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Outburst near-infrared and Chandra observations of the ms-pulsar IGR J17498-2921

ATel #3638; M. A.P. Torres (SRON/CfA), O. Madej (SRON/UU), P. G. Jonker (SRON/CfA/RU), D. Steeghs (Warwick/CfA), S. Greiss (Warwick), N. Morrell and M. Roth (LCO)
on 8 Sep 2011; 11:58 UT
Distributed as an Instant Email Notice Transients
Credential Certification: P.G. Jonker (pjonker@cfa.harvard.edu)

Subjects: Infra-Red, X-ray, Neutron Star, Transient

Referred to by ATel #: 3643

We report on near-infrared and Chandra follow-up observations of the currently active accreting ms-pulsar IGR J17498-2921 (ATel #3551, #3556). An H-band infrared counterpart candidate with H= 15.7 +/- 0.1 mag has been suggested in ATel #3562 on the basis of its coincidence with the Chandra position for the X-ray source (ATel #3559, #3606; cf. ATel #3634).

INFRARED PHOTOMETRY: We have acquired a series of 45s H-band images on 2011 Sep 02 03:13 - 03:41 UT using the Retrocam camera attached to the 2.5m Irenee du Pont telescope at Las Campanas observatory. The frames total 810s on source and were acquired with seeing of 1.0 arcsec. The astrometric and photometric scale were defined using 2MASS sources in the field of view achieving a positional RMS of 0.1 arcsec and a zero-point uncertainty of 0.1 mag.

Our relative photometry shows no significant brightening for the suggested infrared counterpart. We determine a mean magnitude of H = 16.0 +/- 0.1, a value consistent within 2-sigma with the pre-outburst H-band magnitudes reported in ATels #3562, #3634.

X-RAY OBSERVATIONS: Two Chandra HETGS observations were performed starting on 2011-08-29 13:58 and 2011-08-30 13:15 for a total exposure time of 14 and 16 ksec, respectively. The merged MEG and HEG first order spectra fitted between 1 and 7 keV are consistent with an absorbed power-law with an Hydrogen column density of NH= (2.87 +/- 0.04)E22 cm-2 and Gamma = 1.75 +/- 0.02 (68% confidence level errors). The absorbed (unabsorbed) 0.5-10 KeV flux is 2.2E-10 (4.3E-10) erg/s/cm**2.

DISCUSSION: The X-ray outbursts of accretion powered ms-pulsars are expected to be followed by an increase in brightness of several magnitudes at optical and infrared wavelengths. For instance, the 4.3 hr orbital period ms-pulsar XTE J1814-338 increased its brightness by ~ 4 mag in the R-band during outburst (Krauss et al. 2005, ApJ, 627, 910; D'Avanzo et al. 2009, A&A, 508, 297). The non-variable nature of the source suggested as possible infrared and optical counterpart to IGR J17498-2921 (ATels #3562, #3622) is strong evidence that the source is in fact an unrelated foreground star.

We can roughly estimate the optical and infrared apparent magnitudes for the 3.84 hr (ATel #3601) orbital period system IGR J17498-2921 in quiescence by assuming the source is similar to XTE J1814-338 or the 3.8 hr orbital period neutron star system EXO 0748-676. Taking into account the reddening, distance and apparent magnitudes for these sources, we derive absolute magnitudes of M_R=7.3 for J1814 and M_R=6.9, M_J=6.5 for 0748 - we used here the following values from the literature: d=7.7 kpc, E(B-V)=0.42, R=22.3, J=21.3 for 0748 and d=8.0 kpc, E(B-V)=0.29, R=22.5 for J1814. Using E(B-V)=5.0 for IGR J17498-2921 as derived from the NH obtained above would probably constitute an upper limit to the reddening (see for instance the discussion in Jonker & Nelemans 2004, MNRAS, 354, 355). Greiss et al. (2011; ATel #3562) find E(B-V)=2.8 using the red clump method. Using the latter value and a distance of 7.6 kpc (ATel #3568), we expect R ~ 28 and J ~ 23.3 for IGR J17498-2921 in quiescence. Similarly, given the extinction, distance, the approximate absolute H-band magnitude, a typical source outburst brightening of 4-5 magnitudes, and a bright nearby foreground star it is not unsurprising that the H-band outburst counterpart remains undetected so far.