The high-frequency QPOs in GRS 1915+105

¹ Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
² Astronomical Institute "A. Pannekoek"and Center for High-Energy Astrophysics, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: mariano@astro.uva.nl
³ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, 1900 La Plata, Argentina e-mail: mmendez@fcaglp.unlp.edu.ar
⁴ Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), PO Box 50727, 28080 Madrid, Spain e-mail: celia@laeff.esa.es

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

Received: 5 January 2001
Accepted: 26 March 2001

Abstract

We analyzed two 1996 observations of the Galactic Microquasar GRS 1915+105 taken with the Proportional Counter Array on board the Rossi X-ray Timing Explorer. We focus on the properties of the high-frequency QPO as a function of the "dip"oscillations, a factor of 1000 slower. In one observation, 1996 May 5th, we find that the energy spectrum of the QPO at ~65 Hz changes drastically between the high-intervals and the dips, hardening during the dips to the point that the QPO peak is no longer detected at low energies. In the second observation, 1996 May 14th, although it has similar overall characteristics, the 65 Hz QPO is not seen, but another broader QPO peak appears at 27 Hz, only during the dips. This peak is too weak to obtain reliable information about its spectrum. Our results indicate that the presence/absence of high-frequency features in this enigmatic source are intimately linked to the slower oscillations and variations that happen on longer time scales.