415 research outputs found
Preliminary results from antarctic albedo from remote sensing observations
The aim of the study is to analyse the surface albedo of the Ant-arctica and investigate eventual signals of variations in space and time between summer 2000/2001 and 2011/2012 by means of the GLASS albedo product. We followed a step-by-step procedure from micro- to macro-scale. At first, we analysed 95 glaciers around the continent, and we found limited temporal variability. Then, looking at spatial varia-tions, we divided Antarctica based on oceanic basins and by continen-tality. We found spatial signals, since mean albedo values range between 0.79 (Pacific and Atlantic basins) and 0.82 (Indian basin) and between 0.76 (along the shore) and 0.81 (inner continent). An increasing vari-ability was found from the inner continent to the shore, and heteroge-neous patterns among the basins, most likely due to meteorological and environmental conditions (mainly: temperature, precipitation, katabatic winds). Finally, the general patterns observed (considering the specific gla-ciers, the three basins and the three continentality sectors) were verified by the analysis of the whole continent and we did not find a significant change of summer averages over time, as they range between 0.79 and 0.80
Testing the gamma-ray burst variability/peak luminosity correlation on a Swift homogeneous sample
We test the gamma-ray burst correlation between temporal variability and peak
luminosity of the -ray profile on a homogeneous sample of 36 Swift/BAT
GRBs with firm redshift determination. This is the first time that this
correlation can be tested on a homogeneous data sample. The correlation is
confirmed, as long as the 6 GRBs with low luminosity (<5x10^{50} erg s^{-1} in
the rest-frame 100-1000 keV energy band) are ignored. We confirm that the
considerable scatter of the correlation already known is not due to the
combination of data from different instruments with different energy bands, but
it is intrinsic to the correlation itself. Thanks to the unprecedented
sensitivity of Swift/BAT, the variability/peak luminosity correlation is tested
on low-luminosity GRBs. Our results show that these GRBs are definite outliers.Comment: Accepted for Publication in MNRAS. 10 pages, 5 figures, 3 table
The Swift X-ray Telescope Cluster Survey II. X-ray spectral analysis
(Abridged) We present a spectral analysis of a new, flux-limited sample of 72
X-ray selected clusters of galaxies identified with the X-ray Telescope (XRT)
on board the Swift satellite down to a flux limit of ~10-14 erg/s/cm2 (SWXCS,
Tundo et al. 2012). We carry out a detailed X-ray spectral analysis with the
twofold aim of measuring redshifts and characterizing the properties of the
Intra-Cluster Medium (ICM). Optical counterparts and spectroscopic or
photometric redshifts are obtained with a cross-correlation with NED.
Additional photometric redshifts are computed with a dedicated follow-up
program with the TNG and a cross-correlation with the SDSS. We also detect the
iron emission lines in 35% of the sample, and hence obtain a robust measure of
the X-ray redshift zX. We use zX whenever the optical redshift is not
available. Finally, for all the sources with measured redshift,
background-subtracted spectra are fitted with a mekal model. We perform
extensive spectral simulations to derive an empirical formula to account for
fitting bias. The bias-corrected values are then used to investigate the
scaling properties of the X-ray observables. Overall, we are able to
characterize the ICM of 46 sources. The sample is mostly constituted by
clusters with temperatures between 3 and 10 keV, plus 14 low-mass clusters and
groups with temperatures below 3 keV. The redshift distribution peaks around
z~0.25 and extends up to z~1, with 60% of the sample at 0.1<z<0.4. We derive
the Luminosity-Temperature relation for these 46 sources, finding good
agreement with previous studies. The quality of the SWXCS sample is comparable
to other samples available in the literature and obtained with much larger
X-ray telescopes. Our results have interesting implications for the design of
future X-ray survey telescopes, characterised by good-quality PSF over the
entire field of view and low background.Comment: 27 pages, 15 figures; minor typos corrected. To be published in A&A,
Volume 567, July 2014. Websites of the SWXCS project:
http://www.arcetri.astro.it/SWXCS/ and http://swxcs.ustc.edu.cn
Optical and X-ray Rest-frame Light Curves of the BAT6 sample
We present the rest-frame light curves in the optical and X-ray bands of an
unbiased and complete sample of Swift long Gamma-Ray Bursts (GRBs), namely the
BAT6 sample. The unbiased BAT6 sample (consisting of 58 events) has the highest
level of completeness in redshift ( 95%), allowing us to compute the
rest-frame X-ray and optical light curves for 55 and 47 objects, respectively.
We compute the X-ray and optical luminosities accounting for any possible
source of absorption (Galactic and intrinsic) that could affect the observed
fluxes in these two bands. We compare the behaviour observed in the X-ray and
in the optical bands to assess the relative contribution of the emission during
the prompt and afterglow phases. We unarguably demonstrate that the GRBs
rest-frame optical luminosity distribution is not bimodal, being rather
clustered around the mean value Log(L) = 29.9 0.8 when estimated at
a rest frame time of 12 hr. This is in contrast with what found in previous
works and confirms that the GRB population has an intrinsic unimodal luminosity
distribution. For more than 70% of the events the rest-frame light curves in
the X-ray and optical bands have a different evolution, indicating distinct
emitting regions and/or mechanisms. The X-ray light curves normalised to the
GRB isotropic energy (E), provide evidence for X-ray emission still
powered by the prompt emission until late times ( hours after the burst
event). On the other hand, the same test performed for the E-normalised optical light curves shows that the optical emission is a
better proxy of the afterglow emission from early to late times.Comment: Accepted for publication in A&A: 10 pages, 5 figures, 2 table
V39: an unusual object in the field of IC 1613
The variable star V39 in the field of IC 1613 is discussed in the light of
the available photometric and new spectroscopic data. It has strong emission
Balmer lines, and the observed characteristics could be explained by a W Vir
pulsating star with a period of 14.341 d, located at more than 115 kpc, that is
in the very outer halo of our Galaxy. It should have an apparent companion, a
long period (1118d) red variable, belonging to IC 1613. The main uncertainty in
this interpretation is an emission feature at 668.4 nm, which we tentatively
identified as a He I line.Comment: 5 pages; accepted for publication in Astronomy & Astrophysic
Optical and infrared photometry of the blazar PKS0537-441
We present a large collection of photometric data on the Blazar PKS 0537-441
in the VRIJHK bands taken in 2004-2009. At least three flare-like episodes with
months duration, and >3 mag amplitude are apparent. The spectral energy
distribution is consistent with a power law, and no indication of a thermal
component is found. We searched for short time scale variability, and an
interesting event was identified in the J band, with a duration of ~25 minutes.Comment: 10 pages, 3 figures, in press in ApJ
Dust extinction for an unbiased sample of GRB afterglows
In this paper we compute rest-frame extinctions for the afterglows of a
sample of gamma-ray bursts complete in redshift. The selection criteria of the
sample are based on observational high-energy parameters of the prompt emission
and therefore our sample should not be biased against dusty sight-lines. It is
therefore expected that our inferences hold for the general population of
gamma-ray bursts. Our main result is that the optical/near-infrared extinction
of gamma-ray burst afterglows in our sample does not follow a single
distribution. 87% of the events are absorbed by less than 2 mag, and 50% suffer
from less than 0.3-0.4 mag extinction. The remaining 13% of the afterglows are
highly absorbed. The true percentage of gamma-ray burst afterglows showing high
absorption could be even higher since a fair fraction of the events without
reliable redshift measurement are probably part of this class. These events may
be due to highly dusty molecular clouds/star forming regions associated with
the gamma-ray burst progenitor or along the afterglow line of sight, and/or to
massive dusty host galaxies. No clear evolution in the dust extinction
properties is evident within the redshift range of our sample, although the
largest extinctions are at z~1.5-2, close to the expected peak of the star
formation rate. Those events classified as dark are characterized, on average,
by a higher extinction than typical events in the sample. A correlation between
optical/near-infrared extinction and hydrogen-equivalent column density based
on X-ray studies is shown although the observed NH appears to be well in excess
compared to those observed in the Local Group. Dust extinction does not seem to
correlate with GRB energetics or luminosity.Comment: 18 pages, 7 figures, 10 tables, MNRAS, in pres
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