180 research outputs found
BURGOS, J. M. — CAÑAS, J. L. — FERRER, U., Hacia una definición de la filosofía personalista. Palabra, Madrid, 2006, 173 pp. [RESEÑA]
Another short-burst host galaxy with an optically obscured high star formation rate: The case of GRB 071227
We report on radio continuum observations of the host galaxy of the short
gamma-ray burst 071227 (z=0.381) with the Australia Telescope Compact Array
(ATCA). We detect the galaxy in the 5.5 GHz band with an integrated flux
density of Fnu = 43 +/- 11 microJy, corresponding to an unobscured
star-formation rate (SFR) of about 24 Msun/yr, forty times higher than what was
found from optical emission lines. Among the ~30 well-identified and studied
host galaxies of short bursts this is the third case where the host is found to
undergo an episode of intense star formation. This suggests that a fraction of
all short-burst progenitors hosted in star-forming galaxies could be physically
related to recent star formation activity, implying a relatively short merger
time scale.Comment: 6 pages, ApJ, accepted for publicatio
Afterglow rebrightenings as a signature of a long-lasting central engine activity? The emblematic case of GRB 100814A
In the past few years the number of well-sampled optical to NIR light curves
of long Gamma-Ray Bursts (GRBs) has greatly increased particularly due to
simultaneous multi-band imagers such as GROND. Combining these densely sampled
ground-based data sets with the Swift UVOT and XRT space observations unveils a
much more complex afterglow evolution than what was predicted by the most
commonly invoked theoretical models. GRB 100814A represents a remarkable
example of these interesting well-sampled events, showing a prominent late-time
rebrightening in the optical to NIR bands and a complex spectral evolution.
This represents a unique laboratory to test the different afterglow emission
models. Here we study the nature of the complex afterglow emission of GRB
100814A in the framework of different theoretical models. Moreover, we compare
the late-time chromatic rebrightening with those observed in other well-sampled
long GRBs. We analysed the optical and NIR observations obtained with the
seven-channel Gamma-Ray burst Optical and Near-infrared Detector at the 2.2 m
MPG/ESO telescope together with the X-ray and UV data detected by the
instruments onboard the Swift observatory. The broad-band afterglow evolution,
achieved by constructing multi-instrument light curves and spectral energy
distributions, will be discussed in the framework of different theoretical
models. We find that the standard models that describe the broad-band afterglow
emission within the external shock scenario fail to describe the complex
evolution of GRB 100814A, and therefore more complex scenarios must be invoked.
[abridged]Comment: 11 pages, 7 figures, 2 tables; Astronomy & Astrophysics, in pres
Multiwavelength analysis of three SNe associated with GRBs observed by GROND
After the discovery of the first connection between GRBs and SNe almost two
decades ago, tens of SN-like rebrightenings have been discovered and about
seven solid associations have been spectroscopically confirmed to date. Using
GROND optical/NIR data and Swift X-ray/UV data, we estimate the intrinsic
extinction, luminosity, and evolution of three SN rebrightenings in GRB
afterglow light curves at z~0.5. The SNe 2008hw, 2009nz, and 2010ma exhibit
0.80, 1.15, and 1.78 times the optical (r band) luminosity of SN 1998bw,
respectively. While SN 2009nz evolves similarly to SN 1998bw, SNe 2008hw and
2010ma show earlier peak times. The quasi-bolometric light curves were
corrected for the contribution of the NIR bands using data available in the
literature and blackbody fits. The large luminosity of SN 2010ma (1.4x10^43
erg/s) is confirmed, while SNe 2008hw and 2009nz reached a peak luminosity
closer to SN 1998bw. Physical parameters of the SN explosions, such as
synthesised nickel mass, ejecta mass, and kinetic energy, are estimated using
Arnett's analytic approach, which resulted in nickel masses of around 0.4-0.5
Msun. By means of the a very comprehensive data set, we found that the
luminosity and the nickel mass of SNe 2008hw, 2009nz, and 2010ma resembles
those of other known GRB-associated SNe. This findings strengthens previous
claims of GRB-SNe being brighter than type-Ic SNe unaccompanied by GRBs.Comment: 11 pages, 9 figures, accepted for publication in Astronomy &
Astrophysics, abstract abridge
BL Lacertae objects beyond redshift 1.3 - UV-to-NIR photometry and photometric redshift for Fermi/LAT blazars
Observations of the gamma-ray sky with Fermi led to significant advances
towards understanding blazars, the most extreme class of Active Galactic
Nuclei. A large fraction of the population detected by Fermi is formed by BL
Lacertae (BL Lac) objects, whose sample has always suffered from a severe
redshift incompleteness due to the quasi-featureless optical spectra. Our goal
is to provide a significant increase of the number of confirmed high-redshift
BL Lac objects contained in the 2 LAC Fermi/LAT catalog. For 103 Fermi/LAT
blazars, photometric redshifts using spectral energy distribution fitting have
been obtained. The photometry includes 13 broad-band filters from the far
ultraviolet to the near-IR observed with Swift/UVOT and the multi-channel
imager GROND at the MPG/ESO 2.2m telescope. Data have been taken
quasi-simultaneously and the remaining source-intrinsic variability has been
corrected for. We release the UV-to-near-IR 13-band photometry for all 103
sources and provide redshift constraints for 75 sources without previously
known redshift. Out of those, eight have reliable photometric redshifts at
z>1.3, while for the other 67 sources we provide upper limits. Six of the
former eight are BL Lac objects, which quadruples the sample of confirmed
high-redshift BL Lac. This includes three sources with redshifts higher than
the previous record for BL Lac, including CRATES J0402-2615 with the best-fit
solution at z~1.9.Comment: Uploaded correct Fig 4. Changed counterpart name for 2FGLJ0537.7-5716
from PKS 0541-834 (different source) to SUMSS J053748-57182
GRB 091127: The cooling break race on magnetic fuel
Using high-quality, broad-band afterglow data for GRB 091127, we investigate
the validity of the synchrotron fireball model for gamma-ray bursts, and infer
physical parameters of the ultra-relativistic outflow. We used multi-wavelength
follow-up observations obtained with GROND and the XRT onboard the Swift
satellite. The resulting afterglow light curve is of excellent accuracy, and
the spectral energy distribution is well-sampled over 5 decades in energy.
These data present one of the most comprehensive observing campaigns for a
single GRB afterglow and allow us to test several proposed emission models and
outflow characteristics in unprecedented detail. Both the multi-color light
curve and the broad-band SED of the afterglow of GRB 091127 show evidence of a
cooling break moving from high to lower energies. The early light curve is well
described by a broken power-law, where the initial decay in the optical/NIR
wavelength range is considerably flatter than at X-rays. Detailed fitting of
the time-resolved SED shows that the break is very smooth with a sharpness
index of 2.2 +- 0.2, and evolves towards lower frequencies as a power-law with
index -1.23 +- 0.06. These are the first accurate and contemporaneous
measurements of both the sharpness of the spectral break and its time
evolution. The measured evolution of the cooling break (nu_c propto t^-1.2) is
not consistent with the predictions of the standard model, wherein nu_c propto
t^-0.5 is expected. A possible explanation for the observed behavior is a time
dependence of the microphysical parameters, in particular the fraction of the
total energy in the magnetic field epsilon_B. This conclusion provides further
evidence that the standard fireball model is too simplistic, and time-dependent
micro-physical parameters may be required to model the growing number of
well-sampled afterglow light curves.Comment: accepted to A&A, 13 pages, 5 figure
The late-time afterglow of the extremely energetic short burst GRB 090510 revisited
The discovery of the short GRB 090510 has raised considerable attention
mainly because it had a bright optical afterglow and it is among the most
energetic events detected so far within the entire GRB population. The
afterglow was observed with swift/UVOT and swift/XRT and evidence of a jet
break around 1.5 ks after the burst has been reported in the literature,
implying that after this break the optical and X-ray light curve should fade
with the same decay slope. As noted by several authors, the post-break decay
slope seen in the UVOT data is much shallower than the steep decay in the X-ray
band, pointing to an excess of optical flux at late times. We reduced and
analyzed new afterglow light-curve data obtained with the multichannel imager
GROND. Based on the densely sampled data set obtained with GROND, we find that
the optical afterglow of GRB 090510 did indeed enter a steep decay phase
starting around 22 ks after the burst. During this time the GROND optical light
curve is achromatic, and its slope is identical to the slope of the X-ray data.
In combination with the UVOT data this implies that a second break must have
occurred in the optical light curve around 22 ks post burst, which, however,
has no obvious counterpart in the X-ray band, contradicting the interpretation
that this could be another jet break. The GROND data provide the missing piece
of evidence that the optical afterglow of GRB 090510 did follow a post-jet
break evolution at late times.Comment: submitted to Astronomy & Astrophysics, accepted for publication on
Dec 24, 201
Super-solar metallicity at the position of the ultra-long GRB130925A
Over the last decade there has been immense progress in the follow-up of
short and long GRBs, resulting in a significant rise in the detection rate of
X-ray and optical afterglows, in the determination of GRB redshifts, and of the
identification of the underlying host galaxies. Nevertheless, our theoretical
understanding on the progenitors and central engines powering these vast
explosions is lagging behind, and a newly identified class of `ultra-long' GRBs
has fuelled speculation on the existence of a new channel of GRB formation. In
this paper we present high signal-to-noise X-shooter observations of the host
galaxy of GRB130925A, which is the fourth unambiguously identified ultra-long
GRB, with prompt gamma-ray emission detected for ~20ks. The GRB line of sight
was close to the host galaxy nucleus, and our spectroscopic observations cover
both this region along the bulge/disk of the galaxy, in addition to a bright
star-forming region within the outskirts of the galaxy. From our broad
wavelength coverage we obtain accurate metallicity and dust-extinction
measurements at both the galaxy nucleus, and an outer star-forming region, and
measure a super-solar metallicity at both locations, placing this galaxy within
the 10-20% most metal-rich GRB host galaxies. Such a high metal enrichment has
implications on the progenitor models of both long and ultra-long GRBs,
although the edge-on orientation of the host galaxy does not allow us to rule
out a large metallicity variation along our line of sight. The spatially
resolved spectroscopic data presented in this paper offer important insight
into variations in the metal and dust abundance within GRB host galaxies. They
also illustrate the need for IFU observations on a larger sample of GRB host
galaxies at varies metallicities to provide a more quantitative view on the
relation between the GRB circumburst and the galaxy-whole properties.Comment: 9 pages, 3 figures, A&A in press, matches published versio
GRB 091029: At the limit of the fireball scenario
Using high-quality, broad-band afterglow data for GRB 091029, we test the
validity of the forward-shock model for gamma-ray burst afterglows. We used
multi-wavelength (NIR to X-ray) follow-up observations obtained with the GROND,
BOOTES-3/YA and Stardome optical ground-based telescopes, and the UVOT and the
XRT onboard the Swift satellite. To explain the almost totally decoupled light
curves in the X-ray and optical/NIR domains, a two-component outflow is
proposed. Several models are tested, including continuous energy injection,
components with different electron energy indices and components in two
different stages of spectral evolution. Only the last model can explain both
the decoupled light curves with asynchronous peaks and the peculiar SED
evolution. However, this model has so many unknown free parameters that we are
unable to reliably confirm or disprove its validity, making the afterglow of
GRB 091029 difficult to explain in the framework of the simplest fireball
model.Comment: Accepted to A&
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