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 (M_x). Combined with the mass function of 4.97 M_⊙, the infrared light curves imply M_x ≤ 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 M_x = 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 f_λ ∝ 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%.