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Sparre M.,Copenhagen University | Hartoog O.E.,University of Amsterdam | Kruhler T.,Copenhagen University | Kruhler T.,European Southern Observatory | And 30 more authors.
Astrophysical Journal | Year: 2014

Observations of the afterglows of long gamma-ray bursts (GRBs) allow the study of star-forming galaxies across most of cosmic history. Here we present observations of GRB 111008A, from which we can measure metallicity, chemical abundance patterns, dust-to-metals ratio (DTM), and extinction of the GRB host galaxy at z = 5.0. The host absorption system is a damped Lyα absorber with a very large neutral hydrogen column density of and a metallicity of [S/H] = -1.70 ± 0.10. It is the highest-redshift GRB with such a precise metallicity measurement. The presence of fine-structure lines confirms the z = 5.0 system as the GRB host galaxy and makes this the highest redshift where Fe II fine-structure lines have been detected. The afterglow is mildly reddened with A V = 0.11 ± 0.04 mag, and the host galaxy has a DTM that is consistent with being equal to or lower than typical values in the Local Group. © 2014. The American Astronomical Society. All rights reserved..

Schulze S.,University of Santiago de Chile | Schulze S.,Millennium Center for Supernova Science | Schulze S.,University of Iceland | Malesani D.,Copenhagen University | And 75 more authors.
Astronomy and Astrophysics | Year: 2014

Context. At low redshift, a handful of gamma-ray bursts (GRBs) have been discovered with luminosities that are substantially lower (Liso ≲ 1048.5 erg s-1) than the average of more distant ones (Liso ≳ 1049.5 erg s-1). It has been suggested that the properties of several low-luminosity (low-L) GRBs are due to shock break-out, as opposed to the emission from ultrarelativistic jets. This has led to much debate about how the populations are connected. Aims. The burst at redshift z = 0.283 from 2012 April 22 is one of the very few examples of intermediate-L GRBs with a γ-ray luminosity of Liso ∼ 1049.6-49.9 erg s-1 that have been detected up to now. With the robust detection of its accompanying supernova SN 2012bz, it has the potential to answer important questions on the origin of low- and high-L GRBs and the GRB-SN connection. Methods. We carried out a spectroscopy campaign using medium- and low-resolution spectrographs with 6-10-m class telescopes, which covered a time span of 37.3 days, and a multi-wavelength imaging campaign, which ranged from radio to X-ray energies over a duration of ∼270 days. Furthermore, we used a tuneable filter that is centred at Hα to map star-formation in the host and the surrounding galaxies. We used these data to extract and model the properties of different radiation components and fitted the spectral energy distribution to extract the properties of the host galaxy. Results. Modelling the light curve and spectral energy distribution from the radio to the X-rays revealed that the blast wave expanded with an initial Lorentz factor of Γ0 ∼ 50, which is a low value in comparison to high-L GRBs, and that the afterglow had an exceptionally low peak luminosity density of ≲ 2 × 1030 erg s-1 Hz-1 in the sub-mm. Because of the weak afterglow component, we were able to recover the signature of a shock break-out in an event that was not a genuine low-L GRB for the first time. At 1.4 hr after the burst, the stellar envelope had a blackbody temperature of kBT ∼ 16 eV and a radius of ∼7 × 1013 cm (both in the observer frame). The accompanying SN 2012bz reached a peak luminosity of MV = -19.7 mag, which is 0.3 mag more luminous than SN 1998bw. The synthesised nickel mass of 0.58 M⊙, ejecta mass of 5.87 M⊙, and kinetic energy of 4.10 × 1052 erg were among the highest for GRB-SNe, which makes it the most luminous spectroscopically confirmed SN to date. Nebular emission lines at the GRB location were visible, which extend from the galaxy nucleus to the explosion site. The host and the explosion site had close-to-solar metallicity. The burst occurred in an isolated star-forming region with an SFR that is 1/10 of that in the galaxy's nucleus. Conclusions. While the prompt γ-ray emission points to a high-L GRB, the weak afterglow and the low Γ0 were very atypical for such a burst. Moreover, the detection of the shock break-out signature is a new quality for high-L GRBs. So far, shock break-outs were exclusively detected for low-L GRBs, while GRB 120422A had an intermediate Liso of ∼1049.6-49.9 erg s-1. Therefore, we conclude that GRB 120422A was a transition object between low- and high-L GRBs, which supports the failed-jet model that connects low-L GRBs that are driven by shock break-outs and high-L GRBs that are powered by ultra-relativistic jets. © 2014 ESO.

Cano Z.,University of Iceland | De Ugarte Postigo A.,Institute Astrofisica Of Andalucia Iaa | De Ugarte Postigo A.,Niels Bohr Institute | Pozanenko A.,Space Research Institute | And 67 more authors.
Astronomy and Astrophysics | Year: 2014

We present optical and near-infrared (NIR) photometry for three gamma-ray burst supernovae (GRB-SNe): GRB 120729A, GRB 130215A/SN 2013ez, and GRB 130831A/SN 2013fu. For GRB 130215A/SN 2013ez, we also present optical spectroscopy at t - t0 = 16.1 d, which covers rest-frame 3000-6250 Å. Based on Fe ii λ5169 and Si ii λ6355, our spectrum indicates an unusually low expansion velocity of ~4000-6350 km s-1, the lowest ever measured for a GRB-SN. Additionally, we determined the brightness and shape of each accompanying SN relative to a template supernova (SN 1998bw), which were used to estimate the amount of nickel produced via nucleosynthesis during each explosion. We find that our derived nickel masses are typical of other GRB-SNe, and greater than those of SNe Ibc that are not associated with GRBs. For GRB 130831A/SN 2013fu, we used our well-sampled R-band light curve (LC) to estimate the amount of ejecta mass and the kinetic energy of the SN, finding that these too are similar to other GRB-SNe. For GRB 130215A, we took advantage of contemporaneous optical/NIR observations to construct an optical/NIR bolometric LC of the afterglow. We fit the bolometric LC with the millisecond magnetar model of Zhang & Mészáros (2001, ApJ, 552, L35), which considers dipole radiation as a source of energy injection to the forward shock powering the optical/NIR afterglow. Using this model we derive an initial spin period of P = 12 ms and a magnetic field of B = 1.1 × 1015 G, which are commensurate with those found for proposed magnetar central engines of other long-duration GRBs. © ESO, 2014.

De Ugarte Postigo A.,Institute Astrofisica Of Andalucia Iaa Csic | De Ugarte Postigo A.,Niels Bohr Institute | Thone C.C.,Institute Astrofisica Of Andalucia Iaa Csic | Rowlinson A.,University of Amsterdam | And 48 more authors.
Astronomy and Astrophysics | Year: 2014

Context. Short duration gamma-ray bursts (SGRBs) are thought to be related to the violent merger of compact objects, such as neutron stars or black holes, which makes them promising sources of gravitational waves. The detection of a "kilonova"-likesignature associated to the Swift-detected GRB 130603B has suggested that this event is the result of a compact object merger. Aims. Our knowledge on SGRB has been, until now, mostly based on the absence of supernova signatures and the analysis of the host galaxies to which they cannot always be securely associated. Further progress has been significantly hampered by the faintness and rapid fading of their optical counterparts (afterglows), which has so far precluded spectroscopy of such events. Afterglow spectroscopy is the key tool to firmly determine the distance at which the burst was produced, crucial to understand its physics, and study its local environment. Methods. Here we present the first spectra of a prototypical SGRB afterglow in which both absorption and emission features are clearly detected. Together with multi-wavelength photometry we study the host and environment of GRB 130603B. Results. From these spectra we determine the redshift of the burst to be z = 0.3565 ± 0.0002, measure rich dynamics both in absorption and emission, and a substantial line of sight extinction of AV = 0.86 ± 0.15 mag. The GRB was located at the edge of a disrupted arm of a moderately star forming galaxy with near-solar metallicity. Unlike for most long GRBs (LGRBs), NHX/AV is consistent with the Galactic ratio, indicating that the explosion site differs from those found in LGRBs. Conclusions. The merger is not associated with the most star-forming region of the galaxy; however, it did occur in a dense region, implying a rapid merger or a low natal kick velocity for the compact object binary. © ESO, 2014.

Kruhler T.,Copenhagen University | Ledoux C.,European Southern Observatory | Fynbo J.P.U.,Copenhagen University | Vreeswijk P.M.,Weizmann Institute of Science | And 34 more authors.
Astronomy and Astrophysics | Year: 2013

We present the discovery of molecular hydrogen (H2), including the presence of vibrationally-excited H2 in the optical spectrum of the afterglow of GRB 120815A at z = 2:36 obtained with X-shooter at the VLT. Simultaneous photometric broad-band data from GROND and X-ray observations by Swift/XRT place further constraints on the amount and nature of dust along the sightline. The galactic environment of GRB 120815A is characterized by a strong DLA with log(N(Hi)=cm-2) = 21:95 ± 0:10, prominent H 2 absorption in the Lyman-Werner bands (log(N(H2)=cm -2) = 20:54 ± 0:13) and thus a molecular gas fraction log f (H2) = -1:14 ± 0:15. The distance d between the absorbing neutral gas and GRB 120815A is constrained via photo-excitation modeling of fine-structure and meta-stable transitions of Fe ii and Ni ii to d = 0:5 ± 0:1 kpc. The DLA metallicity ([Zn=H] = -1:15 ± 0:12), visual extinction (AV 0:15 mag) and dust depletion ([Zn=Fe] = 1:01 ± 0:10) are intermediate between the values of well-studied, H2- deficient GRB-DLAs observed at high spectral resolution, and the approximately solar metallicity, highly-obscured and H2-rich GRB 080607 sightline. With respect to N(Hi), metallicity, as well as dust-extinction and depletion, GRB 120815A is fairly representative of the average properties of GRB-DLAs. This demonstrates that molecular hydrogen is present in at least a fraction of the more typical GRBDLAs, and H2 and H2 are probably more wide-spread among GRB-selected systems than the few examples of previous detections would suggest. Because H2 transitions are located redwards of the Lyman α absorption, H2 opens a second route for positive searches for molecular absorption also in GRB afterglows at lower redshifts and observed at lower spectral resolution. Further detections of molecular gas in GRB-DLAs would allow statistical studies, and, coupled with host follow-up and sub-mm spectroscopy, provide unprecedented insights into the process and conditions of star-formation at high redshift. © ESO, 2013.

Levan A.J.,University of Warwick | Tanvir N.R.,University of Leicester | Starling R.L.C.,University of Leicester | Wiersema K.,University of Leicester | And 42 more authors.
Astrophysical Journal | Year: 2014

We present comprehensive multiwavelength observations of three gamma-ray bursts (GRBs) with durations of several thousand seconds. We demonstrate that these events are extragalactic transients; in particular, we resolve the long-standing conundrum of the distance of GRB 101225A (the "Christmas-day burst"), finding it to have a redshift z = 0.847 and showing that two apparently similar events (GRB 111209A and GRB 121027A) lie at z = 0.677 and z = 1.773, respectively. The systems show extremely unusual X-ray and optical light curves, very different from classical GRBs, with long-lasting, highly variable X-ray emission and optical light curves that exhibit little correlation with the behavior seen in the X-ray. Their host galaxies are faint, compact, and highly star-forming dwarf galaxies, typical of "blue compact galaxies." We propose that these bursts are the prototypes of a hitherto largely unrecognized population of ultra-long GRBs, which while observationally difficult to detect may be astrophysically relatively common. The long durations may naturally be explained by the engine-driven explosions of stars of much larger radii than normally considered for GRB progenitors, which are thought to have compact Wolf-Rayet progenitor stars. However, we cannot unambiguously identify supernova signatures within their light curves or spectra. We also consider the alternative possibility that they arise from the tidal disruption of stars by massive black holes and conclude that the associated timescales are only consistent with the disruption of compact stars (e.g., white dwarfs) by black holes of relatively low mass (<105 M). © 2014. The American Astronomical Society. All rights reserved.

Xu D.,Copenhagen University | De Ugarte Postigo A.,Copenhagen University | De Ugarte Postigo A.,Institute Astrofisica Of Andalucia | Leloudas G.,Copenhagen University | And 30 more authors.
Astrophysical Journal | Year: 2013

Long-duration gamma-ray bursts (GRBs) at z < 1 are found in most cases to be accompanied by bright, broad-lined Type Ic supernovae (SNe Ic-BL). The highest-energy GRBs are mostly located at higher redshifts, where the associated SNe are hard to detect observationally. Here, we present early and late observations of the optical counterpart of the very energetic GRB 130427A. Despite its moderate redshift, z = 0.3399 ± 0.0002, GRB 130427A is at the high end of the GRB energy distribution, with an isotropic-equivalent energy release of E iso ∼ 9.6 × 1053 erg, more than an order of magnitude more energetic than other GRBs with spectroscopically confirmed SNe. In our dense photometric monitoring, we detect excess flux in the host-subtracted r-band light curve, consistent with that expected from an emerging SN, ∼0.2 mag fainter than the prototypical SN 1998bw. A spectrum obtained around the time of the SN peak (16.7 days after the GRB) reveals broad undulations typical of SNe Ic-BL, confirming the presence of an SN, designated SN 2013cq. The spectral shape and early peak time are similar to those of the high expansion velocity SN 2010bh associated with GRB 100316D. Our findings demonstrate that high-energy, long-duration GRBs, commonly detected at high redshift, can also be associated with SNe Ic-BL, pointing to a common progenitor mechanism. © 2013. The American Astronomical Society. All rights reserved.

Bauer F.E.,University of Santiago de Chile | Bauer F.E.,Space Science Institute | Bauer F.E.,Millennium Center for Supernova Science | Zelaya P.,University of Santiago de Chile | And 4 more authors.
Proceedings of the International Astronomical Union | Year: 2011

We report results for two epochs of spectropolarimetry on the luminous type IIn SN2010jl, taken at ≈36 and 85 days post-explosion with VLT FORS2-PMOS. The high signal-to-noise data demonstrate distinct evolution in the continuum and the broad lines point to a complex origin for the various emission components and to a potentially common polarization signal for the type IIn class even over 1-2 orders of magnitude in luminosity output. © 2012 International Astronomical Union.

Cano Z.,University of Iceland | Maeda K.,Kyoto University | Maeda K.,University of Tokyo | Schulze S.,University of Santiago de Chile | Schulze S.,Millennium Center for Supernova Science
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present the results of modelling archival observations of Type Ib SN 1999dn. In the spectra, two He . i absorption features are seen: a slower component with larger opacity, and a more rapid He . i component with smaller opacity. Complementary results are obtained from modelling the bolometric light curve of SN 1999dn, where a two-zone model (dense inner region, and less dense outer region) provides a much better fit than a one-zone model. A key result we find is that roughly equal amounts of radioactive material are found in both regions. The two-zone analytical model provides a more realistic representation of the structure of the ejecta, including mixing and asymmetries, which offers a physical explanation for how the radioactive material is propelled to, and mixed within, the outer regions. Our result supports the theoretical expectation that the radioactive content in the outflow of a Type Ib supernova (SN) is thoroughly mixed. We fit our model to six additional SNe Ibc, of which the majority of the SNe Ib are best described by the two-zone model, and the majority of the SNe Ic by the one-zone model. Of the SNe Ic, only SN 2007gr was best fitted by the two-zone model, indicating that the lack of helium spectral features for this event cannot be attributed to poor mixing. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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