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 F_γ = 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/L_X≈10³, L_X/L_opt≈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 (M_d=0.6 M_⊙) filling its Roche lobe and orbiting a black hole (M_bh=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.