Lorentz Center - A New View of Accretion onto Sgr A* from 21 Jan 2013 through 25 Jan 2013
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    A New View of Accretion onto Sgr A*
    from 21 Jan 2013 through 25 Jan 2013

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Although astronomers and cosmologists typically assume we do not occupy a privileged place in the universe, our Milky Way Galaxy appears to be an exception. While most supermassive black holes (SMBH) in the local universe accrete at a small fraction (10−7 − 10−5) of the rate of more luminous

quasars, Sgr A*, the SMBH in our Galactic center, is the most under-luminous black hole we know of—a factor of 10−9 below its canonical maximum luminosity. But what makes Sgr A* so special?


To address this question, in 2012 we performed an unprecedented 3 Megasecond observing campaign with the Chandra High-Energy Transmission Grating (HETG) Spectrometer in concert with numerous multiwavelength programs (coverage from radio to gamma rays). This Chandra X-ray Visionary Project (XVP) was designed to have a lasting impact on our understanding of our Galaxy and the physics of black holes.


The aim of the meeting at the Snellius Center was to bring together leaders and members of our seven working groups (WG) and various multiwavelength programs. We identified the following goals that, if met, would signal a successful workshop:


• WG leaders are updated on the progress of the entire project and agree with the overall collaboration strategy for science exploitation and output

• individual WGs make significant progress on their science, and come up with a coherent strategy for their science plans, including data products and publication plans

• an updated catalog of all data products and results to date are presented to the larger XVP collaboration, and disseminated via the XVP website

• a coherent strategy for observations of the impact of G2 is developed, utilizing the collaborative structures already in place from the XVP


Each of these goals was indeed accomplished and the collaboration continues to work toward realizing the full potential of this phenomenal data set.


Specifically, the meeting produced several important developments: One of the most crucial was the creation of a flexible database of all available multiwavelength light curves of the Galactic center from 2012. This database is particularly useful for cross-correlation of flares at different energies, which can distinguish between models for the physical mechanisms that drive the flares, and inform the distribution of the flare population (e.g., see Neilsen et al. 2013). This database will also be useful in constraining flare and accretion theory for Sgr A* (e.g., Dibi et al. 2014). Another critical moment came when one WG noticed that Sgr A*’s quiescent emission was mostly consistent with a Poisson process. This realization led to the discovery of a small amount of excess X-ray variability from the black hole. Ultimately we discovered that results from X-ray spectroscopy, timing, imaging, and statistical analyses all converge to tell us that ~10% of the quiescent emission is actually contributed by undetected flares.


In addition, we learned that the X-ray spectrum from Chandra would allow us to conclusively refute claims that the extremely faint X-ray source is not actually Sgr A*, but a cluster of active stars (published by Wang et al. in Science in 2013). Early discovery of flares from NuSTAR, in one case overlapping with flares detected during the Chandra XVP, opened the prospect for using these observatories in concert to perform detailed timing analysis (recent results on NuSTAR’s first flare discovery can be found in Barriére et al. 2014). Other exciting multiwavelength developments included hints of spectral evolution in the NIR, resolution of Sgr A*’s spatial scale in the radio, and progress on VLBA capability in anticipation of the Event Horizon Telescope, which aims to resolve the shadow of the supermassive black hole.


The meeting took place over five days. Each day was structured to allow sharing of results and projects, as well as lots of time to work. For those who were not able to attend, we posted nearly real-time updates from our meeting to the collaboration website. We started on the first day with a basic overview of the collaboration structure and policies, as well as a description of the Chandra HETG and multiwavelength campaigns; this was followed by seven individual talks (~25 min each) describing each of the working groups, their mandate, science scope/goals, and recent progress. Days 2, 3, and 4 began with focused WG meetings, followed by plenary discussions. Each of these finished with one or two talks describing mutliwavelength monitoring efforts, with particular focus on the infrared, radio, and higher energies (e.g., those probed by HESS and the newly-launched NuSTAR). The final day of the workshop shifted to a discussion of future plans and prospects, with a series of summary talks from the WG leads, as well as future observing campaigns (some focused on the G2 encounter). This format, which emphasized only a few talks and lots of time to work, was far more invigorating than the normal conference schedule.


The Lorentz Center@ Snellius venue was perfect for this workshop, which focused on a combination of group discussion/talks and smaller break-out discussions. The space (the main room, the lounge, and the individual offices) and the staff (Gerda Filippo, in particular) were greatly appreciated by all those who attended. Thank you!


Materials developed during the workshop (proprietary and non-proprietary), as well as developments since the meeting, can be found at the Sgr A* webpage: www.sgra-star.com.


Working Groups: Science Steering Committee; Chandra Data Products; Accretion Flow Quiescent Spectra; Central Parsec Diffuse Spectra; Flares, X-ray Properties; Flares, Multi-wavelength Properties; Flares, VLBI/EHT Observations/Constraints on Geometry; Non-Sgr A* Studies; Theory.



1. “Chandra/HETGS Observations of the Brightest Flare Seen from Sgr A*,” M.A. Nowak, J. Neilsen, S.B. Markoff, F.K. Baganoff, D. Porquet, N. Grosso, Y. Levin, J. Houck, A. Eckart, H. Falcke, L. Ji, J.M. Miller, Q.D. Wang. ApJ, 2012, 759, 95 (NASA ADS, arXiv:1209.6354)

2. “Dissecting X-ray-emitting Gas around the Center of our Galaxy,” Q. D. Wang, M. A. Nowak, S. B. Markoff, F. K. Baganoff, S. Nayakshin, F. Yuan, J. Cuadra, J. Davis, J. Dexter, A. C. Fabian, N. Grosso, D. Haggard, J. Houck, L. Ji, Z. Li, J. Neilsen, D. Porquet, F. Ripple, R. V. Shcherbakov. Science, 2013, 341, 981 (NASA ADS, arxiv:1307.5843)

3. “A Chandra/HETGS Census of X-ray Variability From Sgr A* During 2012,” J. Neilsen, M. A. Nowak, C. Gammie, J. Dexter, S. Markoff, D. Haggard, S. Nayakshin, Q. D. Wang, N. Grosso, D. Porquet, J. A. Tomsick, N. Degenaar, P. C. Fragile, R. Wijnands, J. M. Miller, F. K. Baganoff. ApJ, 2013, 774, 42 (NASA ADS, arxiv:1307.5843)

4. “Toward the event horizon—the supermassive black hole in the Galactic Center,” H. Falcke, S. B. Markoff. Classical and Quantum Gravity, 2013, 30, 4003 (NASA ADS, arxiv:1311.1841)

5. “NuSTAR Detection of High-energy X-Ray Emission and Rapid Variability from Sagittarius A* Flares,” N.M. Barri´ere, J.A. Tomsick, F.K. Baganoff, S.E. Boggs, F.E. Christensen, W.W. Craig, J. Dexter, B. Grefenstette, C.J. Hailey, F.A. Harrison, K.K. Madsen, K. Mori, D. Stern, W.W. Zhang, S. Zhang, A. Zoglauer. ApJ, 2014, 786, 46 (NASA ADS, arxiv:1403.0900)

6. “Exploring Plasma Evolution During Sagittarius A* Flares,” S. Dibi, S. Markoff, R. Belmont, J. Malzac, N. M. Barri´ere, J. A. Tomsick, MNRAS, submitted