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Title:
Discovery of a low-frequency broad quasi-periodic oscillation peak in the power density spectrum of Cygnus X-1 with Granat/SIGMA
Authors:
Vikhlinin, A.; Churazov, E.; Gilfanov, M.; Sunyaev, R.; Dyachkov, A.; Khavenson, N.; Kremnev, R.; Sukhanov, K.; Ballet, J.; Laurent, P.; Salotti, L.; Claret, A.; Olive, J. F.; Denis, M.; Mandrou, P.; Roques, J. P.
Affiliation:
AA(Russian Academy of Sciences, Moscow, Russia), AB(Russian Academy of Sciences, Moscow, Russia), AC(Russian Academy of Sciences, Moscow, Russia), AD(Russian Academy of Sciences, Moscow, Russia), AE(Russian Academy of Sciences, Moscow, Russia), AF(Russian Academy of Sciences, Moscow, Russia), AG(Russian Academy of Sciences, Moscow, Russia), AH(Russian Academy of Sciences, Moscow, Russia), AI(Centre d'Étude Nucleaires de Saclay, Gif-sur-Yvette, France), AJ(Centre d'Étude Nucleaires de Saclay, Gif-sur-Yvette, France)
Publication:
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 424, no. 1, p. 395-400 (ApJ Homepage)
Publication Date:
03/1994
Category:
Astrophysics
Origin:
STI
NASA/STI Keywords:
Cygnus Constellation, Radiant Flux Density, Stellar Oscillations, Black Holes (Astronomy), X Ray Astronomy
DOI:
10.1086/173897
Bibliographic Code:
1994ApJ...424..395V

Abstract

A transient broad (delta(f)/f = 0.8 approximately 1) very low frequency (approximately 0.04-0.07 Hz) and strong (fractional rms variations are at the level of approximately 10-15% of total source intensity) quasi-periodic oscillations (QPO) feature was discovered by the SIGMA telescope onboard the Granat observatory in the power density spectra of Cygnus X-1; the source was during all the observations carried out in 1990-1992 in its standard (low or hard) spectral state (Sunyaev & Truemper 1979) with average 40-150 keV flux, corresponding appproximately to the 'nominal' gamma2 level of the source (Ling et al. 1979). The power density spectra, obtained in the 4 x 10-4-10 Hz frequency range, typically exhibit strong very low frequency noise below a few millihertz increasing toward lower frequencies, a nearly flat region from a few millihertz up to a break frequency fbr = 0.04 approximately 0.1 Hz and a power-law spectrum as f-1 above the break frequency. The QPO feature, when observed, was centered below or near the break frequency fbr.

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