Rapid Optical Fluctuations in the Black Hole Binary, V4641 Sgr

Abstract

We report on unprecedented short-term variations detected in the optical flux from the black hole binary system, V4641 Sgr. Amplitudes of the optical fluctuations were larger at longer time scales, and surprisingly reached ~60% around a period of ~10 min. The power spectra of fluctuations are characterized by a power law. It is the first case in black hole binaries that the optical emission was revealed to show short-term and large-amplitude variations given by such a power spectrum. The optical emission from black hole binaries is generally dominated by the emission from the outer portion of an accretion disc. The rapid optical fluctuations however indicate that the emission from an inner accretion region significantly contributes to the optical flux. In this case, cyclo-synchrotron emission associated with various scales of magnetic flares is the most promising mechanism for the violently variable optical emission.

5 pages, 2 figures, accepted for publication in PASJ

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• A jet model for the broadband spectrum of XTE J1118+480: Synchrotron emission from radio to X-rays in the Low/Hard spectral state
• A Black Hole in the Superluminal source SAX J1819.3-2525 (V4641 Sgr)
• Identification of the donor in the X-ray binary GRS 1915+105
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• Furiously Fast and Red: Sub-second Optical Flaring in V404 Cyg during the 2015 Outburst Peak
• The obscured X-ray binaries V404 Cyg, Cyg X-3, V4641 Sgr, and GRS 1915+105
• Chandra Detections of Two Quiescent Black Hole X-Ray Transients
• New clues on outburst mechanisms and improved spectroscopic elements of the black-hole binary V4641 Sagittarii
• Outburst and Post-Outburst Active Phase of the Black Hole X-ray Binary V4641 Sgr in 2002
• Chemical Abundances in the Secondary Star of the Black Hole Binary V4641 Sagittarii (SAX J1819.3-2525)
• Rapid Optical Variations Correlated with X-rays in the 2015 Second Outburst of V404 Cygni (GS 2023$+$338)
• Searching for Fast Optical Transients using VERITAS Cherenkov Telescopes
• MITSuME observation of V404 Cygni in the 2015 outburst: Two optical variable components with different variability