GRO J1744-28, search for the counterpart: infrared photometry and spectroscopy

Using VLT/ISAAC, we have detected two candidate counterparts to the bursting pulsar GRO J1744-28, one bright and one faint, both within the X-ray error circles found using XMM-Newton and Chandra. In determining the spectral types of the counterparts we applied three different extinction corrections; one for an all-sky value, one for a Galactic bulge value and one for a local value. We find that the local value, with an extinction law of alpha = 3.23 +/- 0.01 is the only correction that results in colours and magnitudes for both the bright and faint counterparts that are consistent with a small range of spectral types, and in the case of the bright counterpart are also consistent with the spectroscopic identification. Photometry of the fainter candidate then indicates that it is a K7/M0 V star at a distance of 3.75 +/- 1kpc. Such a star would require a very low inclination angle (i = 15° a stripped giant, or a main-sequence M3+V star would be consistent with this mass function constraint. In both cases mass transfer, if present, will be by wind accretion as the counterpart will not fill its Roche lobe given the observed orbital period. In this case, the derived magnetic field strength of 2.4 × 1011 G is sufficient to inhibit accretion of captured material by the propeller effect, hence the quiescent state of the system.