4,026 research outputs found
Black hole masses and accretion states in ULXs
We summarize indirect empirical arguments used for estimating black hole (BH)
masses in ultraluminous X-ray sources (ULXs). The interpretation of the X-ray
data is still too model-dependent to provide tight constraints, but masses <~
100 Msun seem the most likely. It is getting clearer that ULXs do not show the
same evolutionary sequence between canonical spectral states as stellar-mass
BHs, nor the same timescale for state transitions. Most ULX spectra are
consistent either with a power-law-dominated state (apparently identical to the
canonical low/hard state), or with a very high state (or slim-disk state).
Despite often showing luminosity variability, there is little evidence of ULXs
settling into a canonical high/soft state, dominated by a standard disk
(disk-blackbody spectrum). It is possible that the mass accretion rate (but not
necessarily the luminosity) is always higher than Eddington; but there may be
additional physical differences between stellar-mass BHs and ULXs, which
disfavour transitions to the standard-disk, radio-quiet state in the latter
class. We speculate that the hard state in ULXs is associated with jet or
magnetic processes rather than an ADAF, can persist up to accretion rates ~
Eddington, and can lead directly to the very high state.Comment: 8 pages; to appear in the proceedings of the conference
"Observational Evidence of Black Holes", Kolkata, February 200
On analytic properties of Meixner-Sobolev orthogonal polynomials of higher order difference operators
In this contribution we consider sequences of monic polynomials orthogonal
with respect to Sobolev-type inner product where is the Meixner linear operator,
, , , and
is the forward difference operator , or the backward difference
operator .
We derive an explicit representation for these polynomials. The ladder
operators associated with these polynomials are obtained, and the linear
difference equation of second order is also given. In addition, for these
polynomials we derive a -term recurrence relation. Finally, we find the
Mehler-Heine type formula for the case
Biomarkers for the prediction of oncologic outcomes in non-muscle invasive bladder cancer: State of affairs and new frontiers
A Secure Steganographic Algorithm Based on Frequency Domain for the Transmission of Hidden Information
This contribution proposes a novel steganographic method based on the compression standard according to the Joint Photographic Expert Group and an Entropy Thresholding technique. The steganographic algorithm uses one public key and one private key to generate a binary sequence of pseudorandom numbers that indicate where the elements of the binary sequence of a secret message will be inserted. The insertion takes eventually place at the first seven AC coefficients in the transformed DCT domain. Before the insertion of the message the image undergoes several transformations. After the insertion the inverse transformations are applied in reverse order to the original transformations. The insertion itself takes only place if an entropy threshold of the corresponding block is satisfied and if the pseudorandom number indicates to do so. The experimental work on the validation of the algorithm consists of the calculation of the peak signal-to-noise ratio (PSNR), the difference and correlation distortion metrics, the histogram analysis, and the relative entropy, comparing the same characteristics for the cover and stego image. The proposed algorithm improves the level of imperceptibility analyzed through the PSNR values. A steganalysis experiment shows that the proposed algorithm is highly resistant against the Chi-square attack
XMM-Newton observations of the spiral galaxy M74 (NGC 628)
The face-on spiral galaxy M74 (NGC 628) was observed by XMM on 2002 February
2. In total, 21 sources are found in the inner 5' from the nucleus (after
rejection of a few sources associated to foreground stars). Hardness ratios
suggest that about half of them belong to the galaxy. The higher-luminosity end
of the luminosity function is fitted by a power-law of slope -0.8. This can be
interpreted as evidence of ongoing star formation, in analogy with the
distributions found in disks of other late-type galaxies. A comparison with
previous Chandra observations reveals a new ultraluminous X-ray transient (L_x
\~ 1.5 x 10^39 erg/s in the 0.3--8 keV band) about 4' North of the nucleus. We
find another transient black-hole candidate (L_x ~ 5 x 10^38 erg/s) about 5'
North-West of the nucleus. The UV and X-ray counterparts of SN 2002ap are also
found in this XMM observation.Comment: submitted to ApJL. Based on publicly available data, see
http://xmm.vilspa.esa.es/external/xmm_news/items/sn_2002_ap/index.shtm
Towards an -theorem for granular gases
The -theorem, originally derived at the level of Boltzmann non-linear
kinetic equation for a dilute gas undergoing elastic collisions, strongly
constrains the velocity distribution of the gas to evolve irreversibly towards
equilibrium. As such, the theorem could not be generalized to account for
dissipative systems: the conservative nature of collisions is an essential
ingredient in the standard derivation. For a dissipative gas of grains, we
construct here a simple functional related to the original ,
that can be qualified as a Lyapunov functional. It is positive, and results
backed by three independent simulation approaches (a deterministic spectral
method, the stochastic Direct Simulation Monte Carlo technique, and Molecular
Dynamics) indicate that it is also non-increasing. Both driven and unforced
cases are investigated
Monitoring subaquatic vegetation using Sentinel-2 imagery in Gallocanta Lake (Aragón, Spain)
Remote sensing allows the study of aquatic vegetation cover in shallow lakes from the different spectral responses of the water as the vegetation grows from the bottom toward the surface. In the case of Gallocanta Lake, its seasonality and shallow depth (less than 2 m) allow us to appreciate the variations in the aquatic vegetation with the apparent color. Six common vegetation indices were tested, and the one with the best response was the so-called NDI45, which uses the normalized ratio between the far red (705 nm) and red (665 nm) bands. Our aims are to show the variations in the surface area covered by vegetation at the bottom of the lagoon, its growth and disappearance when drying occurs, and recolonization in a flooding period. The degree of cover reaches 90% at the most favorable times of the year, generally in summer and coinciding with flooding of the lake. The studied period shows how this method can be used for lacustrine habitat detection and highlights the need for field vegetation inventories in future works, which will allow the spectral measurements to be related to the species present
The ultraluminous X-ray source NGC 1313 X-2 - Its optical counterpart and environment
NGC 1313 X-2 is one of the brightest ultraluminous X-ray sources in the sky,
at both X-ray and optical wavelengths; therefore, quite a few studies of
available ESO VLT and HST data have appeared in the literature. Here, we
present our analysis of VLT/FORS1 and HST/ACS photometric data, confirming the
identification of the B ~ 23 mag blue optical counterpart. We show that the
system is part of a poor cluster with an age of 20 Myr, leading to an upper
mass limit of some 12 M_sun for the mass donor. We attribute the different
results with respect to earlier studies to the use of isochrones in the F435W
and F555W HST/ACS photometric system that appear to be incompatible with the
corresponding Johnson B and V isochrones. The counterpart exhibits significant
photometric variability of about 0.2 mag amplitude, both between the two HST
observations and during the one month of monitoring with the VLT. This includes
variability within one night and suggests that the light is dominated by the
accretion disk in the system and not by the mass donor.Comment: 13 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysic
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