418 research outputs found
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
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
ILLUMINATING THE DARKEST GAMMA-RAY BURSTS WITH RADIO OBSERVATIONS
We present X-ray, optical, near-infrared (IR), and radio observations of gamma-ray bursts (GRBs) 110709B and 111215A, as well as optical and near-IR observations of their host galaxies. The combination of X-ray detections and deep optical/near-IR limits establish both bursts as "dark." Sub-arcsecond positions enabled by radio detections lead to robust host galaxy associations, with optical detections that indicate z ≾ 4 (110709B) and z ≈ 1.8-2.9 (111215A). We therefore conclude that both bursts are dark due to substantial rest-frame extinction. Using the radio and X-ray data for each burst we find that GRB 110709B requires A_V^(host) ≳ 5.3 mag and GRB 111215A requires A_V^(host) ≳ 8.5 mag (assuming z = 2). These are among the largest extinction values inferred for dark bursts to date. The two bursts also exhibit large neutral hydrogen column densities of N H, int ≳ 10^(22) cm^(–2) (z = 2) as inferred from their X-ray spectra, in agreement with the trend for dark GRBs. Moreover, the inferred values are in agreement with the Galactic A_V -N_H relation, unlike the bulk of the GRB population. Finally, we find that for both bursts the afterglow emission is best explained by a collimated outflow with a total beaming-corrected energy of E_γ + E_K ≈ (7-9) × 10^(51) erg (z = 2) expanding into a wind medium with a high density, Ṁ ≈ (6-20) x 10^(-5) M_☉ yr^(–1) (n ≈ 100-350 cm^(–3) at ≈ 10^(17) cm). While the energy release is typical of long GRBs, the inferred density may be indicative of larger mass-loss rates for GRB progenitors in dusty (and hence metal rich) environments. This study establishes the critical role of radio observations in demonstrating the origin and properties of dark GRBs. Observations with the JVLA and ALMA will provide a sample with sub-arcsecond positions and robust host associations that will help to shed light on obscured star formation and the role of metallicity in GRB progenitors
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
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
Evidence for Supernova-Synthesised Dust from the Rising Afterglow of GRB 071025 at z~5
We present observations and analysis of the broadband afterglow of Swift GRB
071025. Using optical and infrared (RIYJHK) photometry, we derive a photometric
redshift of 4.4 < z < 5.2; at this redshift our simultaneous multicolour
observations begin at ~30 s after the GRB trigger in the host frame and during
the initial rising phase of the afterglow. We associate the light curve peak at
580 s in the observer frame with the formation of the forward shock, giving an
estimate of the initial Lorentz factor Gamma_0 ~ 200. The red spectral energy
distribution (even in regions not affected by the Lyman-alpha break) provides
secure evidence of a large dust column. However, the inferred extinction curve
shows a prominent flat component between 2000-3000 Angstroms in the rest-frame,
inconsistent with any locally observed template but well-fit by models of dust
formed by supernovae. Time-dependent fits to the extinction profile reveal no
evidence of dust destruction and limit the decrease in the extinction column to
Delta A_3000 < 0.54 mag after t = 50 s in the rest frame. Our observations
provide evidence of a transition in dust properties at z~5, in agreement with
studies of high-z quasars, and suggest that SN-formed dust continues to
dominate the opacity of typical galaxies at this redshift.Comment: Resubmitted to MNRAS following referee report. Contains additional
figure and some extra analysis/discussio
GRB 071028B, a burst behind large amounts of dust in an unabsorbed galaxy
We report on the discovery and properties of the fading afterglow and
underlying host galaxy of GRB 071028B, thereby facilitating a detailed
comparison between these two. Observations were performed with the Gamma-ray
Burst Optical and Near-infrared Detector at the 2.2 m telescope on the La Silla
Paranal Observatory in Chile. We conducted five observations from 1.9 d to
227.2 d after the trigger and obtained deep images in the g'r'i'z' and JHKs
bands. Based on accurate seven-channel photometry covering the optical to
near-infrared wavelength range, we derive a photometric redshift of z = 0.94
+0.05 -0.10 for the unabsorbed host galaxy of GRB 071028B. In contrast, we show
that the afterglow with an intrinsic extinction of AV(SB) = (0.70 +/- 0.11) mag
is moderately absorbed and requires a relatively flat extinction curve.
According to the reported Swift/BAT observations, the energetics yield an
isotropic energy release of E(gamma,iso.,rest) = (1.4 +2.4 -0.7) x 10^51 erg.Comment: 8 pages, 5 figures, accepted for publication in A&
Optical and near-infrared follow-up observations of four Fermi/LAT GRBs : Redshifts, afterglows, energetics and host galaxies
Fermi can measure the spectral properties of gamma-ray bursts over a very
large energy range and is opening a new window on the prompt emission of these
energetic events. Localizations by the instruments on Fermi in combination with
follow-up by Swift provide accurate positions for observations at longer
wavelengths leading to the determination of redshifts, the true energy budget,
host galaxy properties and facilitate comparison with pre-Fermi bursts.
Multi-wavelength follow-up observations were performed on the afterglows of
four bursts with high energy emission detected by Fermi/LAT : GRB090323,
GRB090328, GRB090510 and GRB090902B. They were obtained in the
optical/near-infrared bands with GROND mounted at the MPG/ESO 2.2m telescope
and additionally of GRB090323 in the optical with the 2 m telescope in
Tautenburg, Germany. Three of the events are classified as long bursts while
GRB090510 is a well localized short GRB with GeV emission. In addition, host
galaxies were detected for three of the four bursts. Spectroscopic follow-up
was initiated with the VLT for GRB090328 and GRB090510. The afterglow
observations in 7 bands are presented for all bursts and their host galaxies
are investigated. Knowledge of the distance and the local dust extinction
enables comparison of the afterglows of LAT-detected GRBs with the general
sample. The spectroscopic redshifts of GRB090328 and GRB090510 were determined
to be z=0.7354+/-0.0003 and z=0.903 +/- 0.001 and dust corrected star-formation
rates of 4.8 Mdot yr^-1 and 0.60 M_dot yr^-1 were derived for their host
galaxies, respectively. The afterglows of long bursts exhibit power-law decay
indices alpha from less than 1 to ~2.3 and spectral indices (beta) values from
0.65 to ~1.2 which are fairly standard for GRB afterglows. Constraints are
placed on the jet half opening angles of less than 2.1 deg to greater than 6.4
deg which allows limits to be placed on the beaming corrected energies. These
range from less than 5x10^50 erg to the one of the highest values ever
recorded, greater than 2.2x10^52 erg for GRB090902B, and are not consistent
with a standard candle. The extremely energetic long Fermi bursts have optical
afterglows which lie in the top half of the brightness distribution of all
optical afterglows detected in the Swift era or even in the top 5% if
incompleteness is considered. The properties of the host galaxies of these LAT
detected bursts in terms of extinction, star formation rates and masses do not
appear to differ from previous samples.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 14
figures
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
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