Search for the NIR counterpart to IGR J17498-2921 in quiescence

ATel #3562; S. Greiss (Warwick), D. Steeghs (Warwick/CfA), T. Maccarone (Southampton), P. G. Jonker (SRON/RU/CfA), M. A.P. Torres (SRON/CfA), O. Gonzalez (ESO), N. Masetti (INAF/IASF, Bologna), A. Rojas (PUC Santiago), the VVV consortium
on 16 Aug 2011; 14:54 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Danny Steeghs (dsteeghs@cfa.harvard.edu)

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In order to search for the counterpart of the recently discovered accreting millisecond pulsar IGR J17498-2921 (Atel #3551), we investigated recent near-infrared (NIR) data of the Galactic Bulge region obtained as part of the VVV survey (Minniti et al. 2010, New Astronomy, Volume 15, 433). The observations took place while the source was in quiescence. The data have been acquired using the 4-m VISTA telescope at Paranal observatory (Chile), using a camera with pixel sampling of 0.34 arcsec.

We analysed JHKs images obtained on the 11th of May 2010, at airmass 1.007 and under an average seeing conditions of 0.9 arcsec. Total on source exposure times were 24s for the J-band images and 8s for both H- and Ks-band frames.

We detect a NIR source 0.6 arcsec from the Chandra position reported in Atel #3559 (RA = 17:49:55.38, Dec = -29:19:19.7) and with the following magnitudes: J = 17.45 +/- 0.12, H = 15.73 +/- 0.10 and Ks = 15.10 +/- 0.12. The coordinates of this source are RA = 17:49:55.33, Dec = -29:19:19.6 with an astrometric fit RMS of 0.1 arcsec. The match is within the Chandra error circle and this is the only significantly detected NIR source consistent with that position. The next closest NIR match is over 3" away from the Chandra X-ray position, which appears inconsistent. We also note that no UKIDSS GPS data is available at this position for comparison.

The colours of this match indicate moderate amounts of reddening towards the counterpart candidate. Extinction values in this region obtained using red clump giants with VVV data (Gonzalez et al. 2011, arXiv:1107.5496) returns the following values: A_J = 2.24 mag, A_H = 1.28 mag and A_Ks = 0.74 mag, when using Nishiyama et al. (2009, ApJ, 696, 1407) extinction law towards the Galactic Center. These values suggest a lower extinction than the N_H absorption column of 3e22 cm^-2 reported in Atel #3555. The estimated absorption column from the X-ray spectrum are model-dependent and may also include local absorption, thus over-estimating the line of sight reddening.

The de-reddened JHKs magnitudes of the NIR counterpart candidate are roughly consistent with a late type K or early M dwarf. Given the recent estimate of an orbital period of about 4 hours (Atel #3561), this could in principle be consistent with a donor star dominated LMXB in quiescence. However, assuming standard absolute magnitudes for such dwarfs, the apparent magnitudes imply a source distance much closer than the bulge. This could be explained by a foreground field star not associated with the LMXB. Alternatively, if this is indeed the LMXB counterpart, the pulsar must be relatively close.

We encourage complementary observations now that the source is active to establish the viability of this source as the counterpart to IGR J17498-2921.