On the Mass of the Black Hole in GS 2000+25

We present J- and K'-band (1.95-2.35 μm) photometry of the quiescent X-ray nova GS 2000+25, obtained in an attempt to constrain the mass of the compact object (MX). Combined with the mass function of 4.97 M☉, the infrared light curves imply MX ≤ 11 M☉ (90% confidence level), regardless of the evolutionary state of the secondary, for a mass ratio q of 4 < q < 30. For a secondary mass in the range M2 = 0.4-0.9 M☉ (as expected for a K dwarf companion), and assuming a negligible contribution from the disk at these wavelengths, we find MX = 8.5 ± 1.5 M☉ and an inclination of 65° ± 9°. The observed infrared color is consistent with that expected of a K dwarf secondary. If the accretion disk continuum is given by fλ ∝ λ-1.8, as observed in other quiescent X-ray novae, we constrain the accretion disk contribution to the K' flux (independently of optical estimates) to ≤12%.