Chandra x-ray and gemini near-infrared observations of the eclipsing millisecond pulsar swift j1749.4-2807 in quiescence

We report on Chandra X-ray and Gemini-North near-infrared K-band observations of the eclipsing accretion-powered millisecond X-ray pulsar SWIFT J1749.4-2807 in quiescence. Using the Chandra observation we derive a source position of right ascension 17:49:31.73 and declination -28:08:05.08. The position is accurate to 0.6 arcsec (90 per cent confidence). We find one source at a magnitude K = 18.44 ± 0.03 with a position fully consistent with the accurate Chandra X-ray localization and a second source at K = 19.2 ± 0.1 that falls close to the edge of the error circle in the deep K-band images. The presence of a few weaker sources as suggested by previous H-band observations presented in the literature cannot be ruled out. There is marginal evidence that the brighter of the these two sources is variable. Follow-up spectroscopy of this potential counterpart will show if this source is the true counterpart to SWIFT J1749.4-2807. If so, baring the presence of complicating effects such as heating of the mass-donor star, it would allow for the mass of the neutron star to be measured through the measurement of periodic radial velocity variations