Previous studies have shown that the incidence rate of intervening strong Mg ii absorbers towards gamma-ray bursts (GRBs) were a factor of 2–4 higher than towards quasars. Exploring the similar sized ...and uniformly selected legacy data sets XQ-100 and XSGRB, each consisting of 100 quasar and 81 GRB afterglow spectra obtained with a single instrument (VLT/X-shooter), we demonstrate that there is no disagreement in the number density of strong Mg ii absorbers with rest-frame equivalent widths \hbox{$W_{\rm r}^{\lambda2796}>1$}Wrλ2796>1 Å towardsGRBs and quasars in the redshift range 0.1 ≲ z ≲ 5. With large and similar sample sizes, and path length coverages of Δz = 57.8 and 254.4 for GRBs and quasars, respectively, the incidences of intervening absorbers are consistent within 1σ uncertainty levels at all redshifts. For absorbers at z < 2.3, the incidence towards GRBs is a factor of 1.5 ± 0.4 higher than the expected number of strong Mg ii absorbers in Sloan Digital Sky Survey (SDSS) quasar spectra, while for quasar absorbers observed with X-shooter we find an excess factor of 1.4 ± 0.2 relative to SDSS quasars. Conversely, the incidence rates agree at all redshifts with reported high-spectral-resolution quasar data, and no excess is found. The only remaining discrepancy in incidences is between SDSS Mg ii catalogues and high-spectral-resolution studies. The rest-frame equivalent-width distribution also agrees to within 1σ uncertainty levels between the GRB and quasar samples. Intervening strong Mg ii absorbers towards GRBs are therefore neither unusually frequent, nor unusually strong.
Abstract
We here present the spectroscopic follow-up observations with VLT/X-shooter of the Swift long-duration gamma-ray burst GRB 160804A at z = 0.737. Typically, GRBs are found in low-mass, ...metal-poor galaxies that constitute the sub-luminous population of star-forming galaxies. For the host galaxy of the GRB presented here, we derive a stellar mass of log (M*/ M⊙) = 9.80 ± 0.07, a roughly solar metallicity (12 + log (O/H) = 8.74 ± 0.12) based on emission line diagnostics, and an infrared luminosity of M3.6/(1 + z) = −21.94 mag, but find it to be dust-poor (E(B − V) < 0.05 mag). This establishes the galaxy hosting GRB 160804A as one of the most luminous, massive and metal-rich GRB hosts at z < 1.5. Furthermore, the gas-phase metallicity is found to be representative of the physical conditions of the gas close to the explosion site of the burst. The high metallicity of the host galaxy is also observed in absorption, where we detect several strong Fe ii transitions as well as Mg ii and Mg i. Although host galaxy absorption features are common in GRB afterglow spectra, we detect absorption from strong metal lines directly in the host continuum (at a time when the afterglow was contributing to < 15 per cent). Finally, we discuss the possibility that the geometry and state of the absorbing and emitting gas are indicative of a galactic scale outflow expelled at the final stage of two merging galaxies.
Context. Long gamma-ray bursts (GRBs) are produced during the dramatic deaths of massive stars with very short lifetimes, meaning that they explode close to the birth place of their progenitors. Over ...a short period they become the most luminous objects observable in the Universe, being perfect beacons to study high-redshift star-forming regions. Aims. We aim to use the afterglow of GRB 161023A at a redshift z = 2.710 as a background source to study the environment of the explosion and the intervening systems along its line of sight. Methods. For the first time, we complement ultraviolet (UV), optical and near-infrared (NIR) spectroscopy with millimetre spectroscopy using the Atacama Large Millimeter Array (ALMA), which allows us to probe the molecular content of the host galaxy. The X-shooter spectrum shows a plethora of absorption features including fine-structure and metastable transitions of Fe, Ni, Si, C, and O. We present photometry ranging from 43 s to over 500 days after the burst. Results. We infer a host-galaxy metallicity of Zn/H = −1.11 ± 0.07, which, corrected for dust depletion, results in X/H = −0.94 ± 0.08. We do not detect molecular features in the ALMA data, but we derive limits on the molecular content of log(NCO/cm−2) < 15.7 and log(NHCO+/cm−-12, which are consistent with those that we obtain from the optical spectra, log(NH2/cm−2)< 15.2 and log(NCO/cm−2) < 14.5. Within the host galaxy, we detect three velocity systems through UV, optical and NIR absorption spectroscopy, all with levels that were excited by the GRB afterglow. We determine the distance from these systems to the GRB to be in the range between 0.7 and 1.0 kpc. The sight line to GRB 161023A shows nine independent intervening systems, most of them with multiple components. Conclusions. Although no molecular absorption was detected for GRB 161023A, we show that GRB millimetre spectroscopy is now feasible and is opening a new window on the study of molecular gas within star-forming galaxies at all redshifts. The most favoured lines of sight for this purpose will be those with high metallicity and dust.
Long-duration γ-ray bursts (GRBs) originate from ultra-relativistic jets launched from
the collapsing cores of dying massive stars. They are characterized by an initial phase
of bright and highly ...variable radiation in the kiloelectronvolt-to-megaelectronvolt band, which is probably produced within the jet and lasts from milliseconds to minutes, known as the prompt emission. Subsequently, the interaction of the jet with the surrounding medium generates shock waves that are responsible for the afterglow emission, which lasts from days to months and occurs over a broad energy range from the radio to the gigaelectronvolt bands. The afterglow emission is generally well explained as synchrotron radiation emitted by electrons accelerated by the external shock. Recently, intense long-lasting emission between 0.2 and 1 teraelectronvolts was observed from GRB 190114C. Here we report multifrequency observations of GRB 190114C, and study the evolution in time of the GRB emission across 17 orders of magnitude in energy, from 5 × 10^(−6) to 10^(12) electronvolts. We find that the broadband spectral energy distribution is double-peaked, with the teraelectronvolt emission constituting a distinct spectral component with power comparable to the synchrotron component. This component is associated with the afterglow and is satisfactorily explained by inverse Compton up-scattering of synchrotron photons by high-energy electrons. We find that the conditions required to account for the observed teraelectronvolt component are typical for GRBs, supporting the possibility that inverse Compton emission is commonly produced in GRBs.
GRB hosts through cosmic time Krühler, T.; Malesani, D.; Fynbo, J. P. U. ...
Astronomy and astrophysics (Berlin),
09/2015, Letnik:
581
Journal Article
Recenzirano
Odprti dostop
We present data and initial results from VLT/X-Shooter emission-line spectroscopy of 96 galaxies selected by long γ-ray bursts (GRBs) at 0.1 <z< 3.6, the largest sample of GRB host spectra available ...to date. Most of our GRBs were detected by Swift and 76% are at 0.5 <z< 2.5 with a median zmed ~ 1.6. Based on Balmer and/or forbidden lines of oxygen, nitrogen, and neon, we measure systemic redshifts, star formation rates (SFR), visual attenuations (AV), oxygen abundances (12 + log (O/H)), and emission-line widths (σ). We study GRB hosts up to z ~ 3.5 and find a strong change in their typical physical properties with redshift. The median SFR of our GRB hosts increases from SFRmed ~ 0.6 M⊙ yr-1 at z ~ 0.6 up to SFRmed ~ 15 M⊙ yr-1 at z ~ 2. A higher ratio of O iii/O ii at higher redshifts leads to an increasing distance of GRB-selected galaxies to the locus of local galaxies in the Baldwin-Phillips-Terlevich diagram. There is weak evidence for a redshift evolution in AV and σ, with the highest values seen at z ~ 1.5 (AV) or z ~ 2 (σ). Oxygen abundances of the galaxies are distributed between 12 + log (O/H) = 7.9 and 12 + log (O/H) = 9.0 with a median 12 + log (O/H)med ~ 8.5. The fraction of GRB-selected galaxies with super-solar metallicities is ~20% at z< 1 in the adopted metallicity scale. This is significantly less than the fraction of total star formation in similar galaxies, illustrating that GRBs are scarce in high metallicity environments. At z ~ 3, sensitivity limits us to probing only the most luminous GRB hosts for which we derive metallicities of Z ≲ 0.5 Z⊙. Together with a high incidence of Z ~ 0.5 Z⊙ galaxies at z ~ 1.5, this indicates that a metallicity dependence at low redshift will not be dominant at z ~ 3. Significant correlations exist between the hosts’ physical properties. Oxygen abundance, for example, relates to AV (12 + log (O/H) ∝ 0.17·AV), line width (12 + log (O/H) ∝ σ0.6), and SFR (12 + log (O/H) ∝ SFR0.2). In the last two cases, the normalization of the relations shift to lower metallicities at z> 2 by ~0.4 dex. These properties of GRB hosts and their evolution with redshift can be understood in a cosmological context of star-forming galaxies and a picture in which the hosts’ properties at low redshift are influenced by the tendency of GRBs to avoid the most metal-rich environments.
Long γ-ray bursts are associated with energetic, broad-lined, stripped-envelope supernovae
and as such mark the death of massive stars. The scarcity of such events nearby and the brightness of the ...γ-ray burst afterglow, which dominates the emission in the first few days after the burst, have so far prevented the study of the very early evolution of supernovae associated with γ-ray bursts
. In hydrogen-stripped supernovae that are not associated with γ-ray bursts, an excess of high-velocity (roughly 30,000 kilometres per second) material has been interpreted as a signature of a choked jet, which did not emerge from the progenitor star and instead deposited all of its energy in a thermal cocoon
. Here we report multi-epoch spectroscopic observations of the supernova SN 2017iuk, which is associated with the γ-ray burst GRB 171205A. Our spectra display features at extremely high expansion velocities (around 115,000 kilometres per second) within the first day after the burst
. Using spectral synthesis models developed for SN 2017iuk, we show that these features are characterized by chemical abundances that differ from those observed in the ejecta of SN 2017iuk at later times. We further show that the high-velocity features originate from the mildly relativistic hot cocoon that is generated by an ultra-relativistic jet within the γ-ray burst expanding and decelerating into the medium that surrounds the progenitor star
. This cocoon rapidly becomes transparent
and is outshone by the supernova emission, which starts to dominate the emission three days after the burst.
Context.
Damped Lyman-
α
(DLA) absorption-line systems at the redshifts of gamma-ray burst (GRB) afterglows offer a unique way to probe the physical conditions within star-forming galaxies in the ...early Universe.
Aims.
Here we built up a large sample of 22 GRBs at redshifts
z
> 2 observed with VLT/X-shooter in order to determine the abundances of hydrogen, metals, dust, and molecular species. This allows us to study the metallicity and dust depletion effects in the neutral interstellar medium at high redshift and to answer the question of whether (and why) there might be a lack of H
2
in GRB-DLAs.
Methods.
We developed new methods based on the Bayesian inference package, PyMC, to F
IT
absorption lines and measure the column densities of different metal species as well as atomic and molecular hydrogen. The derived relative abundances are used to F
IT
dust depletion sequences and determine the dust-to-metals ratio and the host-galaxy intrinsic visual extinction. Additionally, we searched for the absorption signatures of vibrationally-excited H
2
and carbon monoxide.
Results.
We find that there is no lack of H
2
-bearing GRB-DLAs. We detect absorption lines from molecular hydrogen in 6 out of 22 GRB afterglow spectra, with molecular fractions ranging between
f
≃ 5 × 10
−5
and
f
≃ 0.04, and claim tentative detections in three additional cases. For the remainder of the sample, we measure, depending on S/N, spectral coverage and instrumental resolution, more or less stringent upper limits. The GRB-DLAs in our sample have on average low metallicities, X/H
¯
≈ −1.3, comparable to the population of extremely-strong QSO-DLAs (log
N
(H
I
) > 21.5). Furthermore, H
2
-bearing GRB-DLAs are found to be associated with significant dust extinction,
A
V
> 0.1 mag, and dust-to-metals ratios
DTM
> 0.4, confirming the importance of dust grains for the production of molecules. All these systems exhibit neutral hydrogen column densities log
N
(H
I
) > 21.7. The overall fraction of H
2
detections in GRB-DLAs is ≥ 27% (41% including tentative detections), which is three to four times larger than in the general QSO-DLA population. For 2 <
z
< 4, and considering column densities log
N
(H
I
) > 21.7, the H
2
detection fraction is 60–80% in GRB-DLAs and in extremely strong QSO-DLAs. This is likely due to the fact that both GRB- and QSO-DLAs with high neutral hydrogen column densities are probed by sight-lines with small impact parameters, indicating that the absorbing gas is associated with the inner regions of the absorbing galaxy, where the gas pressure is higher and the conversion of H
I
to H
2
takes place. In the case of GRB hosts, this diffuse molecular gas is located at distances ≳ 500 pc from the GRB and hence is unrelated to the star-forming region where the event occurred.
In this work we present spectra of all γ-ray burst (GRB) afterglows that have been promptly observed with the X-shooter spectrograph until 31/03/2017. In total, we have obtained spectroscopic ...observations of 103 individual GRBs observed within 48 hours of the GRB trigger. Redshifts have been measured for 97 per cent of these, covering a redshift range from 0.059 to 7.84. Based on a set of observational selection criteria that minimise biases with regards to intrinsic properties of the GRBs, the follow-up effort has been focused on producing a homogeneously selected sample of 93 afterglow spectra for GRBs discovered by the Swift satellite. We here provide a public release of all the reduced spectra, including continuum estimates and telluric absorption corrections. For completeness, we also provide reductions for the 18 late-time observations of the underlying host galaxies. We provide an assessment of the degree of completeness with respect to the parent GRB population, in terms of the X-ray properties of the bursts in the sample and find that the sample presented here is representative of the full Swift sample. We have constrained the fraction of dark bursts to be <28 per cent and confirm previous results that higher optical darkness is correlated with increased X-ray absorption. For the 42 bursts for which it is possible, we have provided a measurement of the neutral hydrogen column density, increasing the total number of published HI column density measurements by ∼33 per cent. This dataset provides a unique resource to study the ISM across cosmic time, from the local progenitor surroundings to the intervening Universe.
Whether stars could have driven the reionization of the intergalactic medium depends critically on the proportion of ionizing radiation that escapes the galaxies in which it is produced. Spectroscopy ...of gamma-ray burst (GRB) afterglows can be used to estimate the opacity to extreme ultraviolet (EUV) radiation along the lines-of-sight to the bursts. Assuming that long-duration GRBs trace the locations of the massive stars dominating EUV production, the average escape fraction of ionizing radiation can be calculated independently of galaxy size or luminosity. Here we present a compilation of HI column density (N(HI)) measures for 140 GRBs in the range 1.6 < z <6.7. Although the sample is heterogeneous, in terms of spectral resolution and signal-to-noise ratio, fits to the Ly α absorption line provide robust constraints on N(HI), even for spectra of insufficient quality for other purposes. Thus we establish an escape fraction at the Lyman limit of {f(esc)} ≈ 0.005, with a 98 per cent confidence upper limit of {f(esc)} ≈ 0.015. This analysis suggests that stars provide a small contribution to the ionizing radiation budget at z < 5. At higher redshifts firm conclusions are limited by the small size of the GRB sample (7/140), but any decline in average HI column density seems to be modest. We also find no significant correlation of N(HI) with galaxy UV luminosity or host stellar mass. We discuss in some detail potential biases and argue that, while not negligible, systematic errors in f(esc) are unlikely to be more than a factor ∼2 in either direction, and so would not affect the primary conclusions. Given that many GRB hosts are low-metallicity dwarf galaxies with high specific star-formation rates, these results present a particular problem for the hypothesis that such galaxies dominated the reionization of the Universe.
Context. Super-luminous supernovae (SLSNe) are rare events defined as being significantly more luminous than normal terminal stellar explosions. The source of the additional power needed to achieve ...such luminosities is still unclear. Discoveries in the local Universe (i.e. z < 0.1) are scarce, but afford dense multi-wavelength observations. Additional low-redshift objects are therefore extremely valuable. Aims. We present early-time observations of the type I SLSN ASASSN-18km/SN 2018bsz. These data are used to characterise the event and compare to literature SLSNe and spectral models. Host galaxy properties are also analysed. Methods. Optical and near-IR photometry and spectroscopy were analysed. Early-time ATLAS photometry was used to constrain the rising light curve. We identified a number of spectral features in optical-wavelength spectra and track their time evolution. Finally, we used archival host galaxy photometry together with H II region spectra to constrain the host environment. Results. ASASSN-18km/SN 2018bsz is found to be a type I SLSN in a galaxy at a redshift of 0.0267 (111 Mpc), making it the lowest-redshift event discovered to date. Strong C II lines are identified in the spectra. Spectral models produced by exploding a Wolf-Rayet progenitor and injecting a magnetar power source are shown to be qualitatively similar to ASASSN-18km/SN 2018bsz, contrary to most SLSNe-I that display weak or non-existent C II lines. ASASSN-18km/SN 2018bsz displays a long, slowly rising, red “plateau” of >26 days, before a steeper, faster rise to maximum. The host has an absolute magnitude of –19.8 mag (r), a mass of M⋆ = 1.5−0.33+0.08 × 109 M⊙ M ⋆ = 1.5 − 0.33 + 0.08 × 10 9 M ⊙ $ M_\star=1.5_{-0.33}^{+0.08}\times10^9M_\odot $ , and a star formation rate of = 0.50−0.19+2.22 M⊙ yr −1 = 0.50 − 0.19 + 2.22 M ⊙ yr − 1 $ =0.50_{-0.19}^{+2.22}M_\odot\text{yr}^{-1} $ . A nearby H II region has an oxygen abundance (O3N2) of 8.31 ± 0.01 dex.
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