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Fermi GBM Observations of V404 Cyg

ATel #7715; P. Jenke (UAH), C. A. WIlson-Hodge (NASA/MSFC), M. H. Finger (USRA), W. Paciesas (USRA), V. Connaughton (USRA)
on 25 Jun 2015; 22:13 UT
Credential Certification: Peter Jenke (peter.a.jenke@nasa.gov)

Subjects: X-ray, Black Hole, Transient

Referred to by ATel #: 7722, 7734, 7740, 7959

On June 15 at 18:59:01.20 UTC, Fermi/GBM triggered (G. Younes et al., GCN 17932) on a hard X-ray flare from the Cygnus region of the sky. Swift/BAT also triggered (S. D. Barthelmy, et al. GCN 17929) on the same event and identified the source as V404 Cyg. Since June 15, GBM has triggered on 96 flares from this source. The events from June 15 and 16 may be fit with a heavily absorbed power law with index around -2. At later times, the spectrum is well fit by a broken power law with lower index around -1.2 and upper index around -3.0 and the break between 35 and 50 keV with highly variable absorption between 1e23 and 10e23 cm-2.

Assuming a distance of 2.4 kpc (Miller-Jones et al. 2009) the peak luminosity (10-1000 keV) of these flares ranges from 2e37 to 6e38 erg/s consistent with Konus-Wind observations (GCN 17938).

Day averaged GBM Earth Occultation light curves of V404 Cyg shows the source reaching 1.8 Crab in the 12-25 keV band and 3.7 Crab in the 100-300 keV band on June 19. The source rose to 2.1 Crab in the 12-25 keV band but fell to 1.2 Crab in the 100-300 keV band on June 22 , likely indicating spectral softening. All bands show a dip in flux on June 20. Quarter day averaged light curves show substantially more variability with flux levels exceeding 4 Crab in all detected energy bands (12-25 keV, 25-50 keV, 50-100 keV and 100-300 keV) and displaying similar spectral evolution.

Low frequency QPOs (periods of tens to hundreds of seconds) are seen in the data from the direction of V404 Cyg starting on June 19 that are similar to those seen in optical data in the 1989 outburst (Gotthelf et al., 1991).

Further analysis is ongoing and we encouraged additional observations at all wavelengths.