Astrophys.J.665:599-607,2007 We present a detailed analysis of Swift multi-wavelength observations of GRB
070110 and its remarkable afterglow. The early X-ray light curve, interpreted
as the tail of ...the prompt emission, displays a spectral evolution already seen
in other gamma-ray bursts. The optical afterglow shows a shallow decay up to ~2
d after the burst, which is not consistent with standard afterglow models. The
most intriguing feature is a very steep decay in the X-ray flux at ~20 ks after
the burst, ending an apparent plateau. The abrupt drop of the X-ray light curve
rules out an external shock as the origin of the plateau in this burst and
implies long-lasting activity of the central engine. The temporal and spectral
properties of the plateau phase point towards a continuous central engine
emission rather than the episodic emission of X-ray flares. We suggest that the
observed X-ray plateau is powered by a spinning down central engine, possibly a
millisecond pulsar, which dissipates energy at an internal radius before
depositing energy into the external shock.
We present science highlights and performance from the Swift X-ray Telescope (XRT), which was launched on November 20, 2004. The XRT covers the 0.2-10 keV band, and spends most of its time observing ...gamma-ray burst (GRB)afterglows, though it has also performed observations of many other objects. By mid-August 2007, the XRT had observed over 220 GRB afterglows, detecting about 96% of them. The XRT positions enable followup ground-based optical observations, with roughly 60% of the afterglows detected at optical or near IR wavelengths. Redshifts are measured for 33% of X-ray afterglows. Science highlights include the discovery of flaring behavior at quite late times, with implications for GRB central engines; localization of short GRBs, leading to observational support for compact merger progenitors for this class of bursts; a mysterious plateau phase to GRB afterglows; as well as many other interesting observations such as X-ray emission from comets, novae, galactic transients, and other objects.
Nuovo Cim.B121:1163-1170,2006 We present the first results of a program to systematically study the
optical-to-X-ray spectral energy distribution (SED) of Swift GRB afterglows
with known redshift. ...The goal is to study the properties of the GRB explosion
and of the intervening absorbing material. In this report we present the
preliminary analysis on 23 afterglows. Thanks to Swift, we could build the SED
at early times after the GRB (minutes to hours). We derived the Hydrogen column
densities and the spectral slopes from the X-ray spectrum. We then constrained
the visual extinction by requiring that the combined optical/X-ray SED is due
to synchrotron, namely either a single power law or a broken power law with a
slope change by 0.5. We confirm a low dust-to-metal ratio, smaller than in the
SMC, even from the analysis of data taken significantly earlier than previously
possible. Our analysis does not support the existence of ``grey'' dust. We also
find that the synchrotron spectrum works remarkably well to explain afterglow
SEDs. We clearly see, however, that during the X-ray steep decay phases and the
flares, the X-ray radiation cannot be due only to afterglow emission.
Astron.Astrophys.443:L1-L5,2005 We present optical and near-infrared observations of the afterglow of the
gamma-ray burst GRB 050904. We derive a photometric redshift z = 6.3, estimated
from the ...presence of the Lyman break falling between the I and J filters. This
is by far the most distant GRB known to date. Its isotropic-equivalent energy
is 3.4x10^53 erg in the rest-frame 110-1100 keV energy band. Despite the high
redshift, both the prompt and the afterglow emission are not peculiar with
respect to other GRBs. We find a break in the J-band light curve at t_b = 2.6
+- 1.0 d (observer frame). If we assume this is the jet break, we derive a
beaming-corrected energy E_gamma = (4-12)x10^51 erg. This limit shows that GRB
050904 is consistent with the Amati and Ghirlanda relations. This detection is
consistent with the expected number of GRBs at z > 6 and shows that GRBs are a
powerful tool to study the star formation history up to very high redshift.
The blazar PKS0537-441 has been observed by Swift between the end of 2004 and November 2005. The BAT monitored it recurrently for a total of 2.7 Ms, and the XRT and UVOT pointed it on seven occasions ...for a total of 67 ks, making it one of the AGNs best monitored by Swift. The automatic optical and infrared telescope REM has monitored simultaneously the source at all times. In January-February 2005 PKS0537-441 has been detected at its brightest in optical and X-rays: more than a factor of 2 brighter in X-rays and about a factor 60 brighter in the optical than observed in December 2004. The July 2005 observation recorded a fainter X-ray state. The simultaneous optical state, monitored by both Swift UVOT and REM, is high, and in the VRI bands it is comparable to what was recorded in early January 2005, before the outburst. In November 2005, the source subsided both in X-rays and optical to a quiescent state, having decreased by factors of ~4 and ~60 with respect to the January-February 2005 outburst, respectively. Our monitoring shows an overall well correlated optical and X-ray decay. On the shorter time scales (days or hours), there is no obvious correlation between X-ray and optical variations, but the former tend to be more pronounced, opposite to what is observed on monthly time scales. The widely different amplitude of the long term variability in optical and X-rays is very unusual and makes this observation a unique case study for blazar activity. The spectral energy distributions are interpreted in terms of the synchrotron and inverse Compton mechanisms within a jet where the plasma radiates via internal shocks and the dissipation depends on the distance of the emitting region from the central engine (abridged).
Astron.Astrophys.462:913-918,2007 We present observations of the X-ray afterglow of GRB 050408, a gamma-ray
burst discovered by HETE-II. Swift began observing the field 42 min after the
burst, ...performing follow-up over a period of 38 d (thus spanning three decades
in time).The X-ray light curve showed a steepening with time, similar to many
other afterglows. However, the steepening was unusually smooth, over the
duration of the XRT observation, with no clear break time. The early decay was
too flat to be described in terms of standard models. We therefore explore
alternative explanations, such as the presence of a structured afterglow or of
long-lasting energy injection into the fireball from the central GRB engine.
The lack of a sharp break puts constraints on these two models. In the former
case, it may indicate that the angular energy profile of the jet was not a
simple power law, while in the second model it implies that injection did not
stop abruptly. The late decay may be due either to a standard afterglow (that
is, with no energy injection), or to a jetted outflow still being refreshed. A
significant amount of absorption was present in the X-ray spectrum,
corresponding to a rest-frame Hydrogen column density NH = 1.2
(-0.3,+0.4)*10^22 cm^-2, indicative of a dense environment.
We present deep CCD surface photometry in the Gunn g, r, i system and spectroscopy of the cD galaxy at the centre of the cluster A3284 at z = 0.15. The brightness profile of the galaxy has been ...tracked up to 40 arcsec (142 kpc) from the centre, and down to 26 mag/arcsec^2. The core and halo components in the galaxy have been singled out deriving geometrical parameters of the fitting isophotes as well as magnitudes and colours. The spectral properties of the galaxy core indicate a stellar population super metal-rich with Fe/H = +0.5. The cD halo is clearly dominant at 45 kpc from the galactic centre and has exceedingly red colours (g-r = 1.03, g-i = 1.82), about 0.7 mag redder than the core g-i. A match with the models for evolutionary population synthesis by Buzzoni (1989) show that the halo is consistent with a population of unevolved M-dwarf stars lower than 0.7 M_\odot. The M/L ratio in B for the halo is estimated to range between 50 and 200 implying a total mass for the cD galaxy of 1.6-3.1~10^{13} M_\odot and a total B luminosity of 6.0~10^{11} L_\odot.
We present an analysis of the optical data of the Hubble Deep Field South. We derive F300W(AB), F450W(AB), F606W(AB) and F814W(AB) number counts for galaxies in all four bands. The slope is steeper ...at shortest wavelengths: we estimated gamma(F300W(AB))=0.47, gamma(F450W(AB))=0.35, gamma(F606W(AB))=0.28 and gamma(F814W(AB))=0.28. Morphological number counts are actually dominated by late type galaxies, while early type galaxies show a decreasing slope at faint magnitudes. Combining this information with photometric redshifts, we notice that galaxies contributing with a steep slope to morphological number counts (i.e. spiral and irregular galaxies) have z>1, suggesting a moderate merging. However we emphasize that any cut in apparent magnitude at optical wavelengths results in samples biased against elliptical galaxies, affecting as a consequence the redshift distributions and the implications on the evolution of galaxies along the Hubble sequence.
Aims: We aim at detecting and determining the properties of the host galaxy
of the dark GRB 050223.
Methods: We use VLT optical/NIR images coupled to Swift X-ray positioning,
and optical spectra of ...the host galaxy to measure its properties.
Results: We find a single galaxy within the Swift error box of GRB 050223. It
is located at z = 0.584 and its luminosity is L ~ 0.4 L*. Emission lines in the
galaxy spectrum imply an intrinsic SFR > 7 Msun/yr, and a large extinction A_V
> 2 mag within it. We also detect absorption lines, which reveal an underlying
stellar population with an age between 40 Myr and 1.5 Gyr.
Conclusions: The identification of a host galaxy with atypical properties
using only the X-ray transient suggests that a bias may be present in the
former sample of host galaxies. Dust obscuration together with intrinsic
faintness are the most probable causes for the darkness of this burst.
