Context. The spatial distribution and variability of Fe-Kα emission from molecular clouds in the Galactic centre region may provide an important key to the understanding of the recent history of Sgr A*. A very plausible interpretation is that this variability represents an echo in the reflected radiation from the clouds of a past episode of high activity in Sgr A*. Aims. We examine the temporal and spectral properties of nine Fe-Kα bright molecular clouds (three of which newly studied) within about 30 pc of Sgr A*, in order to understand and constrain the primary energising source of the Fe fluorescence. Methods. We collected all the archival XMM-Newton observations targeted at Sgr A*. The variability of the Fe-Kα line at 6.4-keV in specific cloud regions was investigated by spectrally fitting the data derived from the EPIC MOS cameras, after subtracting a modelled background. We have also studied the reflection imprints in time-averaged pn-spectra of each cloud. This involved measuring the equivalent width (EW) of the 6.4 keV line with respect to the underlying scattered continuum and the optical depth of the Fe-K absorption edge at 7.1 keV, superimposed on this same continuum. Finally we stacked the MOS spectra from two extended regions in order to quantify the east-west asymmetry apparent in the low-surface brightness diffuse Fe-Kα line emission. Results. Significant Fe-Kα variability was detected, with a spatial and temporal pattern consistent with that reported in previous studies. The main breakthrough that sets our paper apart from earlier contributions on this topic is the direct measurement of the column density and the Fe abundance of the MCs in our sample. All the spectra were characterised by a high EW of the Fe-Kα line and the presence of absorption at the Fe-K edge, both of which serve as tracers of X-ray illumination. We used the EW measurements to infer the average Fe abundance within the clouds to be 1.6 ± 0.1 times solar. The cloud column densities derived from the spectral analysis were typically of the order of 10 23 cm −2 , which is significantly higher than previous estimates. This in turn has a significant impact on the inferred geometry and time delays within the cloud system. The measured cloud parameters were used to set constraints on the past activity of Sgr A* and to investigate whether a contribution to the Fe fluorescence by cosmic-ray bombardment is plausible. Conclusions. Past X-ray activity of Sgr A* is the most likely source of ionisation within the molecular clouds in the innermost 30 pc of the Galaxy. In this scenario, the X-ray luminosity required to excite these reflection nebulae is of the order of 10 37 -10 38 erg s −1 , significantly lower than that estimated for the Sgr B2 molecular cloud. Moreover, the inferred Sgr A* lightcurve over the past 150 years shows a long-term downwards trend punctuated by occasional counter-trend brightening episodes of at least 5 years duration. Finally, we found that a contribution to the Fe fluorescence by X-ray transient binaries...

Aims. The radiative counterpart of the supermassive black hole at the Galactic center (GC), Sgr A , is subject to frequent flares that are visible simultaneously in X-rays and the near-infrared (NIR). Often, enhanced radio variability from centimeter to sub-millimeter wavelengths is observed to follow these X-ray/NIR eruptions. We present here a multi-wavelength campaign carried out in April 2009, with the aim of characterizing this broadband flaring activity. Methods. Concurrent data from the XMM-Newton/EPIC (2-10 keV), VLT/NACO (2.1 μm, 3.8 μm), APEX/LABOCA (870 μm), and Fermi/LAT (0.1-200 GeV) instruments are employed to derive light curves and spectral energy distributions of new flares from Sgr A . Results. We detected two relatively bright NIR flares, both associated with weak X-ray activity, one of which was followed by a strong sub-mm outburst ∼200 min later. Photometric spectral information on a NIR flare was obtained for the first time with NACO, giving a power-law photon index α = −0.4 ± 0.3 (F ν ∝ ν α ). The first attempt to detect flaring activity from the Fermi GC source 1FGL J1745.6-2900 is also reported. We model NIR, X-ray, and sub-mm flares in the context of non-thermal emission processes. We find that the simplest scenario involving a single expanding plasmoid releasing synchrotron NIR/sub-mm and synchrotron selfCompton X-ray radiation is inadequate to reproduce the data, but we offer suggestions to reconcile the basic elements of the theory and the observations.

Context. Bright Fe-K α line emission at 6.4 keV is a unique characteristic of some of the dense molecular complexes present in the Galactic center region. Whether this X-ray fluorescence is due largely to the irradiation of the clouds by X-ray photons or is, at least in part, the result of cosmic-ray particle bombardment, remains an interesting open question. Aims. We present the results of XMM-Newton observations performed over the last eight years of the region surrounding the Arches cluster in the Galactic center. We study the spatial distribution and temporal behaviour of the Fe-K α emission with the objective of identifying the likely source of the excitation. Methods. We have constructed an Fe-K α fluence map in a narrow energy band of width 128 eV centered on 6.4 keV. We use this to localize the brightest fluorescence features in the vicinity of the Arches cluster. We have investigated the variability of the 6.4-keV line emission of several clouds through spectral fitting of the EPIC MOS data with the use of a modelled background, which avoids many of the systematics inherent in local background subtraction. We also employ spectral stacking of both EPIC PN and MOS data to search for evidence of an Fe-K edge feature imprinted on the underlying X-ray continuum.Results. The lightcurves of the Fe-K α line emission from three bright molecular knots close to the Arches cluster were found to be constant over the 8-year observation window. However, West of the cluster, we found a bright cloud which shows the fastest Fe-K α variability yet seen in a molecular cloud in the Galactic center region. The time averaged spectra of the molecular clouds revealed no convincing evidence of the 7.1-keV edge feature, albeit with only weak constraints. The EW of the 6.4-keV line emitted by the clouds near to the cluster was found to be ∼1.0 keV. Conclusions. The observed Fe-K α line flux and the high value of the EW suggest an origin of the fluorescence in the photoionization of the MCs by X-ray photons, although excitation by cosmic-ray particles is not specifically excluded. For the three clouds nearest to the Arches cluster, the identification of the source of these X-rays as an earlier outburst on Sgr A* is at best tentative, although not entirely ruled out by the observations. On the other hand, the hardness of the nonthermal component associated with the 6.4-keV line emission might be best explained in terms of the bombardment of the clouds by cosmic-ray particles emanating from the Arches cluster itself. The relatively short-timescale variability seen in the 6.4-keV line emission from the cloud to the west of the cluster is most likely the result of its X-ray illumination by a nearby transient X-ray source.

Context. We present a study of the Arches cluster based on XMM-Newton observations performed over the past 8 years. Unexpectedly, we find that the X-ray emission associated with the cluster experienced a marked brightening in March/April 2007. Aims. We investigate the origin of both the X-ray continuum emission emanating from the star cluster and the flare. Methods. To study the time variability of the total X-ray flux, we stacked the PN and MOS data of observations performed within a time interval of a few days leading to the detection of the flaring episode. We then constructed two spectral datasets, one corresponding to the flare interval (March/April 2007) and another to the normal quiescent state of the source. Results. The X-ray light curve of the Arches cluster shows, with high significance (8.6σ), a 70% increase in the X-ray emission in the March/April 2007 timeframe followed by a decline over the following year to the pre-flare level; the short-term duration of the flare is constrained to be longer than four days. The temperature and the line-of-sight column density inferred from the flare spectrum do not differ from those measured in the normal activity state of the cluster, suggesting that the flux enhancement is thermal in origin. Conclusions. We attribute the X-ray variability to in situ stellar activity: early-type stars may be responsible for the flare via wind collisions, whereas late-type stars may contribute by means of magnetic reconnection. These two possibilities are discussed.

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