Chandra Localization of the Accretion-Powered Millisecond Pulsar IGR J17498-2921

ATel #3606; Deepto Chakrabarty (MIT), Craig B. Markwardt (NASA/GSFC), Manuel Linares (MIT), Peter G. Jonker (SRON)
on 29 Aug 2011; 18:08 UT
Credential Certification: Deepto Chakrabarty (deepto@space.mit.edu)

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As part of an ongoing Chandra program for precise localization of transient low-mass X-ray binaries, we obtained a short Chandra observation of the 401 Hz transient accreting millisecond X-ray pulsar IGR J17498-2921 during its 2011 August discovery outburst (ATEL #3551, #3555, #3556). Our observation was made on 2011 August 22, 00:40 TT with an exposure time of 1183.2 s using the HRC-S instrument in timing mode. The source was detected with a count rate of 1.96(4) count/s. The best-fit position is:

RA(J2000) = 17h 49m 55.35s
Dec (J2000) = -29d 19' 19.6"

with an error radius of 0.6 arcsec (90% containment). This position is consistent with the Swift XRT position (7 arcsec error radius; ATEL #3555), the Swift XRT-UVOT enhanced position (1.9 arcsec error radius; ATEL #3558), and the presumed quiescent X-ray counterpart identified in archival Chandra data from 2006 October (0.6 arcsec error radius; ATEL #3559), lying 4.6 arcsec, 1.1 arcsec, and 0.43 arcsec away, respectively. It is also consistent with the candidate infrared counterpart (ATEL #3562), lying 0.25 arcsec away.

With regard to the accuracy of the Chandra position, we note that the Chandra X-ray Center (CXC) recently reported an 8 arcsec shift in the aim point for Chandra data taken since July 11 (CXC Announcement 75, 2011 August 22, http://cxc.harvard.edu/announcements/announce_75.html ). However, CXC has determined that this offset does not affect Chandra's astrometric accuracy (T. Gokas, 2011, private comm.).

We searched for 401 Hz pulsations in the Chandra data by correcting the photon arrival times to the solar system barycenter and folding the data at 400.99 Hz, using the updated pulsar orbital ephemeris of Markwardt and Strohmayer (ATEL #3601). Pulsations were detected at the 99.1 percent confidence level, with a fractional r.m.s. amplitude of 11 percent. This is consistent with the value observed by RXTE (ATEL #3601), although the HRC-S instrument is primarily sensitive to photons at lower energies than RXTE.