Investigating the Physics of Hard X-ray Outbursts from the Galactic Center Supermassive Black Hole Sagittarius A* with NuSTAR
Abstract
The Galactic center supermassive black hole (SMBH) Sagittarius A* (Sgr A*) is remarkably underluminous with a bolometric luminosity about 10-9 times its Eddington luminosity. It is the closest SMBH and thus an ideal target for investigation of galactic nuclei and their activity cycles. This goal can be fulfilled by studying its current, past and possible future outbursts. Its current X-ray quiescent state, with a luminosity of LX ~ 1033 erg s-1, is punctuated by X-ray flares up to a few times 1035 erg s-1, whose origin is poorly understood. With 638.6 ks NuSTAR observation of Sgr A*, I collected nine X-ray flares which are detected up to 79 keV, and studied their timing behavior using the Bayesian block analysis. The broadband (3-79 keV) spectroscopic studies show that the X-ray flares can have a range of photon indexes, which can be explained by the magnetic reconnection scenario. During the flares, Sgr A* is still orders of magnitudes lower than its Eddington luminosity. Whether it has ever experienced more substantial increases in activity as observed in low-luminosity Active Galactic Nuclei is still under discussion. Indication of such past activity of Sgr A* has come from the Galactic center molecular clouds (GCMCs). I use the hard X-ray emission from Sgr B2, the densest GCMC, to constrain the past Sgr A* X-ray outburst, resulting in LX~5×1038 erg s-1 with a photon index of Γ=2.2±0.4. I also discovered different timing variability from different cloud substructures, which can further constrain the past Sgr A* outburst and the cloud structure. Future giant outburst from Sgr A* could be triggered by dust/gas infall. Although the G2 infall event has not caused any increased X-ray activity from Sgr A*, there has been evidence that it caused flare behavior changes. Ongoing hard X-ray monitoring of Sgr A* will address this remaining puzzle.
- Publication:
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American Astronomical Society Meeting Abstracts #227
- Pub Date:
- January 2016
- Bibcode:
- 2016AAS...22741002Z