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Register a new userInvestigating the origin of the faint non-thermal emission of the Arches cluster using the 2015-2016 NuSTAR and XMM-Newton X-ray observations
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Ekaterina Kuznetsova
Publication date
2019
fields
Physics
and research's language is
English
Authors
Ekaterina Kuznetsova
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Recent NuSTAR and XMM-Newton observations of the molecular cloud around the Arches stellar cluster demonstrate a dramatic change both in morphology and intensity of its non-thermal X-ray emission, similar to that observed in many molecular clouds of the Central Molecular Zone at the Galactic Center. These variations trace the propagation of illuminating fronts, presumably induced by past flaring activities of Sgr A$^{star}$. In this paper we present results of a long NuSTAR observation of the Arches complex in 2016, taken a year after the previous XMM+NuSTAR observations which revealed a strong decline in the cloud emission. The 2016 NuSTAR observation shows that both the non-thermal continuum emission and the Fe K$_{alpha}$ 6.4~keV line flux are consistent with the level measured in 2015. No significant variation has been detected in both spectral shape and Fe K$_{alpha}$ equivalent width EW$_{rm 6.4 keV}$, which may be interpreted as the intensity of the Arches non-thermal emission reaching its stationary level. At the same time, the measured 2016 non-thermal flux is not formally in disagreement with the declining trend observed in 2007-2015. Thus, we cannot assess whether the non-thermal emission has reached a stationary level in 2016, and new observations, separated by a longer time period, are needed to draw stringent conclusions. Detailed spectral analysis of three bright clumps of the Arches molecular cloud performed for the first time showed different EW$_{rm 6.4 keV}$ and absorption. This is a strong hint that the X-ray emission from the molecular cloud is a mix of two components with different origins.
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We present results of long NuSTAR (200 ks) and XMM-Newton (100 ks) observations of the Arches stellar cluster, a source of bright thermal (kT~2 keV) X-rays with prominent Fe XXV K_alpha 6.7 keV line emission and a nearby molecular cloud, characterized by an extended non-thermal hard X-ray continuum and fluorescent Fe K_alpha 6.4 keV line of a neutral or low ionization state material around the cluster. Our analysis demonstrates that the non-thermal emission of the Arches cloud underwent a dramatic change, with its homogeneous morphology, traced by fluorescent Fe K_alpha line emission, vanishing after 2012, revealing three bright clumps. The declining trend of the cloud emission, if linearly fitted, is consistent with half-life decay time of ~8 years. Such strong variations have been observed in several other molecular clouds in the Galactic Centre, including the giant molecular cloud Sgr B2, and point toward a similar propagation of illuminating fronts, presumably induced by the past flaring activity of Sgr A*.
The origin of the iron fluorescent line at 6.4 keV from an extended region surrounding the Arches cluster is debated and the non-variability of this emission up to 2009 has favored the low-energy cosmic-ray origin over a possible irradiation by hard X-rays. By probing the variability of the Arches cloud non-thermal emission in the most recent years, including a deep observation in 2012, we intend to discriminate between the two competing scenarios. We perform a spectral fit of XMM-Newton observations collected from 2000 to 2013 in order to build the Arches cloud lightcurve corresponding to both the neutral Fe Kalpha line and the X-ray continuum emissions. We reveal a 30% flux drop in 2012, detected with more than 4 sigma significance for both components. This implies that a large fraction of the studied non-thermal emission is due to the reflection of an X-ray transient source.
We present simultaneous XMM-Newton and NuSTAR observations spanning 3-78 keV of the nearest radio galaxy, Centaurus A (Cen A). The accretion geometry around the central engine in Cen A is still debated, and we investigate possible configurations using detailed X-ray spectral modeling. NuSTAR imaged the central region of Cen A with sub-arcminute resolution at X-ray energies above 10 keV for the first time, but finds no evidence for an extended source or other off-nuclear point-sources. The XMM-Newton and NuSTAR spectra agree well and can be described with an absorbed power-law with a photon index {Gamma} = 1.815 +/- 0.005 and a fluorescent Fe K{alpha} line in good agreement with literature values. The spectrum does not require a high-energy exponential rollover, with a constraint of E_fold > 1 MeV. A thermal Comptonization continuum describes the data well, with parameters that agree with values measured by INTEGRAL, in particular an electron temperature kTe between ~100-300 keV, seed photon input temperatures between 5-50 eV. We do not find evidence for reflection or a broad iron line and put stringent upper limits of R < 0.01 on the reflection fraction and accretion disk illumination. We use archival Chandra data to estimate the contribution from diffuse emission, extra-nuclear point-sources, and the outer X-ray jet to the observed NuSTAR and XMM-Newton X-ray spectra and find the contribution to be negligible. We discuss different scenarios for the physical origin of the observed hard X-ray spectrum, and conclude that the inner disk is replaced by an advection-dominated accretion flow or that the X-rays are dominated by synchrotron self-Compton emission from the inner regions of the radio jet or a combination thereof.
We present a high-quality hard X-ray spectrum of the ultraluminous X-ray source (ULX) NGC 5643 X-1 measured with NuSTAR in May-June 2014. We have obtained this spectrum by carefully separating the signals from the ULX and from the active nucleus of its host galaxy NGC 5643 located 0.8 arcmin away. Together with long XMM-Newton observations performed in July 2009 and August 2014, the NuSTAR data confidently reveal a high-energy cutoff in the spectrum of NGC 5643 X-1 above ~10 keV, which is a characteristic signature of ULXs. The NuSTAR and XMM-Newton data are consistent with the source having a constant luminosity ~1.5E40 erg/s (0.2-12 keV) in all but the latest observation (August 2014) when it brightened to ~3E40 erg/s. This increase is associated with the dominant, hard spectral component (presumably collimated emission from the inner regions of a supercritical accretion disc), while an additional, soft component (with a temperature ~0.3 keV if described by multicolor disk emission), possibly associated with a massive wind outflowing from the disk, is also evident in the spectrum but does not exhibit significant variability.
We present the first broadband 0.3-25.0 kev X-ray observations of the bright ultraluminous X-ray source (ULX) Holmberg II X-1, performed by NuSTAR, XMM-Newton and Suzaku in September 2013. The NuSTAR data provide the first observations of Holmberg II X-1 above 10 keV, and reveal a very steep high-energy spectrum, similar to other ULXs observed by NuSTAR to date. These observations further demonstrate that ULXs exhibit spectral states that are not typically seen in Galactic black hole binaries. Comparison with other sources implies that Holmberg II X-1 accretes at a high fraction of its Eddington accretion rate, and possibly exceeds it. The soft X-ray spectrum (E<10 keV) appears to be dominated by two blackbody-like emission components, the hotter of which may be associated with an accretion disk. However, all simple disk models under-predict the NuSTAR data above ~10 keV and require an additional emission component at the highest energies probed, implying the NuSTAR data does not fall away with a Wien spectrum. We investigate physical origins for such an additional high-energy emission component, and favor a scenario in which the excess arises from Compton scattering in a hot corona of electrons with some properties similar to the very-high state seen in Galactic binaries. The observed broadband 0.3-25.0 keV luminosity inferred from these epochs is Lx = (8.1+/-0.1)e39 erg/s, typical for Holmberg II X-1, with the majority of the flux (~90%) emitted below 10 keV.
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