J/MNRAS/501/2790 The Swift Bulge Survey. first X-ray results (Bahramian+, 2021)
The Swift Bulge Survey: motivation, strategy, and first X-ray results.
Bahramian A., Heinke C.O., Kennea J.A., Maccarone T.J., Evans P.A.,
Wijnands R., Degenaar N., in't Zand J.J.M., Shaw A.W.,
Rivera Sandoval L.E., McClure S., Tetarenko A.J., Strader J., Kuulkers E.,
Sivakoff G.R.
<Mon. Not. R. Astron. Soc., 501, 2790-2809 (2021)>
=2021MNRAS.501.2790B 2021MNRAS.501.2790B (SIMBAD/NED BibCode)
ADC_Keywords: X-ray sources ; Accretion ; Active gal. nuclei ;
Binaries, X-ray ; Milky Way ; Optical
Keywords: accretion, accretion discs - surveys - X-rays: binaries -
stars: neutron - stars: black holes
Abstract:
Very faint X-ray transients (VFXTs) are X-ray transients with peak
X-ray luminosities (LX) of LX~<1036erg/s, which are not well
understood. We carried out a survey of 16 deg2 of the Galactic Bulge
with the Swift Observatory, using short (60s) exposures, and returning
every 2 weeks for 19 epochs in 2017-18 (with a gap from 2017 November
to 2018 February, when the Bulge was in sun-constraint). Our main goal
was to detect and study VFXT behaviour in the Galactic Bulge across
various classes of X-ray sources. In this work, we explain the
observing strategy of the survey, compare our results with the
expected number of source detections per class, and discuss the
constraints from our survey on the Galactic VFXT population. We
detected 91 X-ray sources, 25 of which have clearly varied by a factor
of at least 10. In total, 45 of these X-ray sources have known
counterparts: 17 chromospherically active stars, 12 X-ray binaries, 5
cataclysmic variables (and 4 candidates), 3 symbiotic systems, 2 radio
pulsars, 1 active galactic nuclei, and a young star cluster. The other
46 are of previously undetermined nature. We utilize X-ray hardness
ratios, searches for optical/infrared counterparts in published
catalogues, and flux ratios from quiescence to outburst to constrain
the nature of the unknown sources. Of these 46, 7 are newly discovered
hard transients, which are likely VFXT X-ray binaries. Furthermore, we
find strong new evidence for a symbiotic nature of four sources in our
full sample, and new evidence for accretion power in six X-ray sources
with optical counterparts. Our findings indicate that a large subset
of VXFTs is likely made up of symbiotic systems.
Description:
The Swift Bulge Survey covered 16deg2 of the Galactic bulge in 19
epochs, using 60-s Swift/XRT exposures. The data were taken at 2-week
intervals during 2017 April-October, and 2018 February-March. This
allows us to probe faint transient X-ray behaviour, at fluxes below
those detectable by all-sky monitors, across a significant fraction of
our galaxy. We identify 104 clear Swift/XRT detections, 13 of which we
attribute to optical loading of the detectors, and thus 91 significant
X-ray sources. In total, 25 of these sources varied by a factor of 10
or more (generally in comparison to archival Chandra or XMM data), and
another 14 by at least a factor of 5.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
catalog.dat 601 104 Swift Bulge Survey X-ray Source catalogue
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See also:
IX/57 : The Chandra Source Catalog (CSC), Release 2.0 (Evans+, 2019)
IX/55 : XMM-Newton Serendipitous Source Catalogue 3XMM-DR8 (XMM-SSC, 2018)
Byte-by-byte Description of file: catalog.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- SBSX Number in the Swift Bulge Survey Catalog
5- 20 A16 --- Swift Official Swift designation JHHMMSS.s_DDMMSS
22- 66 A45 --- OName Alternative identification
68- 76 A9 --- Type Nature of the X-ray source
78- 164 A87 --- Comment Comment on the nature and behavior of the
source
166- 175 A10 --- Flag Quality of the detection as determined by
the Swift/XRT analysis pipeline
177- 185 F9.5 deg RAdeg Right ascension (J2000)
187- 194 F8.4 deg DEdeg Declination (J2000)
196- 199 F4.1 --- Poserr 90% uncertainty radius on source
localization
201- 207 F7.4 ct/s Peakrate Peak net count rate in the SBS data
209- 214 F6.4 ct/s e_Peakrate Error on Peakrate (1-σ uncertainty)
216- 219 F4.2 --- Hardness Source spectral hardness (1)
221- 224 F4.2 --- e_Hardness 1-σ lower error on Hardness
226- 229 F4.2 --- E_Hardness 1-σ upper error on Hardness
231- 232 I2 --- Ndet ? Number of SBS epochs in which the source
is detected
234- 235 I2 --- Nobs ? Number of SBS epochs in which the source
area is covered
237- 241 F5.2 --- varsigma ? Significance of source flux variation in
the SBS data Varσ (2)
243- 251 E9.4 cm-2 NH Best-fit hydrogen column density for the
merged source spectrum
253- 261 E9.4 cm-2 e_NH Lower 90% error on NH
263- 271 E9.4 cm-2 E_NH Upper 90% error on NH
273- 277 F5.2 --- Gamma Best-fit photon index for the merged source
spectrum Γ
279- 282 F4.2 --- e_Gamma Lower 90% error on Gamma
284- 287 F4.2 --- E_Gamma Upper 90% error on Gamma
289- 296 E8.3 mW/m2 Funabs Best-fit unabsorbed flux for the merged
source spectrum
298- 305 E8.3 mW/m2 e_Funabs Lower 90% error on Funabs
307- 314 E8.3 mW/m2 E_Funabs Upper 90% error on Funabs
316- 323 E8.3 mW/m2 Fabs Best-fit absorbed flux for the merged
source spectrum
325- 332 E8.3 mW/m2 e_Fabs Lower 90% error on Fabs
334- 341 E8.3 mW/m2 E_Fabs Upper 90% error on Fabs
343- 350 E8.3 mW/m2 Peakabs Peak absorbed flux (3)
352- 359 E8.3 mW/m2 e_Peakabs 1-σ error on Peakabs
361- 382 A22 --- Chandra Chandra designation for a counterpart
identified within the SBS source error
circle (4)
384- 391 E8.3 mW/m2 Facis ? Cataloged ACIS flux in the "broad" band
393- 400 E8.3 mW/m2 b_Facis ? Lower bound on Facis
402- 409 E8.3 mW/m2 B_Facis ? Upper bound on Facis
411- 418 E8.3 mW/m2 Fhrc ? Cataloged HRC flux in the "wide" band
420- 427 E8.3 mW/m2 b_Fhrc ? Lower bound on Fhrc
429- 436 E8.3 mW/m2 B_Fhrc ? Upper bound on Fhrc
438- 458 A21 --- XMM XMM-Newton designation for a counterpart
identified within the SBS source error
circle (5)
460- 467 E8.3 mW/m2 Fepic ? Cataloged XMM-Newton/EPIC flux averaged
among all EPIC cameras
469- 476 E8.3 mW/m2 e_Fepic ? Error on Fepic
478- 485 E8.3 --- chi2XMM ? Cataloged chi2 probability that the time
series of the source can be explained by a
constant flux
487- 502 F16.9 --- Fsbs/Facis ? Flux ratio between the peak flux in the
SBS and the cataloged ACIS flux
504- 518 F15.9 --- e_Fsbs/Facis ? 1-σ error on Fsbs/Facis
520- 532 F13.9 --- Fsbs/Fhrc ? Flux ratio between the peak flux in the
SBS and the cataloged HRC flux
534- 545 F12.9 --- e_Fsbs/Fhrc ? 1-σ error on Fsbs/Fhrc
547- 561 F15.9 --- Fsbs/Fepic ? Flux ratio between the peak flux in the
SBS and the cataloged EPIC flux
563- 576 F14.9 --- e_Fsbs/Fepic ? 1-σ error on Fsbs/Fepic
578- 582 F5.2 mag Vmag ? V-band magnitude of the likely optical
counterpart
584- 588 F5.2 mag Kmag ? K-band magnitude of the likely optical
counterpart
590- 601 F12.9 [-] logFX/FV ? Logarithm of the ratio of the average
X-ray flux to the optical flux
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Note (1): Number of source counts with E>1.5keV over total number of source
counts
Note (2): Varσ is determined as the ratio of difference between highest
observed flux and lowest flux (or flux upper limit) over 1-σ
uncertainty in flux
Note (3): Peak absorbed flux estimated based on the peak count rate assuming
the best-fit model parameters for the merged source spectrum
Note (4): Chandra designation from CSC v2; Evans et al. (2010ApJS..189...37E 2010ApJS..189...37E,
Cat. IX/57)
Note (5): XMM-Newton designation from 3XMM-DR8; Zolotukhin et al.
(2017ApJ...839..125Z 2017ApJ...839..125Z)
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History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 18-Oct-2023