arXiv:astro-ph/0303583v1 26 Mar 2003
Astron. Nachr./AN 324, No. S1, 3–8 (2003) / DOI theDOIprefix/theDOIsuffix
A New X-Ray Flare from the Galactic Nucleus Detected with
A. Goldwurm∗1, E. Brion2, P. Goldoni1, P. Ferrando1, F. Daigne1, A. Decourchelle1, R.
S. Warwick3, and P. Predehl4
1Service d’Astrophysique, DAPNIA/DSM/CEA, CE-Saclay, F-91191 Gif-Sur-Yvette, France
2Centre d’Etude Nucl´ eaire de Bordeaux-Gradignan, All´ ee du Haut Vigneau, 33175 Gradignan, France
3Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
4Max-Planck Institut f¨ ur Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
Received 3 March 2003
Key words Accretion, accretion disks — Black hole physics — Galaxy: center — X-rays: general
The compact radio source Sgr A∗believed to be the counterpart of the massive black hole at the Galactic
nucleus, was observed to undergo rapid and intense flaring activity in X-rays with Chandra in October 2000.
We report here the detection with XMM-Newton EPIC cameras of the early phase of a similar X-ray flare
from this source, which occurred on 2001 September 4. The source 2-10 keV luminosity increased by a
factor ≈ 20 to reach a level of 4 1034erg s−1in a time interval of about 900 s, just before the end of the
observation. The data indicate that the source spectrum was hard during the flare and can be described
by simple power law of slope ≈ 0.7. This XMM-Newton observation confirms the results obtained by
Chandra, suggests that, in Sgr A∗, rapid and intense X-ray flaring is not a rare event and therefore sets some
constraints on the emission mechanism models proposed for this source.
The bright, compact radio source Sgr A∗is believed to be the radiative counterpart of the 2.6 106M⊙
black hole which governs the dynamics of the central pc of our Galaxy (Melia & Falcke 2001). The
compelling evidence for such a massive black hole at the Galactic Center (see Sch¨ odel et al. 2002 for the
most rescent results), contrasts remarkablywith the weak high-energyactivity of this object. In spite of the
fact that stellar winds and hot gas probably provide enough material for a moderate/low level of accretion,
the total bolometric luminosity of the source amounts to less than 10−6of the estimated accretion power
(Melia & Falcke 2001, Goldwurm 2001). This motivated the development of several black hole accretion
flow models with low radiative efficiency, some of which have also been applied to binary systems, low
luminosity nuclei of external galaxies and low luminosity active galactic nuclei. These models include
sphericalBondiaccretionin conditionsof magneticfieldsub-equipartitionwitha verysmall Kepleriandisk
located within the inner 50 Schwarzschild radii (RS), large hot two-temperatureaccretion disks dominated
by advection (ADAF) or non-thermal emission from the base of a jet of relativistic electrons and pairs,
and some other variants or combination of the above models. However any such model still predicts some
level of X-ray emission from Sgr A∗and determining the properties of such emission would constrain the
theories of accretion and outflows in the massive black holes and in general in compact objects.
The 20 years search for high energy emission from Sgr A∗(Watson et al. 1981, Predehl & Tr¨ umper
1994, Goldwurm et al. 1994) has recently come to a turning point with the remarkable observations made
∗Corresponding author: e-mail: firstname.lastname@example.org, Phone: +330169082792, Fax: +330169086577
c ? 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim0004-6337/03/S101-0003$ 17.50+.50/0
8 A. Goldwurm et al.: Sgr A∗X-Ray flare seen with XMM-Newton
Spectral Fit to X-ray Emission from within 10′′from Sgr A∗during the Flare
Power-law Model No Dust ScatteringDust Scattering
Norm MOS [10−4ph cm−2s−1keV−1] 1.3+2.0
Norm PN [10−4ph cm−2s−1keV−1] 0.3+0.6
Scattering computed for fixed value of AV = 30.
Normalization is the flux density at 1 keV.
Errors are at 68.3% confidence interval for 1 interesting parameter.
0.98 (20)0.95 (20)
Fig. 3 Count spectra from MOS (black data point set) and PN (red data point set) data, extracted from a region of 10′′
radius around Sgr A∗during the flare after subtraction of the non flaring spectra. The spectra are compared to the best
fit model of an absorbed power law without dust scattering (see Table 1)