Sample HQ than in the full sample. See
section 2.3.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 06-Aug-2013