No abstract

High-resolution X-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified » E 3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. The emission line expected for Perseus based on the XMM-Newton signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the Hitomi data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported high flux level. Taking into account the XMM measurement uncertainties for this region, the inconsistency with Hitomi is at a 99% significance for a broad dark matter line and at 99.7% for a narrow line from the gas. We do not find anomalously high fluxes of the nearby faint K line or the Ar satellite line that were proposed as explanations for the earlier 3.5 keV detections. We do find a hint of a broad excess near the energies of high-n transitions of S XVI (  E 3.44 keV rest-frame)-a possible signature of charge exchange in the molecular nebula and another proposed explanation for the unidentified line. While its energy is consistent with XMM pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment.

We analyze two XMM-Newton observations toward the high-density, high-latitude, neutral hydrogen cloud MBM 20 and a nearby low-density region that we call the Eridanus hole. MBM 20 lies at a distance between 100 and 200 pc from the Sun, and its density is sufficient to shield about 75% of the foreground emission in the 3 4 keV energy band. The combination of the two observations makes it possible to separate the foreground component, due to the Local Bubble and, possibly, charge exchange within the solar system, from the background component, due primarily to the Galactic halo and unidentified point sources. The two observations are in good agreement with each other and with ROSAT observations of the same part of the sky; the O vii and O viii intensities are 3.89 AE 0.56 and 0.68 AE 0.24 photons cm À2 s À1 sr À1 for MBM 20, respectively, and 7.26 AE 0.34 and 1.63 AE 0.17 photons cm À2 s À1 sr À1 for the Eridanus hole. The spectra agree with a simple three-component model: one unabsorbed and one absorbed plasma component, and a power law, due to unresolved distant point sources. Assuming that the two plasma components are in thermal equilibrium, we obtain a temperature of 0.096 keV for the foreground component and 0.197 keV for the background one. Assuming the foreground component is due solely to Local Bubble emission, we obtain lower and upper limits for the plasma density of 0.0079 and 0.0095 cm À3 and limits of 16,200 and 19,500 cm À3 K for the plasma pressure, in good agreement with theoretical predictions. Similarly, assuming that the absorbed plasma component is due to Galactic halo emission, we obtain a plasma density ranging from 0.0009 to 0.0016 cm À3 and a pressure between 3.8 ; 10 3 and 6.7 ; 10 3 cm À3 K.

We assess the possibility of detecting and characterizing the physical state of the missing baryons at low redshift by analyzing the X-ray absorption spectra of the gamma-ray burst (GRB) afterglows, measured by a microcalorimeter-based detector with 3 eV resolution and 1000 cm 2 effective area and capable of fast repointing, similar to that on board of the recently proposed X-ray satellites EDGE and XENIA. For this purpose we have analyzed mock absorption spectra extracted from different hydrodynamical simulations used to model the properties of the warm hot intergalactic medium (WHIM). These models predict the correct abundance of O vi absorption lines observed in UV and satisfy current X-ray constraints. According to these models space missions such as EDGE and XENIA should be able to detect ∼60 WHIM absorbers per year through the O vii line. About 45% of these have at least two more detectable lines in addition to O vii that can be used to determine the density and the temperature of the gas. Systematic errors in the estimates of the gas density and temperature can be corrected for in a robust, largely model-independent fashion. The analysis of the GRB absorption spectra collected in three years would also allow to measure the cosmic mass density of the WHIM with ∼15% accuracy, although this estimate depends on the WHIM model. Our results suggest that GRBs represent a valid, if not preferable, alternative to active galactic nuclei to study the WHIM in absorption. The analysis of the absorption spectra nicely complements the study of the WHIM in emission that the spectrometer proposed for EDGE and XENIA would be able to carry out thanks to its high sensitivity and large field of view.

DXL (Diffuse X-rays from the Local Galaxy) is a sounding rocket mission designed to quantify and characterize the contribution of Solar Wind Charge eXchange (SWCX) to the Diffuse X-ray Background and study the properties of the Local Hot Bubble (LHB). Based on the results from the DXL mission, we quantified and removed the contribution of SWCX to the diffuse X-ray background measured by the ROSAT All Sky Survey (RASS). The "cleaned" maps were used to investigate the physical properties of the LHB. Assuming thermal ionization equilibrium, we measured a highly uniform temperature distributed around kT =0.097 keV±0.013 keV (FWHM)±0.006 keV (systematic). We also generated a thermal emission measure map and used it to characterize the three-dimensional (3D) structure of the LHB which we found to be in good agreement with the structure of the local cavity measured from dust and gas.