Tracing the power-law component in the energy spectrum of black hole candidates as a function of the QPO frequency

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
² Dipartimento di Scienze Università dell'Insubria, via Valleggio, 22100 Como, Italy
³ Astronomical Institute “Anton Pannekoek” University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, NL 1098 SJ Amsterdam, Netherlands
⁴ School of Natural Sciences, Institute for Advanced Study, Princeton, NJ 08540, USA

Corresponding author: T. Belloni, belloni@merate.mi.astro.it

Received: 12 July 2002
Accepted: 18 October 2002

Abstract

We investigated the relation between the centroid frequency of the quasi-periodic oscillation observed in the power density spectra of a sample of galactic black-hole candidates with the power-law photon index obtained from spectral fits. Our aim is to avoid inner accretion disk radius determination directly from spectral fits, given the uncertainties of the absolute values obtained in that way, but to base our analysis on the likely association of QPO frequency to a characteristic radius. We used archival RXTE data of GRS 1915+105 and published parameters for GRO 1655-40, XTE J1550-564, XTE J1748-288 and 4U 1630-47. While for low values of the QPO frequency, the two parameters are clearly correlated for each source, there is evidence for a turnoff in the correlation above a characteristic frequency, different for different sources. We discuss the possible nature of this turnoff.