arXiv subjects

A. Melandri

Publications and source records attributed to A. Melandri.

At least 37 records · Page 2Linked to original sources

arXiv2023

The first JWST spectrum of a GRB afterglow: No bright supernova in observations of the brightest GRB of all time, GRB 221009A

We present JWST and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, obtained with JWST/NIRSPEC (0.6-5.5 micron) and MIRI (5-12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power-law, with $F_ν \propto ν^{-β}$, we obtain $β\approx 0.35$, modified by substantial dust extinction with $A_V = 4.9$. This suggests extinction above the notional Galactic value, possibly due to patchy extinction within the Milky Way or dust in the GRB host galaxy. It further implies that the X-ray and optical/IR regimes are not on the same segment of the synchrotron spectrum of the afterglow. If the cooling break lies between the X-ray and optical/IR, then the temporal decay rates would only match a post jet-break model, with electron index $p<2$, and with the jet expanding into a uniform ISM medium. The shape of the JWST spectrum is near-identical in the optical/nIR to X-shooter spectroscopy obtained at 0.5 days and to later time observations with HST. The lack of spectral evolution suggests that any accompanying supernova (SN) is either substantially fainter or bluer than SN 1998bw, the proto-type GRB-SN. Our HST observations also reveal a disc-like host galaxy, viewed close to edge-on, that further complicates the isolation of any supernova component. The host galaxy appears rather typical amongst long-GRB hosts and suggests that the extreme properties of GRB 221009A are not directly tied to its galaxy-scale environment.

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arXiv2023

GRB 221009A: Discovery of an Exceptionally Rare Nearby and Energetic Gamma-Ray Burst

We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB 221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image (MAXI), and Neutron Star Interior Composition Explorer Mission (NICER). This energetic GRB was located relatively nearby (z = 0.151), allowing for sustained observations of the afterglow. The large X-ray luminosity and low Galactic latitude (b = 4.3 degrees) make GRB 221009A a powerful probe of dust in the Milky Way. Using echo tomography we map the line-of-sight dust distribution and find evidence for significant column densities at large distances (~> 10kpc). We present analysis of the light curves and spectra at X-ray and UV/optical wavelengths, and find that the X-ray afterglow of GRB 221009A is more than an order of magnitude brighter at T0 + 4.5 ks than any previous GRB observed by Swift. In its rest frame GRB 221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10^4 long GRBs were as energetic as GRB 221009A; such a large E_gamma,iso implies a narrow jet structure, but the afterglow light curve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB 221009A occur at a rate of ~<1 per 1000 yr - making this a truly remarkable opportunity unlikely to be repeated in our lifetime.

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arXiv2023

GRB 160410A: the first Chemical Study of the Interstellar Medium of a Short GRB

Short Gamma-Ray Bursts (SGRBs) are produced by the coalescence of compact binary systems which are remnants of massive stars. GRB 160410A is classified as a short-duration GRB with extended emission and is currently the farthest SGRB with a redshift determined from an afterglow spectrum and also one of the brightest SGRBs to date. The fast reaction to the Neil Gehrels Swift Observatory alert allowed us to obtain a spectrum of the afterglow using the X-shooter spectrograph at the Very Large Telescope (VLT). The spectrum shows several absorption features at a redshift of z=1.7177, in addition, we detect two intervening systems at z=1.581 and z=1.444. The spectrum shows ly-alpha in absorption with a column density of log N(HI)=21.2+/-0.2 cm$^{-2}$ which, together with FeII, CII, SiII, AlII and OI, allow us to perform the first study of chemical abundances in a SGRB host galaxy. We determine a metallicity of [X/H]=-2.3+/-0.2 for FeII and -2.5+/-0.2 for SiII and no dust depletion. We also find no evidence for extinction in the afterglow spectral energy distribution (SED) modeling. The environment has a low degree of ionisation and the CIV and SiIV lines are completely absent. We do not detect an underlying host galaxy down to deep limits. Additionally, we compare GRB 160410A to GRB 201221D, another high-z short GRB that shows absorption lines at z=1.045 and an underlying massive host galaxy.

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arXiv2022

GRB 080928 afterglow imaging and spectro-polarimetry

Among the large variety of astrophysical sources that we can observe, gamma-ray bursts (GRBs) are the most energetic of the whole Universe. The definition of a general picture describing the physics behind GRBs has always been a compelling task, but the results obtained so far from observations have revealed a puzzling landscape. The lack of a clear, unique paradigm calls for further observations and additional, independent techniques for this purpose. Polarimetry constitutes a very useful example as it allows us to investigate some features of the source such as the geometry of the emitting region and the magnetic field configuration. To date, only a handful of bursts detected by space telescopes have been accompanied by ground-based spectro-polarimetric follow-up, and therefore such an analysis of more GRBs is of crucial importance in order to increase the sample of bursts with multi-epoch polarisation analysis. In this work, we present the analysis of the GRB 080928 optical afterglow, with observations performed with the ESO-VLT FORS1 instrument. We find that the GRB optical afterglow was not significantly polarised on the first observing night. The polarisation degree ($P$) grew on the following night to a level of $P \sim$ 4.5%, giving evidence of polarised radiation at a 4 $σ$ confidence level. The GRB 080928 light curve is not fully consistent with standard afterglow models, making any comparison with polarimetric models partly inconclusive. The most conservative interpretation is that the GRB emission was characterised by a homogeneous jet and was observed at an angle of 0.6 $< θ_{obs}/θ_{jet} <$ 0.8. Moreover, the non-zero polarisation degree on the second night suggests the presence of a dominant locally ordered magnetic field in the emitting region.

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arXiv2022

A blast from the infant Universe: the very high-z GRB 210905A

We present a detailed follow-up of the very energetic GRB 210905A at a high redshift of z = 6.312 and its luminous X-ray and optical afterglow. We obtained a photometric and spectroscopic follow-up in the optical and near-infrared (NIR), covering both the prompt and afterglow emission from a few minutes up to 20 Ms after burst. With an isotropic gamma-ray energy release of Eiso = 1.27E54 erg, GRB 210905A lies in the top ~7% of gamma-ray bursts (GRBs) in terms of energy released. Its afterglow is among the most luminous ever observed. It starts with a shallow evolution that can be explained by energy injection, and it is followed by a steeper decay, while the spectral energy distribution is in agreement with slow cooling in a constant-density environment within the standard fireball theory. A jet break at ~ 46.2+-16.3 d (~6.3 d rest-frame) has been observed in the X-ray light curve; however, it is hidden in the H band due to the contribution from the likely host galaxy, the fourth GRB host at z > 6 known to date. We derived a half-opening angle of 8.4+-1.0 degrees, which is the highest ever measured for a z>6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of 1E52 erg is also among the highest ever measured. The moderately large half-opening angle argues against recent claims of an inverse dependence of the half-opening angle on the redshift. The total jet energy is likely too large to be sustained by a standard magnetar, and it suggests that the central engine of this burst was a newly formed black hole. Despite the outstanding energetics and luminosity of both GRB 210905A and its afterglow, we demonstrate that they are consistent with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift.

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arXiv2022

The peculiar short-duration GRB 200826A and its supernova

Gamma-ray bursts (GRBs) are classified as long and short events. Long GRBs (LGRBs) are associated with the end states of very massive stars, while short GRBs (SGRBs) are linked to the merger of compact objects. GRB 200826A was a peculiar event, because by definition it was a SGRB, with a rest-frame duration of ~ 0.5 s. However, this event was energetic and soft, which is consistent with LGRBs. The relatively low redshift (z = 0.7486) motivated a comprehensive, multi-wavelength follow-up campaign to characterize its host, search for a possible associated supernova (SN), and thus understand the origin of this burst. To this aim we obtained a combination of deep near-infrared (NIR) and optical imaging together with spectroscopy. Our analysis reveals an optical and NIR bump in the light curve whose luminosity and evolution is in agreement with several LGRB-SNe. Analysis of the prompt GRB shows that this event follows the $E_{\rm p,i}-E_{\rm iso}$ relation found for LGRBs. The host galaxy is a low-mass star-forming galaxy, typical for LGRBs, but with one of the highest star-formation rates (SFR), especially with respect to its mass ($\log M_\ast/M_\odot = 8.6$, SFR $\sim 4.0 \,M_\odot$/yr). We conclude that GRB 200826A is a typical collapsar event in the low tail of the duration distribution of LGRBs. These findings support theoretical predictions that events produced by collapsars can be as short as 0.5 s in the host frame and further confirm that duration alone is not an efficient discriminator for the progenitor class of a GRB.

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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.

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arXiv2021

Outflows from GRB hosts are ubiquitous: Kinematics of z<0.3 GRB-SN hosts resolved with FLAMES

The hosts of long duration gamma-ray bursts are predominantly starburst galaxies at subsolar metallicity. At redshifts z<1, this implies that most of them are low-mass galaxies similar to the populations of blue compact dwarfs and dwarf irregulars. What triggers the massive star-formation (SF) needed for producing a GRB progenitor is still largely unknown, as are the resolved gas properties and kinematics of these galaxies and their formation history. Here we present a sample of six spatially resolved GRB hosts at z<0.3 observed with 3D spectroscopy at high spectral resolution (R=8,000-13,000) using FLAMES/VLT. We analyzed the resolved gas kinematics of the full sample and the abundances in a subsample. Only two galaxies show a regular disk-like rotation field, another two are dispersion-dominated, the remaining two have a double emission component associated with different parts of the galaxy, which might indicate a recent merger. All galaxies show evidence for broad components underlying the main emission peak (sigma = 50-110 km/s). This broad component is more metal-rich than the narrow components, it is blueshifted in most cases, and it follows a different velocity structure. We find a weak correlation between the SF rate and the width of the broad component, its flux compared to the narrow component, and the maximum outflow velocity of the gas, but we do not find any correlation with the SF density, metallicity or stellar mass. We associate this broad component with a metal-rich outflow from star-forming regions. The GRB is not located in the brightest region of the host, but is always associated with some star-forming region showing a clear wind component. Our study shows the potential of 3D spectroscopy to study the SF processes in galaxies hosting extreme transients, the need for high S/N, and the perils using unresolved or only partially resolved data for these kinds of studies.

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arXiv2021

Swift/UVOT follow-up of Gravitational Wave Alerts in the O3 era

In this paper, we report on the observational performance of the Swift Ultra-violet/Optical Telescope (UVOT) in response to the Gravitational Wave alerts announced by the Advanced Laser Interferometer Gravitational Wave Observatory and the Advanced Virgo detector during the O3 period. We provide the observational strategy for follow-up of GW alerts and provide an overview of the processing and analysis of candidate optical/UV sources. For the O3 period, we also provide a statistical overview and report on serendipitous sources discovered by Swift/UVOT. Swift followed 18 gravitational-wave candidate alerts, with UVOT observing a total of 424 deg^2. We found 27 sources that changed in magnitude at the 3 sigma level compared with archival u or g-band catalogued values. Swift/UVOT also followed up a further 13 sources reported by other facilities during the O3 period. Using catalogue information, we divided these 40 sources into five initial classifications: 11 candidate active galactic nuclei (AGN)/quasars, 3 Cataclysmic Variables (CVs), 9 supernovae, 11 unidentified sources that had archival photometry and 6 uncatalogued sources for which no archival photometry was available. We have no strong evidence to identify any of these transients as counterparts to the GW events. The 17 unclassified sources are likely a mix of AGN and a class of fast-evolving transient, and one source may be a CV.

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arXiv2021

Gamma Ray Burst studies with THESEUS

Gamma-ray Bursts (GRBs) are the most powerful transients in the Universe, over-shining for a few seconds all other $γ$-ray sky sources. Their emission is produced within narrowly collimated relativistic jets launched after the core-collapse of massive stars or the merger of compact binaries. THESEUS will open a new window for the use of GRBs as cosmological tools by securing a statistically significant sample of high-$z$ GRBs, as well as by providing a large number of GRBs at low-intermediate redshifts extending the current samples to low luminosities. The wide energy band and unprecedented sensitivity of the Soft X-ray Imager (SXI) and X-Gamma rays Imaging Spectrometer (XGIS) instruments provide us a new route to unveil the nature of the prompt emission. For the first time, a full characterisation of the prompt emission spectrum from 0.3 keV to 10 MeV with unprecedented large count statistics will be possible revealing the signatures of synchrotron emission. SXI spectra, extending down to 0.3 keV, will constrain the local metal absorption and, for the brightest events, the progenitors' ejecta composition. Investigation of the nature of the internal energy dissipation mechanisms will be obtained through the systematic study with XGIS of the sub-second variability unexplored so far over such a wide energy range. THESEUS will follow the spectral evolution of the prompt emission down to the soft X-ray band during the early steep decay and through the plateau phase with the unique ability of extending above 10 keV the spectral study of these early afterglow emission phases.

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arXiv2020

Swift Multiwavelength Follow-up of LVC S200224ca and the Implications for Binary Black Hole Mergers

On 2020 February 24, during their third observing run ("O3"), the Laser Interferometer Gravitational-wave Observatory and Virgo Collaboration (LVC) detected S200224ca: a candidate gravitational wave (GW) event produced by a binary black hole (BBH) merger. This event was one of the best-localized compact binary coalescences detected in O3 (with 50%/90% error regions of 13/72 deg$^2$), and so the Neil Gehrels Swift Observatory performed rapid near-UV/X-ray follow-up observations. Swift-XRT and UVOT covered approximately 79.2% and 62.4% (respectively) of the GW error region, making S200224ca the BBH event most thoroughly followed-up in near-UV (u-band) and X-ray to date. No likely EM counterparts to the GW event were found by the Swift BAT, XRT, or UVOT, nor by other observatories. Here we report on the results of our searches for an EM counterpart, both in the BAT data near the time of the merger, and in follow-up UVOT/XRT observations. We also discuss the upper limits we can place on EM radiation from S200224ca, and the implications these limits have on the physics of BBH mergers. Namely, we place a shallow upper limit on the dimensionless BH charge, $\hat{q} < 1.4 \times10^{-4}$, and an upper limit on the isotropic-equivalent energy of a blast wave $E < 4.1\times10^{51}$ erg (assuming typical GRB parameters).

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arXiv2020

Swift-XRT follow-up of gravitational wave triggers during the third aLIGO/Virgo observing run

The Neil Gehrels Swift Observatory followed up 18 gravitational wave (GW) triggers from the LIGO/Virgo collaboration during the O3 observing run in 2019/2020, performing approximately 6500 pointings in total. Of these events, four were finally classified (if real) as binary black hole (BH) triggers, six as binary neutron star (NS) events, two each of NSBH and Mass Gap triggers, one an unmodelled (Burst) trigger, and the remaining three were subsequently retracted. Thus far, four of these O3 triggers have been formally confirmed as real gravitational wave events. While no likely electromagnetic counterparts to any of these GW events have been identified in the X-ray data (to an average upper limit of 3.60 x 10^{-12} erg cm^{-2} s^{-1} over 0.3-10 keV), or at other wavelengths, we present a summary of all the Swift-XRT observations performed during O3, together with typical upper limits for each trigger observed. The majority of X-ray sources detected during O3 were previously uncatalogued; while some of these will be new (transient) sources, others are simply too faint to have been detected by earlier survey missions such as ROSAT. The all-sky survey currently being performed by eROSITA will be a very useful comparison for future observing runs, reducing the number of apparent candidate X-ray counterparts by up to 95 per cent.

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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.

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arXiv2020

A comparison between short GRB afterglows and AT2017gfo: shedding light on kilonovae properties

Multi-messenger astronomy received a great boost following the discovery of kilonova AT2017gfo, the optical counterpart of the gravitational wave source GW170817 associated with the short gamma-ray burst GRB 170817A. AT2017gfo was the first kilonova that could be extensively monitored in time both photometrically and spectroscopically. Previously, only few candidates have been observed against the glare of short GRB afterglows. In this work, we aim to search the fingerprints of AT2017gfo-like kilonova emissions in the optical/NIR light curves of 39 short GRBs with known redshift. For the first time, our results allow us to study separately the range of luminosity of the blue and red components of AT2017gfo-like kilonovae in short GRBs. In particular, the red component is similar in luminosity to AT2017gfo, while the blue kilonova can be more than 10 times brighter. Finally, we find further evidence to support all the claimed kilonova detections and we exclude an AT2017gfo-like kilonova in GRBs 050509B and 061201.

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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.

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arXiv2019

Swift-XRT Follow-up of Gravitational Wave Triggers in the Second Advanced LIGO/Virgo Observing Run

The Neil Gehrels Swift Observatory carried out prompt searches for gravitational wave (GW) events detected by the LIGO/Virgo Collaboration (LVC) during the second observing run ("O2"). Swift performed extensive tiling of eight LVC triggers, two of which had very low false-alarm rates (GW 170814 and the epochal GW 170817), indicating a high confidence of being astrophysical in origin; the latter was the first GW event to have an electromagnetic counterpart detected. In this paper we describe the follow-up performed during O2 and the results of our searches. No GW electromagnetic counterparts were detected; this result is expected, as GW 170817 remained the only astrophysical event containing at least one neutron star after LVC's later retraction of some events. A number of X-ray sources were detected, with the majority of identified sources being active galactic nuclei. We discuss the detection rate of transient X-ray sources and their implications in the O2 tiling searches. Finally, we describe the lessons learned during O2, and how these are being used to improve the \swift\ follow-up of GW events. In particular, we simulate a population of GRB afterglows to evaluate our source ranking system's ability to differentiate them from unrelated and uncatalogued X-ray sources. We find that $\approx$60-70% of afterglows whose jets are oriented towards Earth will be given high rank (i.e., "interesting" designation) by the completion of our second follow-up phase (assuming their location in the sky was observed), but that this fraction can be increased to nearly 100% by performing a third follow-up observation of sources exhibiting fading behavior.

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arXiv2019

GRB 161219B-SN 2016jca: a powerful stellar collapse

We report observations and analysis of the nearby gamma-ray burst GRB\,161219B (redshift $z=0.1475$) and the associated Type Ic supernova (SN) 2016jca. GRB\,161219B had an isotropic gamma-ray energy of $\sim 1.6 \times 10^{50}$\,erg. Its afterglow is likely refreshed at an epoch preceding the first photometric points (0.6\,d), which slows down the decay rates. Combined analysis of the SN light curve and multiwavelength observations of the afterglow suggest that the GRB jet was broad during the afterglow phase (full opening angle $\sim 42^\circ \pm 3^\circ$). Our spectral series shows broad absorption lines typical of GRB supernovae (SNe), which testify to the presence of material with velocities up to $\sim 0.25$c. The spectrum at 3.73\,d allows for the very early identification of a SN associated with a GRB. Reproducing it requires a large photospheric velocity ($35,000 \pm 7000$\,\kms). The kinetic energy of the SN is estimated through models to be \KE $\approx 4 \times 10^{52}$\,erg in spherical symmetry. The ejected mass in the explosion was \Mej $\approx 6.5 \pm 1.5$\,\Msun, much less than that of other GRB-SNe, demonstrating diversity among these events. The total amount of \Nifs\ in the explosion was $0.27 \pm 0.05$\,\Msun. The observed spectra require the presence of freshly synthesised \Nifs\ at the highest velocities, at least 3 times more than a standard GRB-SN. We also find evidence for a decreasing \Nifs\ abundance as a function of decreasing velocity. This suggests that SN\,2016jca was a highly aspherical explosion viewed close to on-axis, powered by a compact remnant. Applying a typical correction for asymmetry, the energy of SN\,2016jca was $\sim$ (1--3) $\times 10^{52}$\,erg, confirming that most of the energy produced by GRB-SNe goes into the kinetic energy of the SN ejecta.

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arXiv2019

X-ray absorbing column densities of a complete sample of short Gamma Ray Bursts

We update a flux-limited complete sample of Swift-based SGRBs (SBAT4, D'Avanzo et al. 2014), bringing it to 25 events and doubling its previous redshift range. We then evaluate the column densities of the events in the updated sample, in order to compare them with the NH distribution of LGRBs, using the sample BAT6ext (Arcodia et al. 2016). We rely on Monte Carlo simulations of the two populations and compare the computed NH distributions with a two sample Kolmogorov Smirnov (K-S) test. We then study how the K-S probability varies with respect to the redshift range we consider. We find that the K-S probability keeps decreasing as redshift increases until at z$\sim$1.8 the probability that short and long GRBs come from the same parent distribution drops below 1$\%$. This testifies for an observational difference among the two populations. This difference may be due to the presence of highly absorbed LGRBs above z$\sim$1.3, which have not been observed in the SGRB sample yet, although this may be due to our inability to detect them, or to the relatively small sample size.

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