arXiv subjects

S. Gallozzi

Publications and source records attributed to S. Gallozzi.

At least 37 records · Page 2Linked to original sources

arXiv2018

The broad-band properties of the intermediate synchrotron peaked BL Lac S2 0109+22 from radio to VHE gamma rays

The MAGIC telescopes observed S2 0109+22 in 2015 July during its flaring activity in high energy gamma rays observed by Fermi-LAT. We analyse the MAGIC data to characterise the very high energy (VHE) gamma-ray emission of S2 0109+22, which belongs to the subclass of intermediate synchrotron peak (ISP) BL Lac objects. We study the multi-frequency emission in order to investigate the source classification. Finally, we compare the source long-term behaviour to other VHE gamma-ray emitting (TeV) blazars. We performed a temporal and spectral analysis of the data centred around the MAGIC interval of observation (MJD 57225-57231). Long-term radio and optical data have also been investigated using the discrete correlation function. The redshift of the source is estimated through optical host-galaxy imaging and also using the amount of VHE gamma-ray absorption. The quasi-simultaneous multi-frequency spectral energy distribution (SED) is modelled with the conventional one-zone synchrotron self-Compton (SSC) model. MAGIC observations resulted in the detection of the source at a significance level of $5.3\,σ$. The VHE gamma-ray emission of S2 0109+22 is variable on a daily time scale. VHE gamma-ray luminosity of the source is lower than the average of TeV BL Lacs. The optical polarization, and long-term optical/radio behaviour of the source are different from the general population of TeV blazars. All these findings agree with the classification of the source as an ISP BL Lac object. We estimate the source redshift as $z = 0.36 \pm 0.07$. The SSC parameters describing the SED are rather typical for blazars.

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arXiv2018

Multi-wavelength characterization of the blazar S5~0716+714 during an unprecedented outburst phase

The BL Lac object S5~0716+714, a highly variable blazar, underwent an impressive outburst in January 2015 (Phase A), followed by minor activity in February (Phase B). The MAGIC observations were triggered by the optical flux observed in Phase A, corresponding to the brightest ever reported state of the source in the R-band. The comprehensive dataset collected is investigated in order to shed light on the mechanism of the broadband emission. Multi-wavelength light curves have been studied together with the broadband Spectral Energy Distributions (SEDs). The data set collected spans from radio, optical photometry and polarimetry, X-ray, high-energy (HE, 0.1 GeV < E < 100 GeV) with \textit{Fermi}-LAT to the very-high-energy (VHE, E>100 GeV) with MAGIC. The flaring state of Phase A was detected in all the energy bands, providing for the first time a multi-wavelength sample of simultaneous data from the radio band to the VHE. In the constructed SED the \textit{Swift}-XRT+\textit{NuSTAR} data constrain the transition between the synchrotron and inverse Compton components very accurately, while the second peak is constrained from 0.1~GeV to 600~GeV by \textit{Fermi}+MAGIC data. The broadband SED cannot be described with a one-zone synchrotron self-Compton model as it severely underestimates the optical flux in order to reproduce the X-ray to $γ$-ray data. Instead we use a two-zone model. The EVPA shows an unprecedented fast rotation. An estimation of the redshift of the source by combined HE and VHE data provides a value of $z = 0.31 \pm 0.02_{stats} \pm 0.05_{sys}$, confirming the literature value. The data show the VHE emission originating in the entrance and exit of a superluminal knot in and out a recollimation shock in the inner jet. A shock-shock interaction in the jet seems responsible for the observed flares and EVPA swing. This scenario is also consistent with the SED modelling.

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arXiv2018

Gamma-ray flaring activity of NGC 1275 in 2016-2017 measured by MAGIC

We report on the detection of flaring activity from the Fanaroff-Riley I radio galaxy NGC 1275 in very-high-energy (VHE, E $>$ 100 GeV) gamma rays with the MAGIC telescopes. Observations were performed between 2016 September and 2017 February as part of a monitoring program. The brightest outburst with $\sim1.5$ times the Crab Nebula flux above 100 GeV (C.U.) was observed during the night between 2016 December 31 and 2017 January 1 (fifty times higher than the mean previously measured in two observational campaigns between 2009 and 2011). Significant variability of the day-by-day light curve was measured, the shortest flux-doubling time-scales was found to be of $(611\pm101)$ min. The combined spectrum of the MAGIC data during the strongest flare state and simultaneous data from the Fermi-LAT around 2017 January 1 follows a power-law with an exponential cutoff at the energy $(492\pm35)$ GeV. Simultaneous optical flux density measurements in the R-band obtained with the KVA telescope are also presented and the correlation between the optical and gamma-ray emission is investigated. Due to possible internal pair-production, the fast flux variability constrains the Doppler factor to values which are inconsistent with a large viewing angle as observed in the radio band. We investigate different scenarios for the explanation of fast gamma-ray variability, namely emission from: magnetospheric gaps, relativistic blobs propagating in the jet (mini-jets) or external cloud (or star) entering the jet. We find that the only plausible model to account for the luminosities here observed would be the production of gamma rays in a magnetospheric gap around the central black hole only in the eventuality of an enhancement of the magnetic field threading the hole from its equipartition value with the gas pressure in the accretion flow.

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arXiv2017

ASTRI SST-2M prototype and mini-array simulation chain, data reduction software, and archive in the framework of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is a worldwide project aimed at building the next-generation ground-based gamma-ray observatory. Within the CTA project, the Italian National Institute for Astrophysics (INAF) is developing an end-to-end prototype of the CTA Small-Size Telescopes with a dual-mirror (SST-2M) Schwarzschild-Couder configuration. The prototype, named ASTRI SST-2M, is located at the INAF "M.C. Fracastoro" observing station in Serra La Nave (Mt. Etna, Sicily) and is currently in the scientific and performance validation phase. A mini-array of (at least) nine ASTRI telescopes has been then proposed to be deployed at the Southern CTA site, by means of a collaborative effort carried out by institutes from Italy, Brazil, and South-Africa. The CTA/ASTRI team is developing an end-to-end software package for the reduction of the raw data acquired with both ASTRI SST-2M prototype and mini-array, with the aim of actively contributing to the global ongoing activities for the official data handling system of the CTA observatory. The group is also undertaking a massive Monte Carlo simulation data production using the detector Monte Carlo software adopted by the CTA consortium. Simulated data are being used to validate the simulation chain and evaluate the ASTRI SST-2M prototype and mini-array performance. Both activities are also carried out in the framework of the European H2020-ASTERICS (Astronomy ESFRI and Research Infrastructure Cluster) project. A data archiving system, for both ASTRI SST-2M prototype and mini-array, has been also developed by the CTA/ASTRI team, as a testbed for the scientific archive of CTA. In this contribution, we present the main components of the ASTRI data handling systems and report the status of their development.

astro-ph.IM↗

arXiv2015

Cherenkov Telescope Array Data Management

Very High Energy gamma-ray astronomy with the Cherenkov Telescope Array (CTA) is evolving towards the model of a public observatory. Handling, processing and archiving the large amount of data generated by the CTA instruments and delivering scientific products are some of the challenges in designing the CTA Data Management. The participation of scientists from within CTA Consortium and from the greater worldwide scientific community necessitates a sophisticated scientific analysis system capable of providing unified and efficient user access to data, software and computing resources. Data Management is designed to respond to three main issues: (i) the treatment and flow of data from remote telescopes; (ii) "big-data" archiving and processing; (iii) and open data access. In this communication the overall technical design of the CTA Data Management, current major developments and prototypes are presented.

astro-ph.IM↗

arXiv2014

The primordial environment of super massive black holes: large scale galaxy overdensities around $z\sim6$ QSOs with LBT

We investigated the presence of galaxy overdensities around four $z\sim6$ QSOs, namely SDSS J1030+0524 (z = 6.28), SDSS J1148+5251 (z = 6.41), SDSS J1048+4637 (z = 6.20) and SDSS J1411+1217 (z = 5.95), through deep $r$-, $i$- and $z$- band imaging obtained with the wide-field ($\sim23'\times25'$) Large Binocular Camera (LBC) at the Large Binocular Telescope (LBT). We adopted color-color selections within the $i-z$ vs $r-z$ plane to identify samples of $i$-band dropouts at the QSO redshift and measure their relative abundance and spatial distribution in the four LBC fields, each covering $\sim8\times8$ physical Mpc at $z\sim6$. The same selection criteria were then applied to $z$-band selected sources in the $\sim$1 deg$^2$ Subaru-XMM Newton Deep Survey to derive the expected number of dropouts over a blank LBC-sized field ($\sim$0.14 deg$^2$). The four observed QSO fields host a number of candidates larger than what is expected in a blank field. By defining as $i$-band dropouts objects with $z_{AB}<25$, $i-z>1.4$ and undetected in the $r$-band, we found 16, 10, 9, 12 dropouts in SDSS J1030+0524, SDSS J1148+5251, SDSS J1048+4637, and SDSS J1411+1217, respectively, whereas only 4.3 such objects are expected over a 0.14 deg$^2$ blank field. This corresponds to overdensity significances of 3.3, 1.9, 1.7, 2.5$σ$, respectively. By considering the total number of dropouts in the four LBC fields and comparing it with what is expected in four blank fields of 0.14 deg$^2$ each, we find that high-z QSOs reside in overdense environments at the $3.7σ$ level. This is the first direct and unambiguous measurement of the large scale structures around $z\sim6$ QSOs. [shortened]

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arXiv2014

Deep R-band counts of z~3 Lyman break galaxy candidates with the LBT

Aims. We present a deep multiwavelength imaging survey (UGR) in 3 different fields, Q0933, Q1623, and COSMOS, for a total area of ~1500arcmin^2. The data were obtained with the Large Binocular Camera on the Large Binocular Telescope. Methods. To select our Lyman break galaxy (LBG) candidates, we adopted the well established and widely used color-selection criterion (U-G vs. G-R). One of the main advantages of our survey is that it has a wider dynamic color range for U-dropout selection than in previous studies. This allows us to fully exploit the depth of our R-band images, obtaining a robust sample with few interlopers. In addition, for 2 of our fields we have spectroscopic redshift information that is needed to better estimate the completeness of our sample and interloper fraction. Results. Our limiting magnitudes reach 27.0(AB) in the R band (5σ) and 28.6(AB) in the U band (1σ). This dataset was used to derive LBG candidates at z~3. We obtained a catalog with a total of 12264 sources down to the 50% completeness magnitude limit in the R band for each field. We find a surface density of ~3 LBG candidates arcmin^2 down to R=25.5, where completeness is >=95% for all 3 fields. This number is higher than the original studies, but consistent with more recent samples.

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arXiv2013

Diffuse optical intracluster light as a measure of stellar tidal stripping: the cluster CL0024+17 at $z\sim $0.4 observed at LBT

We have evaluated the diffuse intracluster light (ICL) in the central core of the galaxy cluster CL0024+17 at $z\sim 0.4$ observed with the prime focus camera (LBC) at LBT. The measure required an accurate removal of the galaxies light within $\sim 200$ kpc from the center. The residual background intensity has then been integrated in circular apertures to derive the average ICL intensity profile. The latter shows an approximate exponential decline as expected from theoretical cold dark matter models. The radial profile of the ICL over the galaxies intensity ratio (ICL fraction) is increasing with decreasing radius but near the cluster center it starts to bend and then decreases where the overlap of the halos of the brightest cluster galaxies becomes dominant. Theoretical expectations in a simplified CDM scenario show that the ICL fraction profile can be estimated from the stripped over galaxy stellar mass ratio in the cluster. It is possible to show that the latter quantity is almost independent of the properties of the individual host galaxies but mainly depends on the average cluster properties. The predicted ICL fraction profile is thus very sensitive to the assumed CDM profile, total mass and concentration parameter of the cluster. Adopting values very similar to those derived from the most recent lensing analysis in CL0024+17 we find a good agreement with the observed ICL fraction profile. The galaxy counts in the cluster core have then been compared with that derived from composite cluster samples in larger volumes, up to the clusters virial radius. The galaxy counts in the CL0024+17 core appear flatter and the amount of bending respect to the average cluster galaxy counts imply a loss of total emissivity in broad agreement with the measured ICL fraction.

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arXiv2012

The blue UV slopes of z~4 Lyman break galaxies: implications for the corrected star formation rate density

We plan to analyse dust extinction in Lyman Break Galaxies (LBGs) by introducing a new and more reliable approach to their selection and to the characterization of their distribution of UV slopes beta, using deep IR images from HST. We exploit deep WFC3 IR observations of the ERS and HUDF fields over GOODS-South, combined with HST-ACS optical data, to select z~4 LBGs through a new (B-V) vs. (V-H) colour diagram. The UV slope of the selected galaxies is robustly determined by a linear fit over their observed I, Z, Y, J magnitudes, coherently with the original definition of beta. The same fit is used to determine their rest-frame UV magnitudes M1600 through a simple interpolation. We estimate the effect of observational uncertainties with detailed simulations that we also exploit, under a parametric maximum-likelihood approach, to constrain the probability density function of UV slopes PDF(beta) as a function of rest-frame magnitude. We find 142 and 25 robust LBGs in the ERS and HUDF fields respectively, limiting our sample to S/N(H)>10 objects. Our newly defined criteria improve the selection of z~4 LBGs and allow us to exclude red interlopers at lower redshift, especially z~3-3.5 objects. We find that z~4 LBGs are characterized by blue UV slopes, suggesting a low dust extinction: all L \simeq-2.1, while brighter objects only are slightly redder ( -1.9). We find an intrinsic dispersion ~ 0.3 for PDF(beta) at all magnitudes. The SFRD at z~4 corrected according to these estimates turns out to be lower than previously found: log(SFRD)\simeq-1.09 M_sun/yr/Mpc^3. Finally, we discuss how the UV slope of z~4 galaxies changes as a function of the dust-corrected UV magnitude (i.e. SFR) showing that most galaxies with a high SFR (> 80 M_sun/yr) are highly extincted objects. [Abridged]

astro-ph.CO↗

arXiv2012

The evolving slope of the stellar mass function at 0.6 <= z < 4.5 from deep WFC3 data

We used Early Release Science (ERS) observations taken with the Wide Field Camera 3 (WFC3) in the GOODS-S field to study the galaxy stellar mass function (GSMF) at 0.6<=z<4.5. Deep WFC3 near-IR data (for Y as faint as 27.3, J and H as faint as 27.4 AB mag at 5 sigma), as well as deep Ks (as faint as 25.5 at 5 sigma) Hawk-I band data, provide an exquisite data set with which determine in an unprecedented way the low-mass end of the GSMF, allowing an accurate probe of masses as low as M~7.6 10^9 Msun at z~3. Although the area used is relatively small (~33 arcmin^2), we found generally good agreement with previous studies on the entire mass range. Our results show that the slope of the faint-end increases with redshift, from alpha=-1.44+/-0.03 at z~0.8 to alpha=-1.86+/-0.16 at z~3, although indications exist that it does not steepen further between z~3 and z~4. This result is insensitive to any uncertainty in the M* parameter. The steepness of the GSMF faint-end solves the well-known disagreement between the stellar mass density (SMD) and the integrated star formation history at z>2. However, we confirm the that there appears to be an excess of integrated star formation with respect to the SMD at z<2, by a factor of ~2-3. Our comparison of the observations with theoretical predictions shows that the models forecast a greater abundance of low mass galaxies, at least up to z~3, as well as a dearth of massive galaxies at z~4 with respect to the data, and that the predicted SMD is generally overestimated at z<~2.

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arXiv2011

A low escape fraction of ionizing photons of L>L* Lyman break galaxies at z=3.3

We present an upper limit for the relative escape fraction (f_{esc}^{rel}) of ionizing radiation at z~3.3 using a sample of 11 Lyman Break Galaxies (LBGs) with deep imaging in the U band obtained with the Large Binocular Camera, mounted on the prime focus of the Large Binocular Telescope. We selected 11 LBGs with secure redshift in the range 3.27 =30.7(AB)mag at 1 sigma. We derive a 1 sigma upper limit of f_{esc}^{rel}~5%, which is one of the lowest values found in the literature so far at z~3.3. Assuming that the upper limit for the escape fraction that we derived from our sample holds for all galaxies at this redshift, the hydrogen ionization rate that we obtain (Gamma_{-12}<0.3 s^{-1}) is not enough to keep the IGM ionized and a substantial contribution to the UV background by faint AGNs is required. Since our sample is clearly still limited in size, larger z~3 LBG samples, at similar or even greater depths are necessary to confirm these results on a more firm statistical basis.

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arXiv2011

Spectroscopic confirmation of two Lyman break galaxies at redshift beyond 7

We report the spectroscopic confirmation of two Lyman break galaxies at redshift > 7. The galaxies were observed as part of an ultra-deep spectroscopic campaign with FORS2 at the ESO/VLT for the confirmation of z~7 ``z--band dropout'' candidates selected from our VLT/Hawk-I imaging survey. Both galaxies show a prominent emission line at 9735A and 9858A respectively: the lines have fluxes of ~ 1.6-1.2 x 10^(-17) erg/s/cm2 and exhibit a sharp decline on the blue side and a tail on the red side. The asymmetry is quantitatively comparable to the observed asymmetry in z ~ 6 Ly-alpha lines, where absorption by neutral hydrogen in the IGM truncates the blue side of the emission line profile. We carefully evaluate the possibility that the galaxies are instead at lower redshift and we are observing either [OII], [OIII] or H-alpha emission: however from the spectroscopic and the photometric data we conclude that there are no other plausible identifications, except for Ly-alpha at redshift >7, implying that these are two of the most robust redshift determination for galaxies in the reionization epoch. Based on their redshifts and broad--band photometry, we derive limits on the star formation rate and on the ultraviolet spectral slopes of the two galaxies. We argue that these two galaxies alone are unlikely to have ionized the IGM in their surroundings.

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arXiv2010

An optical and HI study of the dwarf Local Group galaxy VV124=UGC4879. A gas-poor dwarf with a stellar disk?

We present a detailed study of the dwarf galaxy VV124, recently recognized as a isolated member of the Local Group. We have obtained deep (r=26.5) wide-field g,r photometry of individual stars with the LBT under sub-arcsec seeing conditions. The Color-Magnitude Diagram suggests that the stellar content of the galaxy is dominated by an old, metal-poor population, with a significant metallicity spread. A very clean detection of the RGB tip allows us to derive an accurate distance of D=1.3 +/- 0.1 Mpc. Combining surface photometry with star counts, we are able to trace the surface brightness profile of VV124 out to ~ 5' = 1.9 kpc radius (where mu_r=30 mag/arcsec^2), showing that it is much more extended than previously believed. Moreover, the surface density map reveals the presence of two symmetric flattened wings emanating from the central elongated spheroid and aligned with its major axis, resembling a stellar disk seen nearly edge-on. We also present HI observations obtained with the WSRT, the first ever of this object. A total amount of 10^6 M_sun of HI gas is detected in VV124. Compared to the total luminosity, this gives a value of M_HI/L_V=0.11, which is particularly low for isolated Local Group dwarfs. The spatial distribution of the gas does not correlate with the observed stellar wings. The systemic velocity of the HI in the region superposed to the stellar main body of the galaxy is V_h=-25 km/s. The velocity field shows substructures typical of galaxies of this size but no sign of rotation. The HI spectra indicates the presence of a two-phase interstellar medium, again typical of many dwarf galaxies.

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arXiv2010

The bright end of the z ~ 7 UV Luminosity Function from a wide and deep HAWK-I survey

(Abridged) We present here the second half of an ESO Large Programme, which exploits the unique combination of area and sensitivity provided in the near-IR by the camera Hawk-I at the VLT. We have obtained - 30 observing hours with Hawk-I in the Y-band of two high galactic latitude fields. We combined the Y-band data with deep J and K Hawk-I observations, and with FORS1/FORS2 U, B, V, R, I, and Z observations to select z-drop galaxies having Z - Y > 1, no optical detection and flat Y - J and Y - K colour terms. We detect 8 high-quality candidates in the magnitude range Y = 25.5 - 26.5 that we add to the z-drop candidates selected in two Hawk-I pointings over the GOODS-South field. We use this full sample of 15 objects found in -161 arcmin^2 of our survey to constrain the average physical properties and the evolution of the number density of z ~ 7 LBGs. A stacking analysis yields a best-fit SED with photometric redshift z= 6.85 +0.20 -0.15 and an E(B-V)=0.05 +0.15 -0.05. We compute a binned estimate of the z ~ 7 LF and explore the effects of photometric scatter and model uncertainties on the statistical constraints. After accounting for the expected incompleteness through MonteCarlo simulations, we strengthen our previous finding that a Schechter luminosity function constant from z=6 to z=7 is ruled out at a >99% confidence level, even including the effects of cosmic variance. For galaxies brighter than M_1500= -19.0, we derive a luminosity density rho_UV = 1.5^{+2.1}{-0.8} x 10^25 erg/s/Hz/Mpc^3, implying a decrease by a factor 3.5 from z=6 to z=6.8. We find that, under standard assumptions, the emission rate of ionizing photons coming from UV bright galaxies is lower by at least a factor of two than the value required for reionization. Finally, we exploit deep Hawk-I J and K band observations to derive an upper limit on the number density of M1500<~ -22.0 LBGs at z-8 (Y-dropouts).

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arXiv2010

Abell 611: I. weak lensing analysis with LBC

Aims. The Large Binocular Cameras (LBC) are two twin wide field cameras (FOV ~ 23'x 25') mounted at the prime foci of the 8.4m Large Binocular Telescope (LBT). We performed a weak lensing analysis of the z=0.288 cluster Abell 611 on g-band data obtained by the blue-optimized Large Binocular Camera in order to estimate the cluster mass. Methods. Due to the complexity of the PSF of LBC, we decided to use two different approaches, KSB and Shapelets, to measure the shape of background galaxies and to derive the shear signal produced by the cluster. Then we estimated the cluster mass with both aperture densitometry and parametric model fits. Results. The combination of the large aperture of the telescope and the wide field of view allowed us to map a region well beyond the expected virial radius of the cluster and to get a high surface density of background galaxies (23 galaxies/arcmin^2). This made possible to estimate an accurate mass for Abell 611. We find that the mass within 1.5 Mpc is: $(8\pm3)\times 10^{14} M_\odot$ from the aperture mass technique and $(5\pm1)\times 10^{14} M_\odot$ using the model fitting by a NFW mass density profile, for both Shapelets and KSB methods. This analysis demonstrates that LBC is a powerful instrument for weak gravitational lensing studies.

astro-ph.CO↗

arXiv2010

The end of the white dwarf cooling sequence in M 67

In this paper, we present for the first time a proper-motion-selected white dwarf (WD) sample of the old Galactic open cluster M 67, down to the bottom of the WD cooling sequence (CS). The color-magnitude diagram is based on data collected with the LBC-Blue camera at the prime focus of LBT. As first epoch data, we used CFHT-archive images collected 10 years before LBC data. We measured proper motions of all the identified sources. Proper motions are then used to separate foreground and background objects from the cluster stars, including WDs. Finally, the field-object cleaned WD CS in the V vs. B-I color-magnitude diagram is compared with the models. We confirm that the age derived from the location of the bottom of the WD CS is consistent with the turn off age.

astro-ph.SR↗

arXiv2009

Evidence for a fast evolution of the UV luminosity function beyond redshift 6 from a deep HAWK-I survey of the GOODS-S field

We perform a deep search for galaxies in the redshift range 6.5 1, Y-J<1.5 and Y-K<2. The other public data in the UBVRIJHK bands are used to reject possible low redshift interlopers. The output has been compared with extensive Monte Carlo simulations to quantify the observational effects of our selection criteria as well as the effects of photometric errors. We detect 7 high quality candidates in the magnitude range Y=25.5-26.7. This interval samples the critical range for M* at z>6 (M_1500 ~- 19.5 to -21.5). After accounting for the expected incompleteness, we rule out at a 99% confidence level a Luminosity Function constant from z=6 to z=7, even including the effects of cosmic variance. For galaxies brighter than M_1500=-19.0 we derive a luminosity density rho_UV=1.5^{+2.0}_{-0.9} 10^25 erg/s/Hz/Mpc3, implying a decrease by a factor 3.5 from z=6 to z~6.8. On the basis of our findings, we make predictions for the surface densities expected in future surveys surveys, based on ULTRA-VISTA, HST-WFC3 or JWST-NIRCam, evaluating the best observational strategy to maximise their impact.

astro-ph.CO↗

arXiv2009

GRB090426: the farthest short gamma-ray burst?

Aims. With an observed and rest-frame duration of < 2s and < 0.5s, respectively, GRB090426 could be classified as a short GRB. The prompt detection, both from space and ground-based telescopes, of a bright optical counterpart to this GRB offered a unique opportunity to complete a detailed study. Methods. Based on an extensive ground-based observational campaign, we obtained the spectrum of the optical afterglow of GRB090426, measuring its redshift and obtaining information about the medium in which the event took place. We completed follow-up observation of the afterglow optical light curve down to the brightness level of the host galaxy that we firmly identified and studied. We also retrieved and analyzed all the available high-energy data of this event, and compared the results with our findings in the optical. This represents one of the most detailed studies of a short-duration event presented so far. Results. The time properties qualify GRB090426 as a short burst. In this case, its redshift of z = 2.61 would be the highest yet found for a GRB of this class. On the other hand, the spectral and energy properties are more similar to those of long bursts. LBT late-time deep imaging identifies a star-forming galaxy at a redshift consistent with that of the GRB. The afterglow lies within the light of its host and shows evidence of local absorption.

astro-ph.HE↗