694 research outputs found
Observational Features of Black Holes
Recently considered a very attracting possibility to detect retro-MACHOs,
i.e. retro-images of the Sun by a Schwarzschild black hole. In this paper we
discuss glories (mirages) formed near rapidly rotating Kerr black hole horizons
and propose a procedure to measure masses and rotation parameters analyzing
these forms of mirages. In some sense that is a manifestation of gravitational
lens effect in the strong gravitational field near black hole horizon and a
generalization of the retro-gravitational lens phenomenon. We analyze the case
of a Kerr black hole rotating at arbitrary speed for some selected positions of
a distant observer with respect to the equatorial plane of a Kerr black hole.
We discuss glories (mirages) formed near rapidly rotating Kerr black hole
horizons and propose a procedure to measure masses and rotation parameters
analyzing these forms of mirages. Some time ago suggested to search shadows at
the Galactic Center. In this paper we present the boundaries for shadows
calculated numerically. We also propose to use future radio interferometer
RADIOASTRON facilities to measure shapes of mirages (glories) and to evaluate
the black hole spin as a function of the position angle of a distant observer.Comment: Plenary talk presented at Workshop on High Energy Physics&Field
Theory (Protvino, Russia, 2004
XMM-Newton observation of MV Lyr and the sandwiched model confirmation
We present spectral and timing analyses of simultaneous X-ray and UV
observations of the VY Scl system MV Lyr taken by XMM-Newton, containing the
longest continuous X-ray+UV light curve and highest signal-to-noise X-ray
(EPIC) spectrum to date. The RGS spectrum displays emission lines plus
continuum, confirming model approaches to be based on thermal plasma models. We
test the sandwiched model based on fast variability that predicts a
geometrically thick corona that surrounds an inner geometrically thin disc. The
EPIC spectra are consistent with either a cooling flow model or a 2-T
collisional plasma plus Fe emission lines in which the hotter component may be
partially absorbed which would then originate in a central corona or a
partially obscured boundary layer, respectively. The cooling flow model yields
a lower mass accretion rate than expected during the bright state, suggesting
an evaporated plasma with a low density, thus consistent with a corona. Timing
analysis confirms the presence of a dominant break frequency around log(f/Hz) =
-3 in the X-ray Power Density Spectrum (PDS) as in the optical PDS. The complex
soft/hard X-ray light curve behaviour is consistent with a region close to the
white dwarf where the hot component is generated. The soft component can be
connected to an extended region. We find another break frequency around
log(f/Hz) = -3.4 that is also detected by Kepler. We compared flares at
different wavelengths and found that the peaks are simultaneous but the rise to
maximum is delayed in X-rays with respect to UV.Comment: 17 pages, 21 figures, 4 tables, Accepted for publication in MNRA
Signatures of rotating binaries in micro-lensing experiments
Gravitational microlensing offers a powerful method with which to probe a
variety of binary-lens systems, as the binarity of the lens introduces
deviations from the typical (single-lens) Paczy\'nski behaviour in the event
light curves. Generally, a static binary lens is considered to fit the observed
light curve and, when the orbital motion is taken into account, an
oversimplified model is usually employed. In this paper, we treat the
binary-lens motion in a realistic way and focus on simulated events that are
fitted well by a Paczy\'nski curve. We show that an accurate timing analysis of
the residuals (calculated with respect to the best-fitting Paczy\'nski model)
is usually sufficient to infer the orbital period of the binary lens. It goes
without saying that the independently estimated period may be used to further
constrain the orbital parameters obtained by the best-fitting procedure, which
often gives degenerate solutions. We also present a preliminary analysis of the
event OGLE-2011-BLG-1127 / MOA-2011-BLG-322, which has been recognized to be
the result of a binary lens. The period analysis results in a periodicity of
\simeq 12 days, which confirms the oscillation of the observed data around the
best-fitting model. The estimated periodicity is probably associated with an
intrinsic variability of the source star, and therefore there is an opportunity
to use this technique to investigate either the intrinsic variability of the
source or the effects induced by the binary-lens orbital motion.Comment: In press on MNRAS, 2014. 8 pages, 4 figures. On-line material
available on the Journal web-pag
A catalogue sample of low mass galaxies observed in X-rays with central candidate black holes
We present a sample of -ray selected candidate black holes in 51 low mass
galaxies with {and mass up to M} obtained by
cross-correlating the NASA-SLOAN Atlas with the 3XMM catalogue. {We have also
searched in the available catalogues for radio counterparts of the black hole
candidates and find that 19 of the previously selected sources have also a
radio counterpart.} Our results show that about of the galaxies of our
sample host { an -ray source} (associated to a radio counterpart) spatially
coincident with the galaxy center, in agreement with { other recent works}. For
these {\it nuclear} sources, the -ray/radio fundamental plane relation
allows one to estimate the mass of the (central) candidate black holes which
results to be in the range M (with median
value of M and eight candidates having mass
below M). This result, while suggesting that -ray
emitting black holes in low-mass galaxies may have had a key role in the
evolution of such systems, makes even more urgent to explain how such massive
objects formed in galaxies. {Of course, dedicated follow-up observations both
in the -ray and radio bands, as well as in the optical, are necessary in
order to confirm our resultsComment: 15 Pages, 2 Figures, 3 Table
XMM-Newton observation of a sample of four close dSph galaxies
We present the results of the analysis of deep archival \sat\ observations
towards the dwarf spheroidal galaxies Draco, Leo I, Ursa Major II and Ursa
Minor in the Milky Way neighbourhood. The X-ray source population is
characterized and cross-correlated with available databases with the aim to
infer their nature. We also investigate if intermediate-mass black holes are
hosted in the center of these galaxies. In the case of Draco, we detect 96
high-energy sources, two of them being possibly local stars, while no evidence
for any X-ray emitting central compact object is found. Towards the Leo I and
UMa II field of view we reveal 116 and 49 X-ray sources, respectively. None of
them correlates with the putative central black holes and only one is likely
associated with a UMa II local source. The study of the UMi dwarf galaxy shows
54 high-energy sources and a possible association {with a source at the dSph
center}. We put an upper limit to the central compact object luminosity of
4.0210 erg/s. Furthermore, via the correlation with a radio
source near the galactic center, we get that the putative black hole should
have a mass of and be
radiatively inefficient. This confirms a previous result obtained by using
Chandra data alone.Comment: MNRAS, in press, tables available on lin
Astrometric microlensing
Astrometric microlensing will offer in the next future a new channel for
investigating the nature of both lenses and sources involved in a gravitational
microlensing event. The effect, corresponding to the shift of the position of
the multiple image centroid with respect to the source star location, is
expected to occurr on scales from micro-arcoseconds to milli-arcoseconds
depending on the characteristics of the lens-source system. Here, we consider
different classes of events (single/binary lens acting on a single/binary
source) also accounting for additional effects including the finite source
size, the blending and orbital motion. This is particularly important in the
era of Gaia observations which is making possible astrometric measurements with
unprecedent quality.Comment: On IJMP D, 15 pages, 6 Figure
Estimating Finite Source Effects in Microlensing Events due to Free-Floating Planets with the Euclid Survey
In recent years free-loating planets (FFPs) have drawn a great interest among
astrophysicists. Gravitational microlensing is a unique and exclusive method
for their investigation which may allow obtaining precious information about
their mass and spatial distribution. The planned Euclid space-based observatory
will be able to detect a substantial number of microlensing events caused by
FFPs towards the Galactic bulge. Making use of a synthetic population
algorithm, we investigate the possibility of detecting finite source effects in
simulated microlensing events due to FFPs. We find a significant efficiency for
finite source effect detection that turns out to be between 20% and 40% for a
FFP power law mass function index in the range [0.9, 1.6]. For many of such
events it will also be possible to measure the angular Einstein radius and
therefore constrain the lens physical parameters. These kinds of observations
will also offer a unique possibility to investigate the photosphere and
atmosphere of Galactic bulge stars.Comment: 8 pages, 5 figures, published in Advances in Astronomy, Volume 2015,
Article ID 40230
Starspot induced effects in microlensing events with rotating source star
We consider the effects induced by the presence of hot and cold spots on the
source star in the light curves of simulated microlensing events due to either
single or binary lenses taking into account the rotation of the source star and
the orbital motion of the lens system. Our goal is to study the anomalies
induced by these effects on simulated microlensing light curves.Comment: 5 pages, 2 figures, accepted for publication in MNRA
Investigating the free-floating planet mass by Euclid observations
The detection of anomalies in gravitational microlensing events is nowadays
one of the main goals among the microlensing community. In the case of
single-lens events, these anomalies can be caused by the finite source effects,
that is when the source disk size is not negligible, and by the Earth rotation
around the Sun (the so-called parallax effect). The finite source and parallax
effects may help to define the mass of the lens, uniquely. Free-floating
planets (FFPs) are extremely dim objects, and gravitational microlensing
provides at present the exclusive method to investigate these bodies. In this
work, making use of a synthetic population algorithm, we study the possibility
of detecting the finite source and parallax effects in simulated microlensing
events caused by FFPs towards the Galactic bulge, taking into consideration the
capabilities of the space-based Euclid telescope. We find a significant
efficiency for detecting the parallax effect in microlensing events with
detectable finite source effect, that turns out to be about 51% for mass
function index .Comment: Astrophysics and Space Science 201
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