A model of object Geminga as a degenerate white dwarf orbiting around a black hole

Abstract

Geminga (2CG195+04) is a very bright γ-ray source with the flux Fγ = 2.4×10−9 erg s−1 cm−2 for E≥50 MeV (refs 1–3). It has been recently identified with X-ray4 and optical5 sources. The optical object is the bluest for its magnitude5, indicating compactness of the emitting region. The high ratios L/LX≈103, LX/Lopt≈200 and the absence of radio emission are very unusual. The observed periodicity is P≈1 min and it changes at a rate corresponding to a doubling time of about several hundred years. I propose here a model which quantitatively explains the values of P and Ṗ. It consists of a degenerate dwarf (Md=0.6 M⊙) filling its Roche lobe and orbiting a black hole (Mbh=4.4 M⊙) with a period P=1 min. Gravitational radiation dissipates the angular momentum, which leads to mass transfer with a current rate ∼10−3 M⊙ yr−1 and a period change Ṗ which agrees with the observed value. At a distance of 100 pc the flux of this radiation would be ∼5×10−3 erg s−1 cm−2 (at ν=1/30 Hz). This is about seven orders of magnitude greater than from any other known stationary source.