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Register a new userVariation of the X-ray non-thermal emission in the Arches cloud
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Authors
Maica Clavel
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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.
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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.
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*.
We report on a detailed spectral characterization of the non-thermal X-ray emission for a large sample of gamma-ray pulsars in the second Fermi-LAT catalogue. We outline the criteria adopted for the selection of our sample, its completeness, and critically describe different approaches to estimate the spectral shape and flux of pulsars. We perform a systematic modelling of the pulsars X-ray spectra using archival observations with XMM-Newton, Chandra, and NuSTAR and extract the corresponding non-thermal X-ray spectral distributions. This set of data is made available online and is useful to confront with predictions of theoretical models.
We report the analysis result of UV/X-ray emission from AR~Scorpii, which is an intermediate polar (IP) composed of a magnetic white dwarf and a M-type star, with the XMM-Newton data. The X-ray/UV emission clearly shows a large variation over the orbit, and their intensity maximum (or minimum) is located at the superior conjunction (or inferior conjunction) of the M-type star orbit. The hardness ratio of the X-ray emission shows a small variation over the orbital phase, and shows no indication of the absorption by an accretion column. These properties are naturally explained by the emission from the M-type star surface rather than from the accretion column on the WDs star similar to the usual IPs. Beside, the observed X-ray emission also modulates with WDs spin with a pulse fraction of $sim 14%$. The peak position is aligned in the optical/UV/X-ray band. This supports the hypothesis that the electrons in AR~Scorpii are accelerated to a relativistic speed, and emit non-thermal photons via the synchrotron radiation. In the X-ray bands, the evidence of the power-law spectrum is found in the pulsed component, although the observed emission is dominated by the optically thin thermal plasma emissions with several different temperatures. It is considered that the magnetic dissipation/reconnection process on the M-type star surface heats up the plasma to a temperature of several keV, and also accelerates the electrons to the relativistic speed. The relativistic electrons are trapped in the WDs closed magnetic field lines by the magnetic mirror effect. In this model, the observed pulsed component is explained by the emissions from the first magnetic mirror point.
We present a spectral analysis of the lobes and X-ray jets of Cygnus A, using more than 2 Ms of $textit{Chandra}$ observations. The X-ray jets are misaligned with the radio jets and significantly wider. We detect non-thermal emission components in both lobes and jets. For the eastern lobe and jet, we find 1 keV flux densities of $71_{-10}^{+10}$ nJy and $24_{-4}^{+4}$ nJy, and photon indices of $1.72_{-0.03}^{+0.03}$ and $1.64_{-0.04}^{+0.04}$ respectively. For the western lobe and jet, we find flux densities of $50_{-13}^{+12}$ nJy and $13_{-5}^{+5}$ nJy, and photon indices of $1.97_{-0.10}^{+0.23}$ and $1.86_{-0.12}^{+0.18}$ respectively. Using these results, we modeled the electron energy distributions of the lobes as broken power laws with age breaks. We find that a significant population of non-radiating particles is required to account for the total pressure of the eastern lobe. In the western lobe, no such population is required and the low energy cutoff to the electron distribution there needs to be raised to obtain pressures consistent with observations. This discrepancy is a consequence of the differing X-ray photon indices, which may indicate that the turnover in the inverse-Compton spectrum of the western lobe is at lower energies than in the eastern lobe. We modeled the emission from both jets as inverse-Compton emission. There is a narrow region of parameter space for which the X-ray jet can be a relic of an earlier active phase, although lack of knowledge about the jets electron distribution and particle content makes the modelling uncertain.
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