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Candidate Near-IR Counterpart to the X-ray Transient Pulsar GRO J1744-28

ATel #1014; Z. Wang (McGill), D. Chakrabarty (MIT), M. Muno (Caltech)
on 27 Feb 2007; 22:45 UT
Credential Certification: Z. Wang (wangzx@physics.mcgill.ca)

Subjects: Infra-Red, Binary, Neutron Star, Transient

We previously obtained near-IR multiband images of the field containing the quiescent X-ray transient pulsar GRO J1744-28 (the "Bursting Pulsar") on 2004 Feb 11 using the PANIC camera on the 6.5-meter Magellan/Baade telescope at Las Campanas Observatory in Chile. The exposure times were 5 min in J-band and 2.5 min in each of H- and K-bands. We find a candidate counterpart within the error circle (0.6" radius; this includes the uncertainty from our astrometric calibration) of the reported X-ray position (Muno et al. 2007, ATEL #1013). The magnitudes of this source were J = 19.21, H = 16.16, and Ks = 14.69, with 0.05 mag uncertainty (1-sigma). However, the target field is crowded; the number density of stars that were as bright as our candidate in Ks band was 0.13 stars/arcsec^2, indicating a 14% chance coincidence probability.

Assuming a source distance of 8 kpc for GRO J1744-28, the object's colors are consistent with a normal K2 III giant reddened by Av = 21 mag. However, the low binary mass function of the pulsar (1.3*10^-4 M_sun; Finger et al. 1996, Nature, 381, 291) implies a low mass range (0.2-0.7 M_sun) for the companion, suggesting that the companion is a giant star that has lost most of its hydrogen-rich envelope (Rappaport & Joss 1997, ApJ, 486, 435). Using the predicted colors and magnitudes of Rappaport & Joss, we derive a source distance of approximately 4 kpc for the source, but with an anomalously high Av (=22 mag). However, this distance is roughly consistent with the value derived from the Eddington luminosity limit of the X-ray source (Giles et al. 1996, ApJ, 469, L25).

Given that the X-ray brightness of the quiescent source was higher than usual on 2007 Feb 22, we encourage further near-IR imaging observations to search for correlated near-IR flux changes in order to verify this candidate counterpart.