Equivalent width, shape and proper motion of the iron fluorescent line emission from the molecular clouds as an indicator of the illuminating source X-ray flux history

Observations of the diffuse emission in the 8--22 keV energy range, elongated parallel to the Galactic plane (Sunyaev et al. 1993) and detection of the strong 6.4 keV fluorescent line with $\sim$ 1 keV equivalent width from some giant molecular clouds (e.g. Sgr B2) in the Galactic Centre region (Koyama 1994) suggest that the neutral matter of these clouds is (or was) illuminated by powerful X-ray radiation, which gave rise to the reprocessed radiation. The source of this radiation remains unknown. Transient source close to the Sgr B2 cloud or short outburst of the X-ray emission from supermassive black hole at the Galactic Centre are the two prime candidates under consideration. We argue that new generation of X-ray telescopes combining very high sensitivity and excellent energy and angular resolutions would be able to discriminate between these two possibilities studying time dependent changes of the morphology of the surface brightness distribution, the equivalent width and the shape of the fluorescent line in the Sgr B2 and other molecular clouds in the region. We note also that detection of broad and complex structures near the 6.4 keV line in the spectra of distant AGNs, which are X-ray weak now, may prove the presence of violent activity of the central engines of these objects in the past. Accurate measurements of the line shape may provide an information on the time elapsed since the outburst. Proper motion (super or subluminal) of the fluorescent radiation wave front can give additional information on the location of the source. Observations of the described effects can provide unique information on the matter distribution inside Sgr B2 and other giant molecular clouds.Comment: 14 pages, 10 figures, accepted for publication in MNRA

(No) dimming of X-ray clusters beyond z~1 at fixed mass: crude redhshifts and masses from raw X-ray and SZ data

Scaling relations in the LCDM Cosmology predict that for a given mass the clusters formed at larger redshift are hotter, denser and therefore more luminous in X-rays than their local z~0 counterparts. This effect overturns the decrease in the observable X-ray flux so that it does not decrease at z > 1, similar to the SZ signal. Provided that scaling relations remain valid at larger redshifts, X-ray surveys will not miss massive clusters at any redshift, no matter how far they are. At the same time, the difference in scaling with mass and distance of the observable SZ and X-ray signals from galaxy clusters at redshifts $z\lesssim 2$ offers a possibility to crudely estimate the redshift and the mass of a cluster. This might be especially useful for preselection of massive high-redshift clusters and planning of optical follow-up for overlapping surveys in X-ray (e.g., by SRG/eRosita) and SZ (e.g. Planck, SPT and ACT).Comment: 7 pages, 5 figures, MNRAS accepte

Hard X-ray emission of the Earth's atmosphere: Monte Carlo simulations

We perform Monte Carlo simulations of cosmic ray-induced hard X-ray radiation from the Earth's atmosphere. We find that the shape of the spectrum emergent from the atmosphere in the energy range 25-300 keV is mainly determined by Compton scatterings and photoabsorption, and is almost insensitive to the incident cosmic-ray spectrum. We provide a fitting formula for the hard X-ray surface brightness of the atmosphere as would be measured by a satellite-born instrument, as a function of energy, solar modulation level, geomagnetic cutoff rigidity and zenith angle. A recent measurement by the INTEGRAL observatory of the atmospheric hard X-ray flux during the occultation of the cosmic X-ray background by the Earth agrees with our prediction within 10%. This suggests that Earth observations could be used for in-orbit calibration of future hard X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by cosmic rays in the crusts of the Moon, Mars and Mercury should be significantly different from that emitted by the Earth's atmosphere.Comment: 12 pages, 16 figures, MNRAS accepte

Cumulative hard X-ray spectrum of local AGN: a link to the cosmic X-ray background

We determine the cumulative spectral energy distribution (SED) of local AGN in the 3-300 keV band and compare it with the spectrum of the cosmic X-ray background (CXB) in order to test the widely accepted paradigm that the CXB is a superposition of AGN and to place constraints on AGN evolution. We performed a stacking analysis of the hard X-ray spectra of AGN detected in two recent all-sky surveys, performed by the IBIS/ISGRI instrument aboard INTEGRAL and by the PCA instrument aboard RXTE, taking into account the space densities of AGN with different luminosities and absorption column densities. We derived the collective SED of local AGN in the 3-300 keV energy band. Those AGN with luminosities below 10^43.5 erg/s (17-60 keV) provide the main contribution to the local volume hard X-ray emissivity, at least 5 times more than more luminous objects. The cumulative spectrum exhibits (although with marginal significance) a cutoff at energies above 100-200 keV and is consistent with the CXB spectrum if AGN evolve over cosmic time in such a way that the SED of their collective high-energy emission has a constant shape and the relative fraction of obscured AGN remains nearly constant, while the AGN luminosity density undergoes strong evolution between z~1 and z=0, a scenario broadly consistent with results from recent deep X-ray surveys. The first direct comparison between the collective hard X-ray SED of local AGN and the CXB spectrum demonstrates that the popular concept of the CXB being a superposition of AGN is generally correct. By repeating this test using improved AGN statistics from current and future hard X-ray surveys, it should be possible to tighten the constraints on the cosmic history of black hole growth.Comment: 12 pages, 9 figures. Revised version accepted for publication in A&

Can Sgr A* flares reveal the molecular gas density PDF?

Illumination of dense gas in the Central Molecular Zone (CMZ) by powerful X-ray flares from Sgr A* leads to prominent structures in the reflected emission that can be observed long after the end of the flare. By studying this emission we learn about past activity of the supermassive black hole in our Galactic Center and, at the same time, we obtain unique information on the structure of molecular clouds that is essentially impossible to get by other means. Here we discuss how X-ray data can improve our knowledge of both sides of the problem. Existing data already provide: i) an estimate of the flare age, ii) a model-independent lower limit on the luminosity of Sgr A* during the flare and iii) an estimate of the total emitted energy during Sgr A* flare. On the molecular clouds side, the data clearly show a voids-and-walls structure of the clouds and can provide an almost unbiased probe of the mass/density distribution of the molecular gas with the hydrogen column densities lower than few $10^{23}\;{\rm cm^{-2}}$. For instance, the probability distribution function of the gas density $PDF(\rho)$ can be measured this way. Future high energy resolution X-ray missions will provide the information on the gas velocities, allowing, for example a reconstruction of the velocity field structure functions and cross-matching the X-ray and molecular data based on positions and velocities.Comment: 13 pages, 7 figures; Accepted for publication in MNRA

Not that long time ago in the nearest galaxy: 3D slice of molecular gas revealed by a 110 years old flare of Sgr A*

A powerful outburst of X-ray radiation from the supermassive black hole Sgr A* at the center of the Milky Way is believed to be responsible for the illumination of molecular clouds in the central ~100 pc of the Galaxy (Sunyaev et al., 1993, Koyama et al., 1996). The reflected/reprocessed radiation comes to us with a delay corresponding to the light propagation time that depends on the 3D position of molecular clouds with respect to Sgr A*. We suggest a novel way of determining the age of the outburst and positions of the clouds by studying characteristic imprints left by the outburst in the spatial and time variations of the reflected emission. We estimated the age of the outburst that illuminates the Sgr A molecular complex to be ~110 yr. This estimate implies that we see the gas located ~10 pc further away from us than Sgr A*. If the Sgr B2 complex is also illuminated by the same outburst, then it is located ~130 pc closer than our Galactic Center. The outburst was short (less than a few years) and the total amount of emitted energy in X-rays is $\displaystyle \sim 10^{48}\rho_3^{-1}$ erg, where $\rho_3$ is the mean hydrogen density of the cloud complex in units of $10^3 {\rm cm^{-3}}$. Energetically, such fluence can be provided by a partial tidal disruption event or even by a capture of a planet. Further progress in more accurate positioning and timing of the outburst should be possible with future X-ray polarimetric observations and long-term systematic observations with Chandra and XMM-Newton. A few hundred-years long X-ray observations would provide a detailed 3D map of the gas density distribution in the central $\sim 100$ pc region.Comment: 10 pages, 7 figures, accepted for publication in MNRA

Polarization and long-term variability of Sgr A* X-ray echo

We use a model of the molecular gas distribution within ~100 pc from the center of the Milky Way (Kruijssen, Dale & Longmore) to simulate time evolution and polarization properties of the reflected X-ray emission, associated with the past outbursts from Sgr A*. While this model is too simple to describe the complexity of the true gas distribution, it illustrates the importance and power of long-term observations of the reflected emission. We show that the variable part of X-ray emission observed by Chandra and XMM from prominent molecular clouds is well described by a pure reflection model, providing strong support of the reflection scenario. While the identification of Sgr A* as a primary source for this reflected emission is already a very appealing hypothesis, a decisive test of this model can be provided by future X-ray polarimetric observations, that will allow placing constraints on the location of the primary source. In addition, X-ray polarimeters (like, e.g., XIPE) have sufficient sensitivity to constrain the line-of-sight positions of molecular complexes, removing major uncertainty in the model.Comment: 17 pages, 10 figures, accepted for publication in MNRA