118 research outputs found
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
The bright optical/NIR afterglow of the faint GRB 080710 - Evidence for a jet viewed off axis
We investigate the optical/near-infrared light curve of the afterglow of GRB
080710 in the context of rising afterglows. Optical and near-infrared
photometry was performed using the seven channel imager GROND and the
Tautenburg Schmidt telescope. X-ray data were provided by the X-ray Telescope
onboard the Swift satellite. The optical/NIR light curve of the afterglow of
GRB 080710 is dominated by an initial increase in brightness, which smoothly
turns over into a shallow power law decay. The initially rising achromatic
light curve of the afterglow of GRB 080710 can be accounted for with a model of
a burst viewed off-axis or a single jet in its pre deceleration phase and in an
on-axis geometry. An unified picture of the afterglow light curve and prompt
emission properties can be obtained with an off-axis geometry, suggesting that
late and shallow rising optical light curves of GRB afterglows might be
produced by geometric effects.Comment: 9 pages, 4 figures, accepted by A and
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
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 Metal Abundances in Two Galaxies at z ~ 3.57 revealed by the GRB 090323 Afterglow Spectrum
We report on the surprisingly high metallicity measured in two absorption systems at high redshift, detected in the Very Large Telescope spectrum of the afterglow of the gamma-ray burst GRB090323. The two systems, at redshift z =3.5673 and z =3.5774 (separation ∆v ≈ 660 km s−1), are dominated by the neutral gas in the interstellar medium of the parent galaxies. From the singly ionized zinc and sulfur, we estimate oversolar metallicities of [Zn/H] = +0.29± 0.10 and [S/H] = +0.67± 0.34, in the blue and red absorber, respectively. These are the highest metallicities ever measured in galaxies at z\u3e 3. We propose that the two systems trace two galaxies in the process of merging, whose star formation and metallicity are heightened by the interaction. This enhanced star formation might also have triggered the birth of the GRB progenitor. As typically seen in star-forming galaxies, the fine-structure absorption Siii∗ is detected, both in G0 and G1. From the rest-frame UV emission in the GRB location, we derive arelatively high, not corrected for dust extinction, star-formation rate SFR ≈ 6M yr−1. These properties suggest a possible connection between some high-redshift GRB host galaxies and high-z massive sub-millimeter galaxies, which are characterized by disturbed morphologies and high metallicities. Our result provides additional evidence that the dispersion in the chemical enrichment of the Universe at high redshift is substantial, with the existence of very metal rich galaxies less than two billion years after the Big Bang
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
Prompt Emission of GRB 121217A from Gamma-Rays to the Near-Infrared
The mechanism that causes the prompt-emission episode of gamma-ray bursts (GRBs) is still widely debated despite there being thousands of prompt detections. The favoured internal shock model relates this emission to synchrotron radiation. However, it does not always explain the spectral indices of the shape of the spectrum, which is often fit with empirical functions, such as the Band function. Multi-wavelength observations are therefore required to help investigate the possible underlying mechanisms that causes the prompt emission. We present GRB 121217A, for which we were able to observe its near-infrared (NIR) emission during a secondary prompt-emission episode with the Gamma-Ray burst Optical Near-infrared Detector (GROND) in combination with the Swift and Fermi satellites, which cover an energy range of 5 orders of magnitude (10(exp 3) keV to 100 keV). We determine a photometric redshift of z = 3.1 +/- 0.1 with a line-of-sight with little or no extinction (AV approx. 0 mag) utilising the optical/NIR SED. From the afterglow, we determine a bulk Lorentz factor of Gamma(sub 0) approx. 250 and an emission radius of R < 1018 cm. The prompt-emission broadband spectral energy distribution is well fit with a broken power law with beta1 = 0.3 +/- 0.1 and beta2 = 0.6 +/- 0.1 that has a break at E = 6.6 +/- 0.9 keV, which can be interpreted as the maximum injection frequency. Self-absorption by the electron population below energies of Ea < 6 keV suggest a magnetic field strength of B approx. 10(exp 5) G. However, all the best fit models underpredict the flux observed in the NIR wavelengths, which also only rebrightens by a factor of approx. 2 during the second prompt emission episode, in stark contrast to the X-ray emission, which rebrightens by a factor of approx. 100. This suggests an afterglow component is dominating the emission. We present GRB 121217A, one of the few GRBs that has multi-wavelength observations of the prompt-emission period and shows that it can be understood with a synchrotron radiation model. However, due to the complexity of the GRB's emission, other mechanisms that result in Band-like spectra cannot be ruled out
On the nature of the extremely fast optical rebrightening of the afterglow of GRB 081029
Context. After the launch of the Swift satellite, the Gamma-Ray Burst (GRB)
optical light-curve smoothness paradigm has been questioned thanks to the
faster and better sampled optical follow-up, which has unveiled a very complex
behaviour. This complexity is triggering the interest of the whole GRB
community. The GROND multi-channel imager is used to study optical and
near-infrared (NIR) afterglows of GRBs with unprecedented optical and
near-infrared temporal and spectral resolution. The GRB 081029 has a very
prominent optical rebrightening event and is an outstanding example of the
application of the multi-channel imager to GRB afterglows. Aims. Here we
exploit the rich GROND multi-colour follow-up of GRB 081029 combined with XRT
observations to study the nature of late-time rebrightenings that appear in the
optical-NIR light-curves of some GRB afterglows. Methods. We analyse the
optical and NIR observations obtained with the seven-channel Gamma-Ray burst
Optical and Near-infrared Detector (GROND) at the 2.2 m MPI/ESO telescope and
the X-ray data obtained with the XRT telescope on board the Swift observatory.
The multi-wavelength temporal and spectral evolution is discussed in the
framework of different physical models. Results. The extremely steep optical
and NIR rebrightening observed in GRB 081029 cannot be explained in the
framework of the standard forward shock afterglow model. The absence of a
contemporaneous X-ray rebrightening and the evidence of a strong spectral
evolution in the optical-NIR bands during the rise suggest two separate
components that dominate in the early and late-time lightcurves, respectively.
The steepness of the optical rise cannot be explained even in the framework of
the alternative scenarios proposed in the literature unless a late-time
activity of the central engine is assumed.Comment: 9 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
- …