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Chandra Spectroscopy of V404 Cyg in Outburst

ATel #7745; A. L. King (Stanford University), J. M Miller (University of Michigan), W. Morningstar (Stanford University)
on 2 Jul 2015; 00:28 UT
Credential Certification: Jon Miller (jonmm@umich.edu)

Subjects: X-ray, Binary, Black Hole

Referred to by ATel #: 7959

We report on Chandra HETGS observations of V404 Cyg, during its current outburst. Exposures of 29 ks and 25 ks were obtained on 2015-06-22 starting at 13:39:21, and 2015-06-23 starting at 21:25:2. The source was placed off of the ACIS-S array in order to prevent possible damage to the detector, given the enormous flares detected from the source. The ACIS-S array was also operated in "continuous clocking" mode, to prevent or minimize photon pile-up. This configuration complicates data reduction and analysis, and these preliminary results should be regarded with caution.

The most striking feature of spectra from both observations is the presence of very strong, narrow emission lines. In the 1-8 keV band, He-like and H-like lines from Ne, Mg, Si, S, Ca and Fe are present. Emission from low ionization charge states of Fe is also prominent. Fits are improved when elemental abundances are allowed to vary. The lines can be fit well using "apec" models for diffuse emission. The low energy and high energy spectra may require different plasma temperatures, of roughly kT = 1.5 keV, and kT = 7-10 keV, respectively.

The data also appear to require a very hard power-law, with an index of Gamma = 1.1 (or harder, in some fits). If this is real, and not an effect of the instrumental set-up, then both the lines and continuum signal that much of observed flux may be scattered or reprocessed. Analysis of continuum and Fe K variability in Ginga spectra of V404 Cyg suggested that the illuminated material was distant from the central engine (Oosterbroek et al. 1996). The narrow lines we have observed appear to confirm this.

The emission measure of the lower- temperature "apec" component, for instance, requires a high gas density, commensurate with a disk wind, if the gas is contained within the binary system. Illumination of this wind may produce the line spectrum. Alternatively, the steady L ~ few E+34 erg/s accretion mode typical for V404 Cyg may be one in which the gas is unbound (e.g. Blandford & Begelman 1999), and accumulated gas local to the system may be illuminated or shocked during the outburst. It is also possible that the source undergoes super-Eddington phases that eject shells of material.

Analysis continues and a journal paper is in preparation.

We thank Belinda Wilkes and the Chandra team for executing this DDT observation.

Blandford, R., Begelman, M., 1999, MNRAS, 303, L1
Oosterbroek, T., et al., 1996, A&A, 309, 781