806 research outputs found
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 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 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 . For instance, the probability distribution
function of the gas density 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 erg, where is the mean hydrogen density of the
cloud complex in units of . 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 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
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