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QPO and VLFN in the power spectrum of V404 Cyg detected with INTEGRAL

ATel #7726; Artem V. Prosvetov, Sergei A. Grebenev (Space Research Institute, Moscow)
on 28 Jun 2015; 13:41 UT
Credential Certification: Sergei Grebenev (sergei@hea.iki.rssi.ru)

Subjects: Radio, Infra-Red, Optical, Black Hole, Transient, Potentially Hazardous Asteroid

Referred to by ATel #: 7740, 7758, 7959

In addition to extreme flaring activity observed from the recurrent X-ray nova V404 Cyg/GS 2023+338 in radio, optical and X-ray bands (ATEL #7647, #7658, #7662, #7681, #7694, #7702, #7708, #7716, #7717) the source shows notable short-term variability on time scales of minutes and seconds.

Here we report the detection with INTEGRAL of two strong QPO peaks in the power spectrum of its hard X-ray emission (see the spectra obtained in the 20-80 and 80-200 keV bands). They are based on observations with the IBIS/ISGRI telescope on June 15-17, the ``white'' noise was subtracted. The QPO peaks are at frequencies 0.124+/-0.003 and 0.251+/-0.003 Hz, FWHM are equal to 0.009 and 0.024 Hz respectively. The second peak is at least 2 times stronger. The QPO frequencies are the same for two bands, but parameters of the low-frequency noise (LFN) which can be described by the King profile are different, in particular, the break frequency changes between 20-80 and 80-200 keV band in ~1.7 times, from 0.66 to 0.39 Hz. Another peculiarity of the spectra is the presence of a strong very low-frequency noise component (VLFN) in addition to the usual LFN component. It is power-law in a shape with a slope of about 1.6. Note however that another bright hard X-ray source Cyg X-1 was always in the field and could contaminate to VLFN.

It is remarkable that the presence of QPO in the X-ray power spectra of V404 Cyg/GS 2023+33 is accompanied by strong flaring activity of the source in the radio band (ATEL #7658, #7714). The correlation was observed earlier in other black hole transients.

We encourage follow-up observations of the QPO peaks of this source and their evolution in X-ray and optical bands.