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Chandra identification of the X-ray transient MAXI J0911-635/Swift J0911.9-6452 in NGC 2808

ATel #8971; Jeroen Homan (MIT, SRON), Gregory Sivakoff (University of Alberta), David Pooley (Trinity University, Eureka Scientific), Craig Heinke (University of Alberta), Jay Strader (Michigan State University), Vlad Tudor, James Miller-Jones (ICRAR-Curtin)
on 19 Apr 2016; 19:59 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)

Subjects: Optical, X-ray, Globular Cluster, Transient

Referred to by ATel #: 8986, 9738, 10425, 16358

We triggered a Chandra ToO observation of the X-ray transient MAXI J0911-635/Swift J0911.9-6452 in the globular cluster NGC 2808 (ATel #8872, #8884, #8914). The ~15 ks observation was taken with the HRC-S instrument and started on 2016-04-09 at 06:34:20 UTC.

The HRC-S image reveals a bright X-ray source at the following coordinates (J2000): RA = 09:12:02.43, Dec = -64:52:06.4. The position of this source is consistent with that of source 15 from Servillat et al. (2008, A&A, 490, 641), who analyzed a deeper ACIS-I observation (from June 2007) of the cluster. In the 2007 observation the source had a 0.5-8 keV flux of (4.4+/-2.6)e-14 erg/cm^2/s, with a relatively soft hardness ratio. Using WebPIMMS and assuming a 1.5 keV blackbody spectrum with an Nh of 3.8e21 cm^-2, the HRC-S source count rate in the 2016 observation (4.67 cts/s) converts to a 0.5-8 keV flux of ~5.6e-10 erg/cm^2/s.

We detect two additional weaker X-ray sources in the cluster core, which can be identified as sources 1 and 10 from Servillat et al. (2008).

To set the absolute astrometry, we compare the deeper Chandra ACIS-S data to high resolution ATCA data taken when the source was in quiescence. We recalculated X-ray astrometric precision formulas using equation 14 from Kim et al. (2007, ApJS, 169, 401). Using 10 X-ray sources with relative astrometry better than 0.5" that are matched to radio sources, we find evidence that the existing Chandra positions should be shifted in Declination by -0.14", and that the resulting absolute astrometry is good to 0.12". Source 15 from Servillat et al. (2008) would tentatively be located at RA = 09:12:02.46, Dec = -64:52:06.37 +/- 0.15".

We searched for optical counterparts in quiescence using the HST catalogs from Anderson et al. 2008 (AJ, 135, 2055), which have absolute astrometric positions. We estimate the completeness limit of the HST archival data is at V~22, corresponding to M_V~6.4. Two counterparts were within the (combined absolute and relative) 1 sigma position of the quiescent X-ray source; a main sequence star (V=20.00, V-I=0.66) was 0.11" away and a fainter main sequence star (V=22.14, V-I=0.89) was 0.18" away. Eight other sources were within 0.38" (2 sigma position); four sources (at V between 20.04 and 22.80) were bluer than the globular cluster main sequence; one source at V=19.29, V-I=0.64 is close to the main sequence turnoff; three sources (at V between 21.08 and 21.98) are redder than the main sequence. It is unclear which of these sources, if any, are the optical counterpart.

The optical magnitudes of globular cluster LMXB counterparts during quiescence span a wide range of magnitudes, from M_V~3 for both EXO 1745-248 and IGR J17480-2446 in Terzan 5 (Testa et al. 2012, A&A, 547, A28; Ferraro et al. 2015, ApJL, 807, L1) to M_V~14 in NGC 6397 (Heinke et al. 2014, MNRAS, 444, 443); others include X5 in 47 Tuc, at M_V~8.2 (Edmonds et al. 2002, ApJ, 564, L17), Omega Cen, at M_V~11.8 (Haggard et al. 2004, ApJ, 613, 512), and IGR J1824-24525 in M28 at M_V~8 (Pallanca et al. 2013, ApJ, 773, 122). Most of these counterparts are below the completeness limit of the HST archival data, suggesting that the true optical counterpart has likely not been detected yet, though in quiescence LMXB counterparts do not necessarily have unusual V-I colors (see refs above).

We thank Belinda Wilkes and the Chandra staff for their help in executing this observation.