Infrared observations of the candidate double neutron star system PSR J1811-1736

PSR J1811-1736 (Ps = 104 ms) is an old (~1.89 Gyr) binary pulsar (Porb = 18.8 d) in a highly eccentric orbit (e = 0.828) with an unidentified companion. Interestingly, the pulsar timing solution yields an estimated companion mass of 0.93 M⊙ ≤ MC ≤ 1.5 M⊙, compatible with that of a neutron star. As such, it is possible that PSR J1811-1736 is a double neutron star (DNS) system, one of the very few discovered so far. This scenario can be investigated through deep optical/infrared (IR) observations. We used J- H- and K-band images, obtained as part of the UK Infrared Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS), and available in the recent Data Release 9 Plus, to search for the undetected companion of the PSR J1811-1736 binary pulsar. We detected a possible companion star to PSR J1811-1736 within the 3σ radio position uncertainty (1.32 arcsec), with magnitudes J = 18.61 ± 0.07, H = 16.65 ± 0.03 and K = 15.46 ± 0.02. The star colours are consistent with either a main sequence (MS) star close to the turn-off or a lower red giant branch (RGB) star, at a pulsar distance of ~5.5 kpc and with a reddening of E(B - V) ~4.9. The star mass and radius would be compatible with the constraints on the masses and orbital inclination of the binary system inferred from the mass function and with the lack of radio eclipses near superior conjunction. Thus, it is possible that it is the companion to PSR J1811-1736. However, based on the star density in the field, we estimated quite a large chance coincidence probability of ~0.27 between the pulsar and the star, which makes the association unlikely. No other star is detected within the 3σ pulsar radio position down to J ~ 20.5, H ~19.4 and K ~18.6, which allows us to rule out a MS companion star earlier than a mid-to-late M spectral type