Abstract
ABSTRACT On 2015 June 15, the Swift space observatory discovered that the Galactic black hole candidate V404 Cyg was undergoing another active X-ray phase, after 25 years of inactivity. The 12 telescopes of the MASTER Global Robotic Net located at six sites across four continents were the first ground-based observatories to start optical monitoring of the microquasar after its gamma-ray wake up at 18h 34m 09s U.T. on 2015 June 15. In this paper, we report, for the first time, the discovery of variable optical linear polarization, changing by 4%–6% over a timescale of ∼1 hr, on two different epochs. We can conclude that the additional variable polarization arises from the relativistic jet generated by the black hole in V404 Cyg. The polarization variability correlates with optical brightness changes, increasing when the flux decreases.
Highlights
The X-ray nova GS 2023+338 was discovered on 1989 May 22 by the Ginga X-ray satellite (Makino et al 1989)
Soon after the object was identified with the variable star V404 Cyg, which had been seen to brighten by seven magnitudes in the optical in 1938 (Wagner et al 1989a,b)
It is well established (Rodriguez et al 2015) that V404 Cyg is a binary system consisting of a giant secondary star, older and less massive than the Sun and a primary star which is an accreting black hole candidate (Antokhina et al 1993; Casares et al 1991)
Summary
The X-ray nova GS 2023+338 was discovered on 1989 May 22 by the Ginga X-ray satellite (Makino et al 1989). The twin optical telescope, MASTER-Tunka of the MASTER Global Robotic Net (Lipunov et al 2010), located near Lake Baikal (Lomonosov Moscow State University and Irkutsk State University Tunka astrophysical center), was the first ground-based observatory to point to V404 Cyg after the Swift alert. This was 22 sec after the notice time (Lipunov et al 2015) of the burst alert, via space communication system (i.e. socket messages GCN: The Gamma-ray Coordinates Network alert (Barthelmy et al 1994, 1995)) and after 150 seconds we detected a bright optical flare (Lipunov et al 2015), reaching magnitude 14, with the star brightening by a factor of 2.5 in a mere half-hour. Dozens of telescopes worldwide subsequently observed the object at various wavelengths ( Rodriguez et al (2015); Mooley et al (2015); Munoz-Darias et al (2015); Ferrigno et al (2015); Motta et al (2015); Tetarenko et al (2015a,b); Tsubono et al (2015)), but MASTER is the first to report on the discovery of variable optical linear polarization arising in the relativistic jet generated by the black hole
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