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Register a new userA joint XMM-NuSTAR observation of the galaxy cluster Abell 523: constraints on Inverse Compton emission
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We present the results of a joint XMM-Newton and NuSTAR observation (200 ks) of the galaxy cluster Abell 523 at $z=0.104$. The peculiar morphology of the cluster radio halo and its outlier position in the radio power P(1.4 GHz) - X-ray luminosity plane make it an ideal candidate for the study of radio-X-ray correlations and for the search of inverse Compton (IC) emission. We constructed thermodynamic maps derived from the XMM observations to describe in detail the physical and dynamical state of the ICM. We performed a point-to-point comparison in terms of surface brightness between the X-ray and radio emissions, to quantify their morphological discrepancies. Making use of NuSTARs hard X-ray focusing capability, we looked for IC emission both globally and locally, after modeling the purely thermal component with a multi-temperature description. The thermodynamic maps suggest the presence of a secondary merging process that could be responsible for the peculiar radio halo morphology. This hypothesis is supported by the comparison between the X-ray and radio surface brightnesses, which shows a broad intrinsic scatter and a series of outliers. The global NuSTAR spectrum can be explained by purely thermal gas emission, and there is no convincing evidence that an IC component is needed. The $3sigma$ upper limit on the IC flux in the 20-80 keV band is in the range $left[2.2 - 4.0right] times 10^{-13} , mathrm{erg} , mathrm{s}^{-1} , mathrm{cm}^{-2}$, implying a lower limit on the magnetic field strength in the range $B > [0.23 - 0.31] , mu G$. Locally, we looked for IC emission in the central region of the cluster radio halo finding a $3sigma$ upper limit on the 20-80 keV non-thermal flux of $3.17 times 10^{-14} , mathrm{erg} , mathrm{s}^{-1} , mathrm{cm}^{-2}$, corresponding to a lower limit on the magnetic field strength of $B gtrsim 0.81 , mu G$.
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The search for diffuse non-thermal inverse Compton (IC) emission from galaxy clusters at hard X-ray energies has been undertaken with many instruments, with most detections being either of low significance or controversial. Background and contamination uncertainties present in the data of non-focusing observatories result in lower sensitivity to IC emission and a greater chance of false detection. We present 266ks NuSTAR observations of the Bullet cluster, detected from 3-30 keV. NuSTARs unprecedented hard X-ray focusing capability largely eliminates confusion between diffuse IC and point sources; however, at the highest energies the background still dominates and must be well understood. To this end, we have developed a complete background model constructed of physically inspired components constrained by extragalactic survey field observations, the specific parameters of which are derived locally from data in non-source regions of target observations. Applying the background model to the Bullet cluster data, we find that the spectrum is well - but not perfectly - described as an isothermal plasma with kT=14.2+/-0.2 keV. To slightly improve the fit, a second temperature component is added, which appears to account for lower temperature emission from the cool core, pushing the primary component to kT~15.3 keV. We see no convincing need to invoke an IC component to describe the spectrum of the Bullet cluster, and instead argue that it is dominated at all energies by emission from purely thermal gas. The conservatively derived 90% upper limit on the IC flux of 1.1e-12 erg/s/cm^2 (50-100 keV), implying a lower limit on B>0.2{mu}G, is barely consistent with detected fluxes previously reported. In addition to discussing the possible origin of this discrepancy, we remark on the potential implications of this analysis for the prospects for detecting IC in galaxy clusters in the future.
Long-duration gamma-ray bursts (GRBs) originate from ultra-relativistic jets launched from the collapsing cores of dying massive stars. They are characterised by an initial phase of bright and highly variable radiation in the keV-MeV band that is likely produced within the jet and lasts from milliseconds to minutes, known as the prompt emission. Subsequently, the interaction of the jet with the external medium generates external shock waves, responsible for the afterglow emission, which lasts from days to months, and occurs over a broad energy range, from the radio to the GeV bands. The afterglow emission is generally well explained as synchrotron radiation by electrons accelerated at the external shock. Recently, an intense, long-lasting emission between 0.2 and 1 TeV was observed from the GRB 190114C. Here we present the results of our multi-frequency observational campaign of GRB~190114C, and study the evolution in time of the GRB emission across 17 orders of magnitude in energy, from $5times10^{-6}$ up to $10^{12}$,eV. We find that the broadband spectral energy distribution is double-peaked, with the TeV emission constituting a distinct spectral component that has power comparable to the synchrotron component. This component is associated with the afterglow, and is satisfactorily explained by inverse Compton upscattering of synchrotron photons by high-energy electrons. We find that the conditions required to account for the observed TeV component are not atypical, supporting the possibility that inverse Compton emission is commonly produced in GRBs.
Abell~1142 is a low-mass galaxy cluster at low redshift containing two comparable Brightest Cluster Galaxies (BCG) resembling a scaled-down version of the Coma Cluster. Our Chandra analysis reveals an X-ray emission peak, roughly 100 kpc away from either BCG, which we identify as the cluster center. The emission center manifests itself as a second beta-model surface brightness component distinct from that of the cluster on larger scales. The center is also substantially cooler and more metal rich than the surrounding intracluster medium (ICM), which makes Abell 1142 appear to be a cool core cluster. The redshift distribution of its member galaxies indicates that Abell 1142 may contain two subclusters with each containing one BCG. The BCGs are merging at a relative velocity of ~1200 km/s. This ongoing merger may have shock-heated the ICM from ~ 2 keV to above 3 keV, which would explain the anomalous L_X--T_X scaling relation for this system. This merger may have displaced the metal-enriched cool core of either of the subclusters from the BCG. The southern BCG consists of three individual galaxies residing within a radius of 5 kpc in projection. These galaxies should rapidly sink into the subcluster center due to the dynamical friction of a cuspy cold dark matter halo.
We present the joint Chandra, XMM-Newton and NuSTAR analysis of two nearby Seyfert galaxies, NGC 3081 and ESO 565-G019. These are the only two having Chandra data in a larger sample of ten low redshift ($z le 0.05$), candidates Compton-thick Active Galactic Nuclei (AGN) selected in the 15-150 keV band with Swift-BAT that were still lacking NuSTAR data. Our spectral analysis, performed using physically-motivated models, provides an estimate of both the line-of-sight (l.o.s.) and average (N$_{H,S}$) column densities of the two torii. NGC 3081 has a Compton-thin l.o.s. column density N$_{H,z}$=[0.58-0.62] $times 10^{24}$cm$^{-2}$, but the N$_{H,S}$, beyond the Compton-thick threshold (N$_{H,S}$=[1.41-1.78] $times 10^{24}$cm$^{-2}$), suggests a patchy scenario for the distribution of the circumnuclear matter. ESO 565-G019 has both Compton-thick l.o.s. and N$_{H,S}$ column densities (N$_{H,z}>$2.31 $times 10^{24}$cm$^{-2}$ and N$_{H,S} >$2.57 $times 10^{24}$cm$^{-2}$, respectively). The use of physically-motivated models, coupled with the broad energy range covered by the data (0.6-70 keV and 0.6-40 keV, for NGC 3081 and ESO 565-G019, respectively) allows us to constrain the covering factor of the obscuring material, which is C$_{TOR}$=[0.63-0.82] for NGC 3081, and C$_{TOR}$=[0.39-0.65] for ESO 565-G019.
Abell 548W, one of the galaxy clusters located in the Abell 548 region, has about an order of magnitude lower X-ray luminosity compared to ordinal clusters in view of the well known intracluster medium (ICM) temperature vs X-ray luminosity (kT-L_X) relation. The cluster hosts a pair of diffuse radio sources to the north west and north, both about 10 apart from the cluster center. They are candidate radio relics, frequently associated with merging clusters. A Suzaku deep observation with exposure of 84.4 ks was performed to search signatures for merging in this cluster. The XIS detectors successfully detected the ICM emission out to 16 from the cluster center. The temperature is ~3.6 keV around its center, and ~2 keV at the outermost regions. The hot region (~6 keV) aside the relic candidates shifted to the cluster center reported by XMM-Newton was not seen in the Suzaku data, although its temperature of 3.6 keV itself is higher than the average temperature of 2.5 keV around the radio sources. In addition, a signature of a cool (kT ~0.9 keV) component was found around the north west source. A marginal temperature jump at its outer-edge was also found, consistent with the canonical idea of shock acceleration origin of the radio relics. The cluster has among the highest central entropy of ~400 keV cm^2 and is one of the so-called low surface brightness clusters. Taking into account the fact that its shape itself is relatively circular and smooth and also its temperature structure is nearly flat, possible scenarios for merging is discussed.
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