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J/ApJ/687/1019         Chandra X-ray observations of M71         (Elsner+, 2008)

Chandra X-ray observatory observations of the globular cluster M71. Elsner R.F., Heinke C.O., Cohn H.N., Lugger P.M., Maxwell J.E., Stairs I.H., Ransom S.M., Hessels J.W.T., Becker W., Huang R.H.H., Edmonds P.D., Grindlay J.E., Bogdanov S., Ghosh K., Weisskopf M.C. <Astrophys. J., 687, 1019-1034 (2008)> =2008ApJ...687.1019E
ADC_Keywords: Clusters, globular ; X-ray sources ; Cross identifications Keywords: globular clusters: individual (M71, NGC 6838) - pulsars: individual (PSR J1953+1846A, M71A) - X-rays: binaries - X-rays: stars Abstract: We observe the nearby, low-density globular cluster M71 (NGC 6838) with the Chandra X-Ray Observatory to study its faint X-ray populations. Five X-ray sources are found inside the cluster core radius, including the known eclipsing binary millisecond pulsar (MSP) PSR J1953+1846A. The X-ray light curve of the source coincident with this MSP shows marginal evidence for periodicity at the binary period of 4.2hr. Its hard X-ray spectrum and luminosity resemble those of other eclipsing binary MSPs in 47 Tuc, suggesting a similar shock origin of the X-ray emission. A further 24 X-ray sources are found within the half-mass radius, reaching to a limiting luminosity of 1.5x1030ergs/s (0.3-8keV). From a radial distribution analysis, we find that 18±6 of these 29 sources are associated with M71, somewhat more than predicted, and that 11±6 are background sources, both Galactic and extragalactic. M71 appears to have more X-ray sources in the range LX=1030-1031ergs/s than expected by extrapolating from other studied clusters using either mass or collision frequency. We explore the spectra and variability of these sources and describe the results of ground-based optical counterpart searches. Description: We obtained a 52.4ks Chandra observation (ObsID 5434) of M71 (nominal center of cluster at J195346.1+184642) on 2004 December 20-21 using the Advanced CCD Imaging Spectrometer (ACIS) in very faint (VF), timed-exposure mode with a 3.141s frame time. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file table1.dat 119 68 Chandra X-ray sources detected with rM71≤2rh table2.dat 119 97 Chandra X-ray sources detected with 2rh≥rM71
See also: I/284 : The USNO-B1.0 Catalog (Monet+ 2003) II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003) I/259 : The Tycho-2 Catalogue (Hog+ 2000) B/chandra : The Chandra Archive Log (CXC, 1999-) J/A+AS/144/227 : BV photometry of M71 (Geffert+, 2000) Byte-by-byte Description of file: table1.dat table2.dat
Bytes Format Units Label Explanations
1- 3 A3 --- rl Half-mass radius (r_h_) limits (1) 5- 8 A4 --- ID Source identification (<[EHC2008] [s/i]sNN> in Simbad) 9 A1 --- f_ID [j] Flag for s55 (in table 1) (2) 11- 12 I2 h RAh Hour of right ascension (J2000) 14- 15 I2 min RAm Minute of right ascension (J2000) 17- 22 F6.3 s RAs Second of right ascension (J2000) 24 A1 --- DE- Declination sign (J2000) 25- 26 I2 deg DEd Degree of declination (J2000) 28- 29 I2 arcmin DEm Arcminute of declination (J2000) 31- 35 F5.2 arcsec DEs Arcsecond of declination (J2000) 37- 40 F4.1 arcsec re Radius of the detect cell for collecting X-ray counts 42- 47 F6.1 --- Cx Number of 0.3-8.0keV source counts collected in the detect cell 49- 52 F4.2 arcsec SigX Radius enclosing the true source position with 68% confidence (3) 54- 58 A5 arcmin Chip CCD on which the X-ray source appears (only for table 2) (4) 60- 63 F4.2 arcmin rM71 ? Radius from the nominal center of the cluster (J19:53:46.1+18:46:42; only for table 1) 65- 68 F4.2 arcsec off ? Radial offset of candidate counterparts (5) 70- 75 F6.3 mag Jmag ? J magnitude of a candidate counterpart found in the 2MASS catalog (5) 77- 81 F5.2 mag R1mag ? R1 magnitude of a candidate counterpart found in the USNO-B1.0 catalog (5) 83- 86 F4.1 mag R2mag ? R2 magnitude of a candidate counterpart found in the USNO-B1.0 catalog (5) 88- 93 F6.3 mag Vmag ? VT magnitude of a candidate counterpart found in the Tycho-2 catalog (5) 95-101 F7.4 % Prob ? Probability of chance coincidence (5)(6) 103-119 A17 --- Com Comment (counterpart or location)
Note (1): Limits as follows: rh = sources with rM71≤rh; 2rh = sources with rh<rM71≤2rh; t2 = sources with 2rh≤rM71 (table 2) Note (2): j = This source is on the S2 front-side CCD. All other sources in this table are on the S3 back-side CCD. Note (3): The corresponding radii for inclusion with 95% or 99% confidence are found by multiplying this column by 1.62 or 2.01, respectively. Note (4): In a few cases, the source is dithered across the gap between CCDs. Note (5): Found by searching the HEASARC 2MASS (see; Cat. II/246), USNO-B1.0 (Monet et al. 2003; Cat. I/284), and Tycho-2 (Hog et al. 2000, Cat. I/259) catalogs, requiring that the candidate counterpart lie within the 99% confidence radius of a Chandra source. There are 6414 and 3376 2MASS sources, 15324 and 9196 USNO sources, and 7 and 9 Tycho sources appearing within the S2-S3-S4 and I2-I3 boundaries shown in Figures 3 and 4, respectively. If blank, no candidate counterpart was found in these catalogs. (See section 6.1 for further details). Note (6): The percent probability of a chance coincidence is calculated from: Pcoinc=N(<m)πr299/Asearch where N(<m) is the number of sources in the corresponding catalog inside the search area, Asearch, with a magnitude smaller than that of the candidate counterpart, and r99 is the radius enclosing the X-ray source location with 99% confidence.
History: From electronic version of the journal
(End) Emmanuelle Perret [CDS] 10-Jan-2011
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