Detection of Low‐Hard State Spectral and Timing Signatures from the Black Hole X‐Ray Transient XTE J1650−500 at Low X‐Ray Luminosities

Abstract

Using the Chandra X-Ray Observatory and the Rossi X-Ray Timing Explorer, we have studied the black hole candidate (BHC) X-ray transient XTE J1650-500 near the end of its 2001-2002 outburst after its transition to the low-hard state at X-ray luminosities down to L = 1.5 × 1034 ergs s-1 (1-9 keV, assuming a source distance of 4 kpc). Our results include a characterization of the spectral and timing properties. At the lowest sampled luminosity, we used an 18 ks Chandra observation to measure the power spectrum at low frequencies. For the three epochs at which we obtained Chandra/RXTE observations, the 0.5-20 keV energy spectrum is consistent with a spectral model consisting of a power law with interstellar absorption. We detect evolution in the power-law photon index from Γ = 1.66 ± 0.05 to 1.93 ± 0.13 (90% confidence errors), indicating that the source softens at low luminosities. The power spectra are characterized by strong (20%-35% fractional rms) band-limited noise, which we model as a zero-centered Lorentzian. Including results from an RXTE study of XTE J1650-500 near the transition to the low-hard state by Kalemci and coworkers, the half-width of the zero-centered Lorentzian (roughly where the band-limited noise cuts off) drops from 4 Hz at L = 7 × 1036 ergs s-1 (1-9 keV, absorbed) to 0.067 ± 0.007 Hz at L = 9 × 1034 ergs s-1 to 0.0035 ± 0.0010 Hz at the lowest luminosity. While the spectral and timing parameters evolve with luminosity, it is notable that the general shapes of the energy and power spectra remain the same, indicating that the source stays in the low-hard state. This implies that the X-ray-emitting region of the system likely keeps the same overall structure, while the luminosity changes by a factor of 470. We discuss how these results may constrain theoretical black hole accretion models.