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XMM-Newton detects activity from a weak X-ray transient source in the Galactic Center

ATEL # 2038; G. Ponti (APC-Paris, F), G. Trap, A. Goldwurm, P. Ferrando (CEA-Saclay & APC-Paris, F), R. Terrier (APC-Paris, F), G. Belanger (ESA/ESAC-Villafranca, S), R. Genzel, S. Gillessen, G. Hasinger, K. Dodds-Eden, P. Predehl, B. Aschenbach (MPE-Garching, G), D. Porquet, N. Grosso (OAS-Strasbourg, F), Y. Clenet, D. Rouan (OAPM-Meudon, F), M. Sakano, R. Warwick (Leicester Un., UK), F. Melia (Arizona Un.-Tucson, US), Z. Farhad (Northwestern Un.-Evanston, US), M. Reid (CfA-Cambridge, US)
on 29 Apr 2009; 13:27 UT
Password Certification: Gabriele Ponti (

Subjects: Radio, X-ray, Black Holes, Neutron Stars, Transients

We detected a faint outburst from a weak Galactic Center transient, during a series of three XMM-Newton observations (~40 ks each) performed on 2009 April 1-5 as part of a multi-wavelength campaign on Sgr A*.

The source is located at RA(J2000)=17h 45m 37.97s Dec(J2000)=-29m 00m 23.3s (positional uncertainty of 2.0 arcsec), which is 27 arcsec from Sgr A*. Taking a distance of 8 kpc to the GC places this source at a minimum distance of 1 pc from the supermassive black hole. This position is consistent with those of 2 Chandra sources: CXOGC J174538.0-290022 (1.6" offset) and CXOGC J174537.9-290025 (1.9" offset).

The XMM-Newton spectrum of the source can be well fit by an absorbed power-law with photon index Gamma = 1.4+/-0.3 and column density of neutral material of Nh = (19+/-3) x 10^22 cm-2. The de-absorbed 2-10 keV flux is 2.6 (+1.1-0.4) x 10-12 ergs s-1 cm-2 (chi2=183.4 for 183 dof), which corresponds to a luminosity of 2x10^34 ergs s-1 at 8 kpc, and puts this source in the category of very faint X-ray transients (Wijnands et al. 2006, A&A, 449, 1117). We favour the association with CXOGC J174538.0-290022 because it is the closest and because it has been observed to have a similar luminosity (Muno et al. 2005, ApJ, 622, L113).

Although caught in a burst, the source flux remained approximately constant in the first two XMM-Newton observations. During the third pointing, it had a flare that lasted about 1 hour during which the source count rate almost doubled (from 0.07 to 0.14 counts/s in the EPIC-pn instrument).

Preliminary results of VLA data at 43 GHz taken on April 1st provide a 2 sigma upper limit of 0.4 mJy at the source position. We encourage sensitive X-ray follow-up observations.

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