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A Constant Spectral Index for Sagittarius A* during Infrared/X-Ray Intensity Variations

Hornstein, S. D. and Matthews, K. and Ghez, A. M. and Lu, J. R. and Morris, M. and Becklin, E. E. and Rafelski, M. and Baganoff, F. K. (2007) A Constant Spectral Index for Sagittarius A* during Infrared/X-Ray Intensity Variations. Astrophysical Journal, 667 (2). pp. 900-910. ISSN 0004-637X. doi:10.1086/520762. https://resolver.caltech.edu/CaltechAUTHORS:20100422-095139811

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Abstract

We report the first time series of broadband infrared color measurements of Sgr A*, the variable emission source associated with the supermassive black hole at the Galactic center. Using the laser and natural guide star adaptive optics systems on the Keck II Telescope, we imaged Sgr A* in multiple near-infrared broadband filters with a typical cycle time of ~3 minutes during four observing runs (2005-2006), two of which were simultaneous with Chandra X-ray measurements. In spite of the large range of dereddened flux densities for Sgr A* (2-30 mJy), all of our near-infrared measurements are consistent with a constant spectral index of α = -0.6 ± 0.2 (F_ν ∝ ν^α). Furthermore, this value is consistent with the spectral indices observed at X-ray wavelengths during nearly all outbursts, which is consistent with the synchrotron self-Compton model for the production of the X-ray emission. During the coordinated observations, one infrared outburst occurs ≤36 minutes after a possibly associated X-ray outburst, while several similar infrared outbursts show no elevated X-ray emission. A variable X-ray to IR ratio and constant infrared spectral index challenges the notion that the infrared and X-ray emission are connected to the same electrons. We, therefore, posit that the population of electrons responsible for both the IR and X-ray emission are generated by an acceleration mechanism that leaves the bulk of the electron energy distribution responsible for the infrared emission unchanged, but has a variable high-energy cutoff. Occasionally a tail of electrons ≳1 GeV is generated, and it is this high-energy tail that gives rise to the X-ray outbursts. One possible explanation for this type of variation is from the turbulence induced by a magnetorotational instability, in which the outer scale length of the turbulence varies and changes the high-energy cutoff.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1086/520762DOIArticle
http://iopscience.iop.org/0004-637X/667/2/900?fromSearchPage=truePublisherArticle
ORCID:
AuthorORCID
Ghez, A. M.0000-0003-3230-5055
Lu, J. R.0000-0001-9611-0009
Morris, M.0000-0002-6753-2066
Rafelski, M.0000-0002-9946-4731
Additional Information:© 2007 American Astronomical Society. Received 2006 December 15; accepted 2007 June 9. The authors thank Randy Campbell, Al Conrad, Steven McGee, Madeline Reed, and the entire Keck LGSAO team for their help in conducting these observations, Emiliano Diolaiti for help in modifying the StarFinder code, and Siming Liu for useful discussions. Support for this work was provided by NSF grant AST 04-06816 and the NSF Science and Technology Center for Adaptive Optics, managed by UCSC (AST 98-76783). S. D. H. was also supported by a Dissertation Year Fellowship provided by the University of California. F. K. B. was supported by NASA through Chandra award G05-6093X. The infrared data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M.Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Keck:II (NIRC2), CXO (ACIS-I )
Funders:
Funding AgencyGrant Number
NSFAST 04-06816
University of CaliforniaUNSPECIFIED
NASAG05-6093X
Subject Keywords:black hole physics; Galaxy: center; infrared: stars; techniques: high angular resolution
Issue or Number:2
DOI:10.1086/520762
Record Number:CaltechAUTHORS:20100422-095139811
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20100422-095139811
Official Citation:S. D. Hornstein et al 2007 ApJ 667 900 doi: 10.1086/520762
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:18046
Collection:CaltechAUTHORS
Deposited By: Jason Perez
Deposited On:22 Apr 2010 20:23
Last Modified:08 Nov 2021 23:41

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