Chandra identification of the current X-ray transient in NGC 6440
ATel #7183; Jeroen Homan (MIT), Dave Pooley (Sam Houston State University, Eureka Scientific), and Craig Heinke (University of Alberta)
on 6 Mar 2015; 18:15 UT
Credential Certification: Jeroen Homan (jeroen@space.mit.edu)
Subjects: X-ray, Globular Cluster, Neutron Star, Transient
We triggered a Chandra ToO observation of the transient IGR J17488-2018 in the globular cluster NGC 6440 (ATel #7098, #7106, #7136). The 10.0 ks observation was taken with the HRC-S instrument and started on 2015-03-01 at 02:02:21 UTC.
The HRC-S image reveals a single bright X-ray source at the following coordinates (J2000): RA = 17:48:52.2, Dec = -20:21:32.6. The error on the position is dominated by the astromertic uncertainty of the HRC-S and is ~0.6 arcsec (90% confidence).
This position is consistent with that of the known neutron star transient SAX J1748.8-2021 (in't Zand et al. 2001, ApJ, 563, 41; Pooley et al. 2002, ApJ, 573, 184) and confirms the earlier identification by Bozzo et al. (2015) based on Swift/XRT observations (Atel #7106). No other known Chandra source (from Pooley et al. 2002) is consistent with the position of the current transient. This is the fifth known outburst from this source, after outbursts in 1998, 2001, 2005, and 2009-10.
We observed two type-I X-ray bursts, but did not detect any pulsations at the previously reported frequency of 442 Hz (Altamirano et al. 2008, ApJ, 674, 45).
The HRC-S count rate for the persistent emission was ~60 cts/s. Using the spectral parameters reported in ATel #7106, this converts to an absorbed 0.3-10 keV flux of ~6.9e-9 erg/cm^2/s and a 0.3-10 keV luminosity of ~7.8e37 erg/s (using a distance of 8.5 kpc; Harris et al. 1996, AJ, 112, 1487)
We thank Belinda Wilkes and the Chandra staff for their help in executing this observation.
