Discovery of X-ray flaring activity in the Arches cluster

Capelli, Renzo; Warwick, R. S.; Cappelluti, N.; Gillessen, S.; Predehl, P.; Porquet, D.; Czesla, S.

doi:10.1051/0004-6361/201015758

Cited by 17 publications

(36 citation statements)

References 25 publications

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“…Regions N and S have rather similar spectra, in which contamination by residual thermal emission (not removed by the background subtraction) is clearly present in the form of the helium-like Fe line at 6.7 keV and other helium-like lines of other elements (S, Ar, Ca) at lower energies. In these regions, close to the AC core, we derive a temperature of the warm plasma consistent with that previously measured in the core Tsujimoto et al 2007;Capelli et al 2011). The X-ray spectra of both complexes exhibit high line-of-sight absorption and a relatively high EW for the neutral iron Fe K α line (∼1.0 keV).…”

Section: Fe-k α Line Flux and Variability -Background Modelingsupporting

confidence: 82%

“…3.3 from the analysis of the stacked spectra. These temperatures are marginally lower than the value of 1.7 keV measured for the core of the AC (Capelli et al 2011), hinting at a decrease in the warm plasma temperature as one moves away from the cluster until an "ambient" medium with kT ≈ 1 keV is reached. Because of the reduced signal to noise ratio, all the spectra of the DX region were fitted with the warm plasma temperature fixed at the ambient value, (i.e.…”

Section: Fe-k α Line Flux and Variability -Background Modelingmentioning

confidence: 61%

“…As shown first by Yusef-Zadeh et al (2002b) and later by Law & Yusef-Zadeh (2004), the southern component of the cluster is populated by strong stellar-wind sources, which are also detectable at radio wavelengths (Lang et al 2001). These massive early-type stars could also be present in binary systems, and be the origin of the X-ray flaring activity A38, page 10 of 12 recently discovered by Capelli et al (2011). Massive interacting binaries are efficient in producing X-ray emission in the shock fronts within the winds and also in accelerating CRs.…”

Section: The Cr Particle Bombardment Hypothesismentioning

confidence: 99%

“…A threshold was then set so as to cut all the peaks in the background rate, thereby defining the good time intervals (GTIs) for the time filtering of the observation. Since the selected observations were performed in varying conditions of internal/particle background and orbital phase, we investigated all the lightcurves independently and selected a threshold count rate separately for each observation/camera combination (see also Capelli et al 2011). The specifics of the XMM-Newton observations employed in the present work are reported in Table 1.…”

Section: Observations and Data Reductionmentioning

confidence: 99%

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Fe Kαline emission from the Arches cluster region – evidence for ongoing particle bombardment?

Capelli¹,

Warwick²,

Porquet³

et al. 2011

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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.

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Section: Fe-k α Line Flux and Variability -Background Modelingsupporting

confidence: 82%

Section: Fe-k α Line Flux and Variability -Background Modelingmentioning

confidence: 61%

Section: The Cr Particle Bombardment Hypothesismentioning

confidence: 99%

Section: Observations and Data Reductionmentioning

confidence: 99%

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Fe Kαline emission from the Arches cluster region – evidence for ongoing particle bombardment?

Capelli¹,

Warwick²,

Porquet³

et al. 2011

A&A

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“…11, the central coordinates and sizes of which can be found in the last two rows of Table 2. In the case of the EDE region, all the sky regions coincident with the MCs previously studied have been excised, as have the 6.4-keV bright knots in the Arches cluster region (see Capelli et al 2011b), and the Arches cluster itself (see Capelli et al 2011a).…”

Section: Methodsmentioning

confidence: 99%

The X-ray lightcurve of Sagittarius A* over the past 150 years inferred from Fe-Kαline reverberation in Galactic centre molecular clouds

Capelli

Warwick

Porquet

et al. 2012

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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...

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