NuSTAR and XMM–Newton observations of the Arches cluster in 2015: fading hard X-ray emission from the molecular cloud

Abstract

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 Kα 6.7 keV line emission and a nearby molecular cloud, characterized by an extended non-thermal hard X-ray continuum and fluorescent Fe Kα 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 Kα 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 .

Investigating the origin of the faint non-thermal emission of the Arches cluster using the 2015–2016NuSTARandXMM–NewtonX-ray observations

“…The N H value was determined as (9.3 ± 0.5) × 10 22 cm −2 , which is consistent with N H = (9.5±0.3)×10 22 cm −2 measured by Tatischeff et al (2012). The joint fit gives a solid measurement of the powerlaw Γ = 2.21 ± 0.15 up to 30 − 40 keV, which is in agreement within 1σ uncertainty with Γ = 2.03 ± 0.16 measured by Krivonos et al (2017).…”

“…2. To fit the spectrum, we applied the same spectral model wabs × (APEC + Gaussian + powerlaw) used in Krivonos et al (2017), which contains the thermal emission of the stellar cluster described by the collisionally ionized plasma emission model (APEC ) and the non-thermal emission of the molecular cloud in the form of power-law and Gaussian 6.4 keV line. The thermal emission is characterized by kT, Z/Z ⊙ , and I kT parameters describing, respectively, the temperature in keV, the metallicity relative to the solar one, and the normalization in units of 10 −18 ∫ n e n H dV/(4πD 2 ), where n e and n H are the electron and proton number densities in units of cm −3 , and D is the cluster distance in cm.…”

“…To extend a spectral coverage to the low energies, and to determine the total line-of-sight absorption column density N H , we added spectrum based on XMM-Newton observation carried out on 27 th September 2015 (Table 1), extracted from the same 50 ′′ circular region (Krivonos et al 2017 Arches cluster CXOGCS J 174555.8-284914 30'' 10-20 keV The combined 2015 XMM-Newton and 2015-2016 NuSTAR broad-band spectrum of the Arches cluster region is shown in Fig. 6.…”

“…To describe the Arches non-thermal continuum in the combined 2015 XMM-Newton and NuSTAR spectrum, additionally to a simple power-law model, Krivonos et al (2017) also applied CR-induced emission model developed by Tatischeff et al (2012) and self-consistent X-ray reflection model REFLIONX (Ross & Fabian 2005). We checked that adding 2016 NuSTAR data set to the joint 2015 XMM-Newton and NuSTAR fit allows to better constrain the parameters of these models, however, the improvements are still not enough to significantly confirm or rule out either of them.…”

“…The broad-band NuSTAR spectral analysis of the Arches non-thermal X-ray emission has been previously performed in a wide circular region covering the stellar cluster itself and surrounding molecular cloud (see Sections 3 and 4, Krivonos et al 2014Krivonos et al , 2017. Other studies, mainly based on XMM-Newton observations, used an elliptical region excluding the stellar cluster (see ellipse parameters in Tatischeff et al 2012;Clavel et al 2014;Krivonos et al 2017). Thus, the spectral extraction region often included the whole Arches non-thermal emission, which was characterized by regular and extended shape, traced by iron 6.4 keV line flux (Tatischeff et al 2012).…”