arXiv subjects

J. Japelj

Publications and source records attributed to J. Japelj.

At least 19 recordsLinked to original sources

arXiv2026

SN 2024afyu interpreted as a Pair Instability Supernova

Pair-instability supernovae (PISNe) are the predicted explosions of very massive stars triggered by electron-positron pair production. Numerous transients have been proposed as PISN candidates, yet none has provided unambiguous confirmation of this explosion mechanism. The predicted strengths of nebular emission lines offer a powerful means of testing the PISN scenario. We investigate the nature of SN 2024afyu, a nearby (z = 0.0085), long-lived (trise = 85 +- 11.7 days) SN with peculiar spectral evolution, with the aim of identifying its powering mechanism. We analyse multi-band photometry and optical and near-infrared spectroscopy from shortly after explosion to the nebular phase (around 500 days past peak). Besides early appearance of [Ca II] features, we identify a number of sulfur and silicon emission lines, for which we estimate electron temperatures and elemental masses. SN 2024afyu has an inferred 56Ni mass of around 0.4 to 1.0 solar masses and an inferred sulfur mass of the order of 3 solar masses, substantially larger than expected for conventional core-collapse explosions. SN 2024afyu is photometrically similar (although fainter, M_Peak(r) = -18.9 +- 0.04 mag) but spectroscopically distinct to other proposed PISNe. Yet, existing PISN models broadly reproduce several key characteristics, including the overall spectral appearance and broad photometric evolution. SN 2024afyu is a strong PISN candidate, since alternative scenarios would struggle to explain the combination of broad light curve, large intermediate-mass-element abundance, and general spectroscopic evolution. The discrepancies between the observations and currently available theoretical models highlight the need for new PISN calculations spanning a wider range of progenitor masses, metallicities, mixing prescriptions, and circumstellar environments.

astro-ph.SR↗

arXiv2024

Rapid Response Mode observations of GRB 160203A: Looking for fine-structure line variability at z=3.52

Gamma-ray bursts are the most energetic known explosions. Despite fading rapidly, they allow to measure redshift and important properties of their host-galaxies. We report the photometric and spectroscopic study of GRB 160203A and its host-galaxy. Fine-structure absorption lines, detected in the afterglow at different epochs, allow us to investigate variability due to the strong fading background source. We obtained two optical to near-infrared spectra of the afterglow with X-shooter on ESO/VLT, 18 min and 5.7 hrs after the burst, allowing us to investigate temporal changes of fine-structure absorption lines. We measured HI column density log N(HI/cm-2)=21.75+/-0.10, and several heavy-element ions along the GRB sight-line in the host-galaxy: SiII,AlII,AlIII,CII,NiII,SiIV,CIV,ZnII,FeII, and FeII and SiII fine structure transitions from energetic levels excited by the afterglow, at a redshift z=3.518. We measured [M/H]TOT=-0.78+/-0.13 and [Zn/Fe]FIT=0.69+/-0.15, representing the total(dust-corrected) metallicity and dust depletion, respectively. We detected additional intervening systems along the line of sight at z=1.03,z=1.26,z=1.98,z=1.99,z=2.20 and z=2.83. We could not measure significant variability in the fine-structure lines throughout all the observations and determined an upper limit for the GRB distance from the absorber of d<300 pc, adopting the canonical UV pumping scenario. However, we note that the quality of our data is not sufficient to conclusively rule out collisions as an alternative mechanism. GRB 160203A belongs to a growing sample of GRBs with medium resolution spectroscopy, provided by the Swift/X-shooter legacy program, which enables detailed investigation of the interstellar medium in high-redshift GRB host-galaxies. In particular, this host galaxy shows relatively high metal enrichment and dust depletion already in place when the universe was only 1.8 Gyr old.

astro-ph.HE↗

arXiv2021

The supernova of the MAGIC GRB190114C

We observed GRB190114C (redshift z = 0.4245), the first GRB ever detected at TeV energies, at optical and near-infrared wavelengths with several ground-based telescopes and the Hubble Space Telescope, with the primary goal of studying its underlying supernova, SN2019jrj. The monitoring spanned the time interval between 1.3 and 370 days after the burst, in the observer frame. We find that the afterglow emission can be modelled with a forward shock propagating in a uniform medium modified by time-variable extinction along the line of sight. A jet break could be present after 7 rest-frame days, and accordingly the maximum luminosity of the underlying SN ranges between that of stripped-envelope corecollapse supernovae (SNe) of intermediate luminosity, and that of the luminous GRB-associated SN2013dx. The observed spectral absorption lines of SN2019jrj are not as broad as in classical GRB-SNe, and are rather more similar to those of less-luminous core-collapse SNe. Taking the broad-lined stripped-envelope core-collapse SN2004aw as an analogue, we tentatively derive the basic physical properties of SN2019jrj. We discuss the possibility that a fraction of the TeV emission of this source might have had a hadronic origin and estimate the expected high-energy neutrino detection level with IceCube.

astro-ph.HE↗

arXiv2021

Gaia Photometric Science Alerts

Since July 2014, the Gaia mission has been engaged in a high-spatial-resolution, time-resolved, precise, accurate astrometric, and photometric survey of the entire sky. Aims: We present the Gaia Science Alerts project, which has been in operation since 1 June 2016. We describe the system which has been developed to enable the discovery and publication of transient photometric events as seen by Gaia. Methods: We outline the data handling, timings, and performances, and we describe the transient detection algorithms and filtering procedures needed to manage the high false alarm rate. We identify two classes of events: (1) sources which are new to Gaia and (2) Gaia sources which have undergone a significant brightening or fading. Validation of the Gaia transit astrometry and photometry was performed, followed by testing of the source environment to minimise contamination from Solar System objects, bright stars, and fainter near-neighbours. Results: We show that the Gaia Science Alerts project suffers from very low contamination, that is there are very few false-positives. We find that the external completeness for supernovae, $C_E=0.46$, is dominated by the Gaia scanning law and the requirement of detections from both fields-of-view. Where we have two or more scans the internal completeness is $C_I=0.79$ at 3 arcsec or larger from the centres of galaxies, but it drops closer in, especially within 1 arcsec. Conclusions: The per-transit photometry for Gaia transients is precise to 1 per cent at $G=13$, and 3 per cent at $G=19$. The per-transit astrometry is accurate to 55 milliarcseconds when compared to Gaia DR2. The Gaia Science Alerts project is one of the most homogeneous and productive transient surveys in operation, and it is the only survey which covers the whole sky at high spatial resolution (subarcsecond), including the Galactic plane and bulge.

astro-ph.IM↗

arXiv2021

Gamma-ray bursts as probes of high-redshift Lyman-alpha emitters and radiative transfer models

We present the updated census and statistics of Lyman-$α$ emitting long gamma-ray bursts host galaxies (LAE-LGRBs). We investigate the properties of a sub-sample of LAE-LGRBs and test the shell model commonly used to fit Lyman-$α$ (Ly$α$) emission line spectra. Among the LAE-LGRBs detected to date, we select a golden sample of four LAE-LGRBs allowing us to retrieve information on the host galaxy properties and of its interstellar medium gas. We fit their Ly$α$ spectra using the shell model, and constrain its parameters with the observed values. From the comparison of the statistics and properties of LAE-LGRBs to those of LAE samples in the literature, we find evidences of Ly$α$ suppression in dusty systems, and a fraction of LAE-LGRBs among the overall LGRB hosts lower than that found for Lyman-break galaxy (LBG) samples at similar redshift range. However, we find that LAE-LGRBs are representative of Ly$α$ emission from the bulk of UV-selected galaxies at z~2. We find that the golden sample of LAE-LGRBs are complex systems characterized by multiple emission blobs and by signs of possible galaxy interactions. The fitting procedure fails in recovering the HI column densities (NHI) measured from the afterglow spectra, and the other properties described by the shell-model parameters in the cases with very high NHI. The afterglows of most LGRBs and LAE-LGRBs show high NHI, implying that statistically the bulk of Ly$α$ photons expected to be produced by massive stars in the star-forming region hosting the GRB will be surrounded by such opaque lines of sight. We interpret our results in the context of more sophisticated models and of different dominant Ly$α$ emitting regions. We also compare LAE-LGRBs to LAE Lyman continuum (LyC) leakers in the literature in terms of properties identified as possible indirect indicators of LyC leakage. [Abridged]

astro-ph.GA↗

arXiv2020

Lyman continuum leakage in faint star-forming galaxies at redshift z=3-3.5 probed by gamma-ray bursts

We present the observations of Lyman continuum (LyC) emission in the afterglow spectra of GRB 191004B at $z=3.5055$, together with those of the other two previously known LyC-emitting long gamma-ray bursts (LGRB) (GRB 050908 at $z=3.3467$, and GRB 060607A at $z=3.0749$), to determine their LyC escape fraction and compare their properties. From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of $\log(N_{\rm HI}/cm^{-2})= 17.2 \pm 0.15$, and negligible extinction ($A_{\rm V}=0.03 \pm 0.02$ mag). The only metal absorption lines detected are CIV and SiIV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Ly$α$ emission. From its Ly$α$ emission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to $1 \leq$ SFR $\leq 4.7$ M$_{\odot}\ yr^{-1}$. We fit the Ly$α$ emission with a shell model and find parameters values consistent with the observed ones. The absolute LyC escape fractions we find for GRB 191004B, GRB 050908 and GRB 060607A are of $0.35^{+0.10}_{-0.11}$, $0.08^{+0.05}_{-0.04}$ and $0.20^{+0.05}_{-0.05}$, respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRB presented here have all $M_{\rm 1600} > -19.5$ mag, with the GRB 060607A host at $M_{\rm 1600} > -16$ mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore the time investment is very small compared to galaxy studies. [Abridged]

astro-ph.GA↗

arXiv2020

Observational constraints on the optical and near-infrared emission from the neutron star-black hole binary merger S190814bv

On 2019 August 14, the LIGO and Virgo interferometers detected a high-significance event labelled S190814bv. Preliminary analysis of the GW data suggests that the event was likely due to the merger of a compact binary system formed by a BH and a NS. ElectromagNetic counterparts of GRAvitational wave sources at the VEry Large Telescope (ENGRAVE) collaboration members carried out an intensive multi-epoch, multi-instrument observational campaign to identify the possible optical/near infrared counterpart of the event. In addition, the ATLAS, GOTO, GRAWITA-VST, Pan-STARRS and VINROUGE projects also carried out a search on this event. Our observations allow us to place limits on the presence of any counterpart and discuss the implications for the kilonova (KN) possibly generated by this NS-BH merger, and for the strategy of future searches. Altogether, our observations allow us to exclude a KN with large ejecta mass $M\gtrsim 0.1\,\mathrm{M_\odot}$ to a high ($>90\%$) confidence, and we can exclude much smaller masses in a subsample of our observations. This disfavours the tidal disruption of the neutron star during the merger. Despite the sensitive instruments involved in the campaign, given the distance of S190814bv we could not reach sufficiently deep limits to constrain a KN comparable in luminosity to AT 2017gfo on a large fraction of the localisation probability. This suggests that future (likely common) events at a few hundreds Mpc will be detected only by large facilities with both high sensitivity and large field of view. Galaxy-targeted observations can reach the needed depth over a relevant portion of the localisation probability with a smaller investment of resources, but the number of galaxies to be targeted in order to get a fairly complete coverage is large, even in the case of a localisation as good as that of this event.

astro-ph.SR↗

arXiv2020

VLT/X-shooter spectroscopy of massive young stellar objects in the 30 Doradus region of the Large Magellanic Cloud

The process of massive star ($M\geq8~M_\odot$) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud (LMC). We took advantage of the spectral resolution and wavelength coverage of X-shooter (300-2500 nm), mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect H$α,β$, [O I] 630.0 nm, Ca II infrared triplet, [Fe II] 1643.5 nm, fluorescent Fe II 1687.8 nm, H$_2$ 2121.8 nm, Br$γ$, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of 10 candidates as bona fide MYSOs. We compare our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We compute lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Br$γ$ luminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features such as fluorescent Fe II and CO bandheads which is consistent with the Galactic rate (40\%). Most of our MYSOs show outflow features.

astro-ph.SR↗

arXiv2019

GRB 190114C in the nuclear region of an interacting galaxy -- A detailed host analysis using ALMA, HST and VLT

GRB 190114C is the first GRB for which the detection of very-high energy emission up to the TeV range has been reported. It is still unclear whether environmental properties might have contributed to the production of these very high-energy photons, or if it is solely related to the released GRB emission. The relatively low redshift of the GRB (z=0.425) allows us to study the host galaxy of this event in detail, and to potentially identify idiosyncrasies that could point to progenitor characteristics or environmental properties responsible for such a unique event. We use ultraviolet, optical, infrared and submillimetre imaging and spectroscopy obtained with HST, VLT and ALMA to obtain an extensive dataset on which the analysis of the host galaxy is based. The host system is composed of a close pair of interacting galaxies (Delta v = 50 km s^-1), both of which are well-detected by ALMA in CO(3-2). The GRB occurred within the nuclear region (~170 pc from the centre) of the less massive but more star-forming galaxy of the pair. The host is more massive (log(M/M_odot)=9.3) than average GRB hosts at that redshift and the location of the GRB is rather unique. The enhanced star-formation rate was probably triggered by tidal interactions between the two galaxies. Our ALMA observations indicate that both host galaxy and companion have a high molecular gas fraction, as has been observed before in interacting galaxy pairs. The location of the GRB within the core of an interacting galaxy with an extinguished line-of-sight is indicative of a denser environment than typically observed for GRBs and could have been crucial for the generation of the very-high-energy photons that were observed.

astro-ph.HE↗

arXiv2019

Simulating MOS science on the ELT: Ly$α$ forest tomography

Mapping of the large-scale structure through cosmic time has numerous applications in the studies of cosmology and galaxy evolution. At $z > 2$, the structure can be traced by the neutral intergalactic medium (IGM) by way of observing the Ly$α$, forest towards densely-sampled lines-of-sight of bright background sources, such as quasars and star forming galaxies. We investigate the scientific potential of MOSAIC, a planned multi-object spectrograph on the European Extremely Large Telescope (ELT), for the 3D mapping of the IGM at $z \gtrsim 3$. We simulate a survey of $3 \lesssim z \lesssim 4$ galaxies down to a limiting magnitude of $m_{r}\sim 25.5$ mag in an area of 1 degree$^2$ in the sky. Galaxies and their spectra (including the line-of-sight Ly$α$ absorption) are taken from the lightcone extracted from the Horizon-AGN cosmological hydrodynamical simulation. The quality of the reconstruction of the original density field is studied for different spectral resolutions and signal-to-noise ratios of the spectra. We demonstrate that the minimum $S/N$ (per resolution element) of the faintest galaxies that such survey has to reach is $S/N = 4$. We show that a survey with such sensitivity enables a robust extraction of cosmic filaments and the detection of the theoretically-predicted galaxy stellar mass and star-formation rate gradients towards filaments. By simulating the realistic performance of MOSAIC we obtain $S/N(T_{\rm obs}, R, m_{r})$ scaling relations. We estimate that $\lesssim 35~(65)$ nights of observation time are required to carry out the survey with the instrument's high multiplex mode and with the spectral resolution of $R=1000~(2000)$. A survey with a MOSAIC-concept instrument on the ELT is found to enable the mapping of the IGM at $z > 3$ on Mpc scales, and as such will be complementary to and competitive with other planned IGM tomography surveys. [abridged]

astro-ph.GA↗

arXiv2019

GRB171010A / SN2017htp: a GRB-SN at z=0.33

The number of supernovae known to be connected with long-duration gamma-ray bursts is increasing and the link between these events is no longer exclusively found at low redshift ($z \lesssim 0.3$) but is well established also at larger distances. We present a new case of such a liaison at $z = 0.33$ between GRB\,171010A and SN\,2017htp. It is the second closest GRB with an associated supernova of only three events detected by Fermi-LAT. The supernova is one of the few higher redshift cases where spectroscopic observations were possible and shows spectral similarities with the well-studied SN\,1998bw, having produced a similar Ni mass ($M_{\rm Ni}=0.33\pm0.02 ~\rm{M_{\odot}}$) with slightly lower ejected mass ($M_{\rm ej}=4.1\pm0.7~\rm{M_{\odot}}$) and kinetic energy ($E_{\rm K} = 8.1\pm2.5 \times 10^{51} ~\rm{erg}$). The host-galaxy is bigger in size than typical GRB host galaxies, but the analysis of the region hosting the GRB revealed spectral properties typically observed in GRB hosts and showed that the progenitor of this event was located in a very bright HII region of its face-on host galaxy, at a projected distance of $\sim$ 10 kpc from its galactic centre. The star-formation rate (SFR$_{GRB} \sim$ 0.2 M$_{\odot}$~yr$^{-1}$) and metallicity (12 + log(O/H) $\sim 8.15 \pm 0.10$) of the GRB star-forming region are consistent with those of the host galaxies of previously studied GRB-SN systems.

astro-ph.HE↗

arXiv2019

New constraints on the physical conditions in H$_2$-bearing GRB-host damped Lyman-$α$ absorbers

We report the detections of molecular hydrogen (H$_2$), vibrationally-excited H$_2$ (H$^*_2$), and neutral atomic carbon (CI), in two new afterglow spectra of GRBs\,181020A ($z=2.938$) and 190114A ($z=3.376$), observed with X-shooter at the Very Large Telescope (VLT). Both host-galaxy absorption systems are characterized by strong damped Lyman-$α$ absorbers (DLAs) and substantial amounts of molecular hydrogen with $\log N$(HI, H$_2$) = $22.20\pm 0.05,~20.40\pm 0.04$ (GRB\,181020A) and $\log N$(HI, H$_2$) = $22.15\pm 0.05,~19.44\pm 0.04$ (GRB\,190114A). The DLA metallicites, depletion levels and dust extinctions are [Zn/H] = $-1.57\pm 0.06$, [Zn/Fe] = $0.67\pm 0.03$, and $A_V = 0.27\pm 0.02$\,mag (GRB\,181020A) and [Zn/H] = $-1.23\pm 0.07$, [Zn/Fe] = $1.06\pm 0.08$, and $A_V = 0.36\pm 0.02$\,mag (GRB\,190114A). We then examine the molecular gas content of all known H$_2$-bearing GRB-DLAs and explore the physical conditions and characteristics of these systems. We confirm that H$_2$ is detected in all CI- and H$^*_2$-bearing GRB absorption systems, but that these rarer features are not necessarily detected in all GRB H$_2$ absorbers. We find that a large molecular fraction of $f_{\rm H_2} \gtrsim 10^{-3}$ is required for CI to be detected. The defining characteristic for H$^*_2$ to be present is less clear, though a large H$_2$ column density is an essential factor. We then derive the H$_2$ excitation temperatures of the molecular gas and find that they are relatively low with $T_{\rm ex} \approx 100 - 300$\,K, however, there could be evidence of warmer components populating the high-$J$ H$_2$ levels in GRBs\,181020A and 190114A. Finally, we demonstrate that the otherwise successful X-shooter GRB afterglow campaign is hampered by a significant dust bias excluding the most dust-obscured H$_2$ absorbers from identification [Abridged].

astro-ph.GA↗

arXiv2019

On the dust properties of high redshift molecular clouds and the connection to the 2175 Å extinction bump

We present a study of the extinction and depletion-derived dust properties of gamma-ray burst (GRB) absorbers at $1<z<3$ showing the presence of neutral carbon (\ion{C}{I}). By modelling their parametric extinction laws, we discover a broad range of dust models characterizing the GRB \ion{C}{I} absorption systems. In addition to the already well-established correlation between the amount of \ion{C}{I} and visual extinction, $A_V$, we also observe a correlation with the total-to-selective reddening, $R_V$. All three quantities are also found to be connected to the presence and strength of the 2175\,Å dust extinction feature. While the amount of \ion{C}{I} is found to be correlated with the SED-derived dust properties, we do not find any evidence for a connection with the depletion-derived dust content as measured from [Zn/Fe] and $N$(Fe)$_{\rm dust}$. To reconcile this, we discuss a scenario where the observed extinction is dominated by the composition of dust particles confined in the molecular gas-phase of the ISM. We argue that since the depletion level trace non-carbonaceous dust in the ISM, the observed extinction in GRB \ion{C}{I} absorbers is primarily produced by carbon-rich dust in the molecular cloud and is therefore only observable in the extinction curves and not in the depletion patterns. This also indicates that the 2175\,Å dust extinction feature is caused by dust and molecules in the cold and molecular gas-phase. This scenario provides a possible resolution to the discrepancy between the depletion- and SED-derived amounts of dust in high-$z$ absorbers.

astro-ph.GA↗

arXiv2019

Are LGRBs biased tracers of star formation? Clues from the host galaxies of the $Swift$/BAT6 complete sample of bright LGRBs III: Stellar masses, star formation rates and metallicities at $z>1$

(Abridged) Long gamma-ray bursts (LGRB) have been suggested as promising tracers of star formation owing to their association with the core-collapse of massive stars. The goal of this work is to characterise the population of host galaxies of LGRBs at 1 < z < 2, investigate the conditions in which LGRBs form at these redshifts and assess their use as tracers of star formation. We perform a spectro-photometric analysis to determine the stellar mass, star formation rate, specific star formation rate and metallicity of the complete, unbiased host galaxy sample of the Swift/BAT6 LGRB sample at 1 < z < 2. We compare the distribution of these properties to the ones of typical star-forming galaxies from the MOSDEF and COSMOS2015 Ultra Deep surveys, within the same redshift range. We find that, similarly to z < 1, LGRBs do not directly trace star formation at 1 < z < 2, and they tend to avoid high-mass, high-metallicity host galaxies. We also find evidence for an enhanced fraction of starbursts among the LGRB host sample with respect to the star-forming population of galaxies. Nonetheless we demonstrate that the driving factor ruling the LGRB efficiency is metallicity. The LGRB host distributions can be reconciled with the ones expected from galaxy surveys by imposing a metallicity upper limit of 12+logOH ~ 8.55. Metallicity rules the LGRB production efficiency, which is stifled at Z > 0.7 Zsun. Under this hypothesis we can expect LGRBs to trace star formation at z > 3, once the bulk of the star forming galaxy population are characterised by metallicities below this limit. The moderately high metallicity threshold found is in agreement with the conditions necessary to rapidly produce a fast-rotating Wolf-Rayet star a in close binary system, and could be accommodated by single star models under chemically homogeneous mixing with very rapid rotation and weak magnetic coupling.

astro-ph.HE↗

arXiv2019

Signatures of a jet cocoon in early spectra of a supernova associated with a $γ$-ray burst

Long gamma-ray bursts mark the death of massive stars, as revealed by their association with energetic broad-lined stripped-envelope supernovae. The scarcity of nearby events and the brightness of the GRB afterglow, dominating the first days of emission, have so far prevented the study of the very early stages of the GRB-SN evolution. Here we present detailed, multi-epoch spectroscopic observations of SN 2017iuk, associated with GRB 171205A which display features at extremely high expansion velocities of $\sim$ 100,000 km s$^{-1}$ within the first day after the burst. These high-velocity components are characterized by chemical abundances different from those observed in the ejecta of SN 2017iuk at later times. Using spectral synthesis models developed for the SN 2017iuk, we explain these early features as originating not from the supernova ejecta, but from a hot cocoon generated by the energy injection of a mildly-relativistic GRB jet expanding into the medium surrounding the progenitor star. This cocoon becomes rapidly transparent and is outshone by the supernova emission which starts dominating three days after the burst. These results proves that the jet plays an important role not only in powering the GRB event but also its associated supernova.

astro-ph.HE↗

arXiv2018

Cold gas in the early Universe. Survey for neutral atomic-carbon in GRB host galaxies at 1 < z < 6 from optical afterglow spectroscopy

We present a survey for neutral atomic-carbon (CI) along gamma-ray burst (GRB) sightlines, which probes the shielded neutral gas-phase in the interstellar medium (ISM) of GRB host galaxies at high redshift. We compile a sample of 29 medium- to high-resolution GRB optical afterglow spectra spanning a redshift range through most of cosmic time from $1 < z < 6$. We find that seven ($\approx 25\%$) of the GRBs entering our statistical sample have CI detected in absorption. It is evident that there is a strong excess of cold gas in GRB hosts compared to absorbers in quasar sightlines. We investigate the dust properties of the GRB CI absorbers and find that the amount of neutral carbon is positively correlated with the visual extinction, $A_V$, and the strength of the 2175 Å dust extinction feature, $A_{\mathrm{bump}}$. GRBs with CI detected in absorption are all observed above a certain threshold of $\log N$(HI)$/\mathrm{cm}^{-2}$ + [X/H] > 20.7 and a dust-phase iron column density of $\log N$(Fe)$_{\mathrm{dust}}/\mathrm{cm}^{-2}$ > 16.2. In contrast to the SED-derived dust properties, the strength of the CI absorption does not correlate with the depletion-derived dust properties. This indicates that the GRB CI absorbers trace dusty systems where the dust composition is dominated by carbon-rich dust grains. The observed higher metal and dust column densities of the GRB CI absorbers compared to H$_2$- and CI-bearing quasar absorbers is mainly a consequence of how the two absorber populations are selected, but is also required in the presence of intense UV radiation fields in actively star-forming galaxies.

astro-ph.GA↗

arXiv2018

Evidence for diffuse molecular gas and dust in the hearts of gamma-ray burst host galaxies

Here we built up a sample of 22 GRBs at redshifts $z > 2$ observed with X-shooter 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 ISM at high redshift and to answer the question whether (and why) there might be a lack of H$_2$ in GRB-DLAs. We fit 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 fit dust depletion sequences and determine the dust-to-metals ratio and the host-galaxy intrinsic visual extinction. There is no lack of H$_2$-bearing GRB-DLAs. We detect absorption lines from H$_2$ in 6 out of 22 GRB afterglow spectra, with molecular fractions ranging between $f\simeq 5\cdot10^{-5}$ and $f\simeq 0.04$, and claim tentative detections in three other cases. The GRB-DLAs in the present sample have on average low metallicities ($\mathrm{[X/H]}\approx -1.3$), comparable to the rare population of QSO-ESDLAs (log N(HI) $> 21.5$). H$_2$-bearing GRB-DLAs are found to be associated with significant dust extinction, $A_V > 0.1$ mag, and have dust-to-metals ratios DTM$ > 0.4$. All of these systems exhibit column densities of log N(HI) $> 21.7$. The overall fraction of H$_2$ detections is $\ge 27$% (41% including tentative detections), which is three times larger than in the general population of QSO-DLAs. For $2 21.7$, the H$_2$ detection fraction is 60-80% in GRB-DLAs as well as in extremely strong QSO-DLAs. This is likely a consequence of the fact that both GRB- and QSO-DLAs with high N(HI) probe sight-lines with small impact parameters that indicate that the absorbing gas is associated with the inner regions of the absorbing galaxy, where the gas pressure is higher and the conversion of HI to H$_2$ takes place.

astro-ph.GA↗

arXiv2018

X-shooting GRBs at high redshift: probing dust production history

Evolved asymptotic giant branch (AGB) stars and Type Ia supernovae (SNe) are important contributors to the elements that form dust in the interstellar medium of galaxies, in particular, carbon and iron. However, they require at least a Gyr to start producing these elements, therefore, a change in dust quantity or properties may appear at high redshifts. In this work, we use extinction of gamma-ray burst (GRB) afterglows as a tool to look for variations in dust properties at z>3. We use a spectroscopically selected sample of GRB afterglows observed with the VLT/X-shooter instrument to determine extinction curves out to high redshifts. We present ten new z>3 X-shooter GRBs of which six are dusty. Combining these with individual extinction curves of three previously known z>3 GRBs, we find an average extinction curve consistent with the SMC-Bar. A comparison with spectroscopically selected GRBs at all redshifts indicates a drop in visual extinction (A_V) at z>3.5 with no moderate or high extinction bursts. We check for observational bias using template spectra and find that GRBs up to z~8 are detectable with X-shooter up to A_V~0.3 mag. Although other biases are noted, a uniformly low dust content above z>3.5 indicates a real drop, suggesting a transition in dust properties and/or available dust building blocks. The remarkable increase in dust content at z<3.5 could arise due to carbon and possibly iron production by the first carbon-rich AGB and Type Ia SNe, respectively. Alternatively, z>3.5 dust drop could be the result of low stellar masses of GRB host galaxies.

astro-ph.GA↗