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For stars hosting Circumstellar Habitable Zone (CHZ) exoplanets, we investigate the time-evolution of their ultraviolet habitable zone (UHZ), the annular region around a star where an exoplanet could experience a suitable ultraviolet environment for the presence and emergence of life, and the possible intersection of the UHZ with the CHZ. To estimate their UV luminosity evolution, and therefore the evolution of their UHZ, we analyse Swift-UV/Optical telescope observations and adopt the near-UV luminosity evolutionary tracks derived using GALEX observations of young moving groups. We find that an intersection between CHZ and UHZ could exist (or have existed) around all stars of our sample at different epochs, except for the coldest M-dwarfs (temperature < 2800 K, e.g. Trappist-1). For hotter M-dwarfs the formation of RNA precursors through cyanosulfidic chemistry triggered by near-UV radiation could occur during the first 1-2 Gyrs. The radial-extension and time-duration of the CHZ-UHZ intersection increase with the stellar effective temperature and the exoplanet atmospheric transmissivity at near-UV wavelengths. Within our sample, Proxima Centauri represents a golden target for the quest of life outside the Solar system because it experienced a long-lasting and more extended, compared to similar M-dwarfs, CHZ-UHZ intersection., Comment: published in MNRAS Letters, 7 pages, 4 figures

Over the past two decades, our knowledge of the high-redshift (z > 5) radio quasars has expanded, thanks to dedicated high-resolution very long baseline interferometry (VLBI) observations. Distant quasars provide unique information about the formation and evolution of the first galaxies and supermassive black holes in the Universe. Powerful relativistic jets are likely to have played an essential role in these processes. However, the sample of VLBI-observed radio quasars is still too small to allow meaningful statistical conclusions. We extend the list of the VLBI observed radio quasars to investigate how the source structure and physical parameters are related to radio loudness. We assembled a sample of 10 faint radio quasars located at 5 < z < 6 with their radio-loudness indices spanning between 0.9-76. We observed the selected targets with the European VLBI Network (EVN) at 1.7 GHz. The milliarcsecond-scale resolution of VLBI at this frequency allows us to probe the compact innermost parts of radio-emitting relativistic jets. In addition to the single-band VLBI observations, we collected single-dish and low-resolution radio interferometric data to investigate the spectral properties and variability of our sources. The detection rate of this high-redshift, low-flux-density sample is 90%, with only one target (J0306+1853) remaining undetected. The other 9 sources appear core-dominated and show a single, faint and compact radio core on this angular scale. The derived radio powers are typical of FRII radio galaxies and quasars. By extending our sample with other VLBI-detected z > 5 sources from the literature, we found that the core brightness temperatures and monochromatic radio powers tend to increase with radio loudness.

The study of the high-energy (MeV-GeV) part of GRBs spectrum can play a crucial role in investigating the physics of the prompt emission, but it is often hampered by low statistic and the paucity of GeV observations. In this work, we analyze the prompt emission spectra of the 22 brightest GRBs which have been simultaneously observed by Fermi/GBM and Fermi/LAT, spanning 6 orders of magnitude in energy. The high-energy photon spectra can be modelled with a power-law $N(E)\propto E^{-\beta}$ possibly featuring an exponential cutoff. We find that, with the inclusion of the LAT data, the spectral index $\beta$ is softer than what typically inferred from the analysis of Fermi/GBM data alone. Under the assumption that the emission is synchrotron, we derive the index $p\sim2.79$ of the power-law energy distribution of accelerated particles ($N(\gamma)\propto \gamma^{-p}$). In 9 out of 22 GRB spectra, we find a significant presence of an exponential cut-off at high-energy, ranging between 14 and 298 MeV. By interpreting the observed cut-off as a sign of pair-production opacity, we estimate the jet bulk Lorentz factor $\Gamma$, finding values in the range 130-330. These values are consistent with those inferred from the afterglow light curve onset time. Finally, by combining the information from the high-energy prompt emission spectrum with the afterglow lightcurve, we provide a method to derive the distance R from the central engine where the prompt emission occurs. These results highlight the importance of including high-energy data, when available, in the study of prompt spectra and their role in addressing the current challenges of the GRB standard model., Comment: 17 pages, 17 figures. Accepted for publication in A&A

The dozens of rocky exoplanets discovered in the Circumstellar Habitable Zone (CHZ) currently represent the most suitable places to host life as we know it outside the Solar System. However, the presumed presence of liquid water on the CHZ planets does not guarantee suitable environments for the emergence of life. According to experimental studies, the building blocks of life are most likely produced photochemically in presence of a minimum ultraviolet (UV) flux. On the other hand, high UV flux can be life-threatening, leading to atmospheric erosion and damaging biomolecules essential to life. These arguments raise questions about the actual habitability of CHZ planets around stars other than Solar-type ones, with different UV to bolometric luminosity ratios. By combining the "principle of mediocricy" and recent experimental studies, we define UV boundary conditions (UV-habitable Zone, UHZ) within which life can possibly emerge and evolve. We investigate whether exoplanets discovered in CHZs do indeed experience such conditions. By analysing Swift-UV/Optical Telescope data, we measure the near ultraviolet (NUV) luminosities of 17 stars harbouring 23 planets in their CHZ. We derive an empirical relation between NUV luminosity and stellar effective temperature. We find that eighteen of the CHZ exoplanets actually orbit outside the UHZ, i.e., the NUV luminosity of their M-dwarf hosts is decisively too low to trigger abiogenesis - through cyanosulfidic chemistry - on them. Only stars with effective temperature >3900 K illuminate their CHZ planets with enough NUV radiation to trigger abiogenesis. Alternatively, colder stars would require a high-energy flaring activity., Comment: 8 pages, 3 figures

The highly variable and energetic pulsed emission of a long gamma-ray burst (GRB) is thought to originate from local, rapid dissipation of kinetic or magnetic energy within an ultra-relativistic jet launched by a newborn compact object, formed during the collapse of a massive star. The spectra of GRB pulses are best modelled by power-law segments, indicating the dominance of non-thermal radiation processes. Spectral lines in the X-ray and soft $\gamma$-ray regime for the afterglow have been searched for intensively, but never confirmed. No line features ever been identified in the high energy prompt emission. Here we report the discovery of a highly significant ($> 6 \sigma$) narrow emission feature at around $10$ MeV in the brightest ever GRB 221009A. By modelling its profile with a Gaussian, we find a roughly constant width $\sigma \sim 1$ MeV and temporal evolution both in energy ($\sim 12$ MeV to $\sim 6$ MeV) and luminosity ($\sim 10^{50}$ erg/s to $\sim 2 \times 10^{49}$ erg/s) over 80 seconds. We interpret this feature as a blue-shifted annihilation line of relatively cold ($k_\mathrm{B}T\ll m_\mathrm{e}c^2$) electron-positron pairs, which could have formed within the jet region where the brightest pulses of the GRB were produced. A detailed understanding of the conditions that can give rise to such a feature could shed light on the so far poorly understood GRB jet properties and energy dissipation mechanism., Comment: Submitted

Studies of the most distant AGNs allow us to test our current understanding of the physics present in radio-jetted AGNs across a range of environments. The decrease in apparent luminosity with distance is the primary difficulty to overcome in the study of these distant AGNs, which requires highly sensitive instruments. Our goal is to employ new long wavelength radio data to better parametrise the broad-band SED of GB 1508+5714, a high-redshift (z=4.30) AGN. Its high redshift, high intrinsic luminosity, and classification as a blazar allow us to test emission models that consider the efficient cooling of jet electrons via inverse Compton losses in interactions with the dense CMB photon field at high redshifts. A significant detection of this effect in GB 1508+5714 may partly explain the apparent sparsity of high-redshift radio galaxies in wide-field surveys; detections of this kind are only becoming possible with the current generation of SKA precursors. We used international LOFAR telescope to image the long wavelength radio emission around the high-redshift blazar GB 1508+5714 on arcsecond scales at frequencies between 128 MHz and 160 MHz. This allowed us to compare the spatially resolved structure with higher frequency observations, and to construct spectral index maps. The LOFAR image shows a compact unresolved core and two resolved emission regions around 2 arcsec to the east and to the west of the radio core. We find structure consistent with previous VLA observations, as well as a previously unreported emission region to the east. We interpret the arcsecond-scale radio structure of GB 1508+5714 as a FR II-like radio galaxy at a small viewing angle. Our SED modelling shows that a scenario featuring significant quenching effects caused by interaction with the CMB provides a good description of the data, and notably explains the suppressed radio emission., Comment: 11 pages, 10 figures, 2 tables

We report on the Swift/XRT observation and classification of eleven blazar candidates at $z>4$. These sources were selected as part of a sample of extremely radio-loud quasars, in order to focus on quasars with jets oriented roughly close to our line-of-sight. Deriving their viewing angles and their jets bulk Lorentz factors was crucial for a strict blazar classification, and it was possible only thanks to X-ray observations. Out of eleven sources, five show strong and hard X-ray fluxes, that allow their blazar classification, two are uncertain, three host relativistic jets that we observe just outside their beaming cone (i.e. are not strictly blazars), while one went undetected by Swift/XRT. Following this approach, we were able to trace the $>10^9M_\odot$ active supermassive black hole population hosted in jetted active galactic nuclei. At $z\geq4$ the massive jetted sources are likely predominant in the overall quasar population: this calls for a deep review of our understanding of the first supermassive black holes formation and evolution. Jets are indeed key actors in fast accretion, and must be searched for across the whole high redshift quasar population. A note of caution must be added: radio-loudness and in general radio features at high redshifts seem not to perfectly reflect high-energy properties. A strong effect due to interaction with CMB radiation is surely in place, that quenches the radio emission with respect to the X-rays, but also more frequent occasions for the jet to be bent seem to play a relevant role in this matter. Classifications and population studies thus must be carefully performed, in order not to be confused by these inconsistencies., Comment: Accepted for publication on A&A. 10 pages, 4 figures, 2 tables

Introduced in 1998 to attempt a first unified view of the broad-band emission properties of blazars, the blazar sequence has been extensively used in the past 25 years to guide observations as well as physical interpretation of the overall emission from these galaxies. In this review, we describe the evolution of the sequence along with the tremendous advances in the observational field, in particular in the gamma-ray band. A new version of the sequence built on TeV-detected objects is also presented. Two extreme classes of objects (MeV and hard-TeV blazars) are included in the discussion, given their relevance for future observatories. Finally, the current physical understanding at the base of the sequence is presented along with the major criticisms to the blazar sequence., Comment: 18 pages, 9 figures, 2 tables, invited review in Galaxies special issue on "Extragalactic TeV Astronomy"

We study the ratio $R$ between the luminosity of the torus and that of the accretion disk, inferred from the relativistic model KERRBB for a sample of approximately 2000 luminosity-selected radio-quiet Type I active galactic nuclei from the Sloan Digital Sky Survey catalog. We find a mean ratio $R \approx 0.8$ and a considerable number of sources with $R \gtrsim 1$. Our statistical analysis regarding the distribution of the observed ratios suggests that the largest values might be linked to strong relativistic effects due to a large black hole spin ($a > 0.8$), despite the radio-quiet nature of the sources. The mean value of $R$ sets a constraint on the average torus aperture angle (in the range $30^{\circ} < \theta_{\rm T} < 70^{\circ}$) and, for about one-third of the sources, the spin must be $a > 0.7$. Moreover, our results suggest that the strength of the disk radiation (i.e., the Eddington ratio) could shape the torus geometry and the relative luminosity ratio $R$. Given the importance of the involved uncertainties on this statistical investigation, an extensive analysis and discussion have been made to assess the robustness of our results., Comment: 13 pages, 10 figures, accepted for publication in Astronomy & Astrophysics

We report on the JVLA observations of three high redshift Active Galactic Nuclei (AGN), having a black hole mass estimated to be among the largest known. Two of them, SDSS J0100+2802 and SDSS J0306+1853 at redshift 6.326 and 5.363 respectively, are radio-quiet AGN according to the classic definition, while the third (B2 1023+25 at z=5.284) is a powerful blazar. The JVLA data clearly show a radio structure in the first source, and a radio emission with a relatively steep radio spectrum in the second one, demonstrating the presence of a radio jet and a diffuse component. Therefore, being radio-quiet does not exclude the presence of a powerful relativistic jet with important consequences on the population studies and on the ratio between jetted and non-jetted AGN. We can estimate the viewing angle of these jets, and this allows us to find, albeit with some uncertainty, the density of black holes with a mass in excess of $10^{10}M_\odot$ at high redshifts. We found that their density in jetted AGN is very large in the redshift bin 5-6, comparable with the overall AGN population of the same optical luminosity. Jets might thus play a crucial role in the fast formation and evolution of the most massive black holes in the early Universe. They are more common than what expected from wide radio surveys with mJy flux sensitivity. Deeper JVLA or VLBI observations are key to discover a possible relativistic jet population hiding in plain sight at very high-redshift. The discovery of powerful relativistic jets associated with the most massive black holes in the early Universe re-opens the question: is the jet instrumental for a rapid growth of the black hole or instead is the black hole mass the main driver for the jet formation?, Comment: In press on A&A. 13 pages, 9 figures, 5 tables

We report on two Chandra observations of the quasar PSO J0309+27, the most distant blazar observed so far (z=6.1), performed eight months apart, in March and November 2020. Previous Swift-XRT observation showed that this object is one of the brightest X-ray sources beyond redshift 6.0 ever observed so far. This new data-set confirmed the high flux level and unveiled a spectral change occurred on a very short timescale (250s rest-frame), caused by a significant softening of the emission spectrum. This kind of spectral variability, on a such short interval, has never been reported in the X-ray emission of a flat spectrum radio quasar. A possible explanation is given by the emission produced by the inverse Compton scatter of the quasar UV photons by the cold electrons present in a fast shell moving along the jet. Although this bulk comptonization emission should be an unavoidable consequence of the standard leptonic jet model, this would be the first time that it is observed., Comment: Accepted for publication in ApJ; 10 pages, 7 figures

We report the time-resolved spectral analysis of a bright near-infrared and moderate X-ray flare of Sgr A*. We obtained light curves in the $M$-, $K$-, and $H$-bands in the mid- and near-infrared and in the $2-8~\mathrm{keV}$ and $2-70~\mathrm{keV}$ bands in the X-ray. The observed spectral slope in the near-infrared band is $\nu L_\nu\propto \nu^{0.5\pm0.2}$; the spectral slope observed in the X-ray band is $\nu L_\nu \propto \nu^{-0.7\pm0.5}$. We tested synchrotron and synchrotron self-Compton (SSC) scenarios. The observed near-infrared brightness and X-ray faintness, together with the observed spectral slopes, pose challenges for all models explored. We rule out a scenario in which the near-infrared emission is synchrotron emission and the X-ray emission is SSC. A one-zone model in which both the near-infrared and X-ray luminosity are produced by SSC and a model in which the luminosity stems from a cooled synchrotron spectrum can explain the flare. In order to describe the mean SED, both models require specific values of the maximum Lorentz factor $\gamma_{max}$, which however differ by roughly two orders of magnitude: the SSC model suggests that electrons are accelerated to $\gamma_{max}\sim 500$, while cooled synchrotron model requires acceleration up to $\gamma_{max}\sim5\times 10^{4}$. The SSC scenario requires electron densities of $10^{10}~\mathrm{cm^{-3}}$ much larger than typical ambient densities in the accretion flow, and thus require in an extraordinary accretion event. In contrast, assuming a source size of $1R_s$, the cooled synchrotron scenario can be realized with densities and magnetic fields comparable with the ambient accretion flow. For both models, the temporal evolution is regulated through the maximum acceleration factor $\gamma_{max}$, implying that sustained particle acceleration is required to explain at least a part of the temporal evolution of the flare., Comment: accepted for publication in Astronomy & Astrophysics; preview abstract shortened due to arXiv requirements

Gamma-ray Bursts (GRBs) prompt emission spectra are often fitted with the empirical ''Band" function, namely two power laws smoothly connected. The typical slope of the low energy (sub-MeV) power law is $\alpha_{B}\simeq -1$. In a small fraction of long GRBs this power law splits into two components such that the spectrum presents, in addition to the typical $\sim$ MeV $\nu F_{\nu}$ peak, a break at the order of a few keV or hundreds keV. The typical power law slopes below and above the break are -0.6 and -1.5 respectively. If the break is a common feature, the value of $\alpha_{B}$ could be an ''average'' of the spectral slopes below and above the break in GRBs fitted with Band function. We analyze the spectra of 27 (9) bright long (short) GRBs detected by the Fermi satellite finding a low energy break between 80 keV and 280 keV in 12 long GRBs, but in none of the short events. Through spectral simulations we show that if the break is moved closer (farther) to the peak energy a relatively harder (softer) $\alpha_{B}$ is found by fitting the simulated spectra with the Band function. The hard average slope $\alpha_{B}\simeq-0.38$ found in short GRBs suggests that the break is close to the peak energy. We show that for 15 long GRBs best fitted by the Band function only, the break could be present, but it is not identifiable in the Fermi/GBM spectrum, because either at low energies, close to the detector limit for relatively soft $\alpha_{B}\lesssim-1$, or in the proximity of the energy peak for relatively hard $\alpha_{B}\gtrsim-1$. A spectrum with two breaks could be typical of GRB prompt emission, though hard to identify with current detectors. Instrumental design such that conceived for the THESEUS space mission, extending from 0.3 keV to several MeV and featuring a larger effective area with respect to Fermi/GBM, can reveal a larger fraction of GRBs with a spectral energy break.

We present a definitive assessment of the role of Inverse Compton scattering of Cosmic Microwave Background photons (IC/CMB) in the context of radio galaxies. Owing to the steep increase of the CMB radiation energy density, IC/CMB is supposed to become progressively more important with respect to radio synchrotron cooling as the redshift increases. For typical energies at play, this process will up-scatter the CMB photons into the X-ray band, and is thus expected to yield a redshift-dependent, concurrent X-ray brightening and radio dimming of the jet-powered structures. Here we show how a conclusive proof of this effect hinges on high-resolution imaging data in which the extended lobes can be distinguished from the compact hot spots where synchrotron-self-Compton dominates the X-ray emission regardless of redshift. We analyze Chandra and Very Large Array data of 11 radio galaxies between 1.33 radio galaxies below <1 mJy, IC/CMB alone can not be responsible for a deficit in high-z, radio-loud AGN if--as we argue--such AGN typically have bright, compact hot spots., Comment: 16 pages, 5 figures, 5 tables. Accepted for publication in MNRAS

THESEUS is a medium size space mission of the European Space Agency, currently under evaluation for a possible launch in 2032. Its main objectives are to investigate the early Universe through the observation of gamma-ray bursts and to study the gravitational waves electromagnetic counterparts and neutrino events. On the other hand, its instruments, which include a wide field of view X-ray (0.3-5 keV) telescope based on lobster-eye focusing optics and a gamma-ray spectrometer with imaging capabilities in the 2-150 keV range, are also ideal for carrying out unprecedented studies in time domain astrophysics. In addition, the presence onboard of a 70 cm near infrared telescope will allow simultaneous multi-wavelegth studies. Here we present the THESEUS capabilities for studying the time variability of different classes of sources in parallel to, and without affecting, the gamma-ray bursts hunt., Comment: Submitted to Experimental Astronomy

We collect data at all frequencies for the new sources classified as unknown active galactic nuclei (AGNs) in the latest Burst Alert Telescope (BAT) all-sky hard X-ray catalog. Focusing on the 36 sources with measured redshift, we compute their spectral energy distribution (SED) from radio to $\gamma$-rays with the aim to classify these objects. We apply emission models that attempt to reproduce the obtained SEDs, including: i) a standard thin accretion disk together with an obscuring torus and a X-ray corona; ii) a two temperature thick advection-dominated flow; iii) an obscured AGN model, accounting for absorption along the line of sight at kiloelectronvolt energies and in the optical band; and iv) a phenomenological model to describe the jet emission in blazar-like objects. We integrate the models with the SWIRE template libraries to account for the emission of the host galaxy. For every source we found a good agreement between data and our model. Considering that the sources were selected in the hard X-ray band, which is rather unaffected by absorption, we expected and found a large fraction of absorbed radio-quiet AGNs (31 out of 36) and some additional rare radio-loud sources (5 out of 36), since the jet emission in hard X-rays is important for aligned jets owing to the boost produced by the beaming effect. With our work we can confirm the hypothesis that a number of galaxies, whose optical spectra lack AGN emission features, host an obscured active nucleus. The approach we used proved to be efficient in rapidly identifying objects, which commonly used methods were not able to classify., Comment: 11 pages, LaTeX; Acknowledgments added

Counted among the most powerful cosmic events, supernovae (SNe) and gamma-ray bursts (GRBs) can be highly disruptive for life: Their radiation can be harmful for biota or induce extinction by removing most of the protective atmospheric ozone layer from terrestrial planets (TPs). Nearby high-energy transient astrophysical events have been proposed as possible triggers of mass extinctions on Earth. We assess the habitability of the Milky Way (MW) throughout its cosmic history against potentially disruptive astrophysical transients with the aim of identifying the safest places and epochs within our Galaxy. We also test the hypothesis that one long GRB played a leading role in the late Ordovician mass-extinction event (~445 Myr ago). We characterised the habitability of the MW throughout its cosmic history as a function of galactocentric distance of TPs. We estimated the dangerous effects of transient astrophysical events (long and short GRBs and SNe) with a model that connects their rate to the specific star formation and metallicity evolution within the Galaxy throughout its cosmic history. Our model also accounts for the probability that TPs form around FGK and M stars. Until about six billion years ago, the outskirts of the Galaxy were the safest places to live, despite the relatively low density of TPs. In the last about four billion years, regions between 2 and 8 kpc from the center, which had a higher density of TPs, became the best places for a relatively safer biotic life growth. We confirm the hypothesis that one long GRB played a leading role in the late Ordovician mass-extinction event. In the last 500 Myr, the safest neighborhood in the Galaxy was a region at a distance of 2 to 8 kpc from the Galactic center, whereas the MW outskirts were sterilized by two to five long GRBs., Comment: 11 pages, 10 figures, 4 tables. Accepted for pubblication in Astronomy & Astrophysics

$\gamma$-ray bursts (GRBs) are short-lived transients releasing a large amount of energy ($10^{51}-10^{53} $ erg) in the keV-MeV energy range. GRBs are thought to originate from internal dissipation of the energy carried by ultra-relativistic jets launched by the remnant of a massive star's death or a compact binary coalescence. While thousands of GRBs have been observed over the last thirty years, we still have an incomplete understanding of where and how the radiation is generated in the jet. Here we show a relation between the spectral index and the flux found by investigating the X-ray tails of bright GRB pulses via time-resolved spectral analysis. This relation is incompatible with the long standing scenario which invokes the delayed arrival of photons from high-latitude parts of the jet. While the alternative scenarios cannot be firmly excluded, the adiabatic cooling of the emitting particles is the most plausible explanation for the discovered relation, suggesting a proton-synchrotron origin of the GRB emission., Comment: Published in Nature Communications

We report on deep Chandra X-ray Telescope imaging observations of 4C 63.20, one of the few known radio galaxies at z>3.5. The X-ray counterpart is resolved into a core plus two off-nuclear sources that (combined) account for close to 30% of the total X-ray flux. Their morphology and orientation are consistent with a diffuse, lobe-like nature, albeit compact hotspots cannot be ruled out. The broadband spectral energy distribution of 4C 63.20 can be reproduced with a jet model where the majority of the radio flux can be ascribed to synchrotron emission from the hotspots, whereas the (non-nuclear) X-ray emission is produced via Inverse Compton (IC) off of Cosmic Microwave Background (CMB) photons within the extended lobes. This scenario is broadly consistent with the expectation from highly magnetized lobes in a hotter CMB, and supports the view that IC/CMB may quench less extreme radio lobes at high redshifts., Comment: 10 pages, 8 figures, accepted to MNRAS

We present the discovery of PSO J030947.49+271757.31, the radio brightest (23.7 mJy at 1.4 GHz) active galactic nucleus (AGN) at z>6.0. It was selected by cross-matching the NRAO VLA Sky Survey and the Panoramic Survey Telescope and Rapid Response System PS1 databases and its high-z nature was confirmed by a dedicated spectroscopic observation at the Large Binocular Telescope. A pointed Neil Gehrels $Swift$ Observatory XRT observation allowed us to measure a flux of $\sim$3.4$\times$10$^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the [0.5-10] keV energy band, which also makes this object the X-ray brightest AGN ever observed at z>6.0. Its flat radio spectrum ($\alpha_{\nu r}$<0.5), very high radio loudness (R>10$^3$), and strong X-ray emission, compared to the optical, support the hypothesis of the blazar nature of this source. %i.e. a radio-loud (RL) AGN with the relativistic jet pointed toward us. Assuming that this is the only blazar at this redshift in the surveyed area of sky, we derive a space density of blazars at z$\sim$6 and with M$_{1450 \mbox{\AA}}$ < -25.1 of 5.5$^{+11.2}_{-4.6}$$\times$10$^{-3}$ Gpc$^{-3}$. From this number, and assuming a reasonable value of the bulk velocity of the jet ($\Gamma$=10), we can also infer a space density of the entire radio-loud AGN population at z$\sim$6 with the same optical/UV absolute magnitude of 1.10$^{+2.53}_{-0.91}$ Gpc$^{-3}$. Larger samples of blazars will be necessary to better constrain these estimates., Comment: 7 pages, 8 figures, 3 tables, Accepted to publication in A&A Letters. Typo corrections

We discuss the new surprising observational results that indicate quite convincingly that the prompt emission of Gamma-Ray Bursts (GRBs) is due to synchrotron radiation produced by a particle distribution that has a low energy cut-off. The evidence of this is provided by the low energy part of the spectrum of the prompt emission, that shows the characteristic F(nu) \propto nu^(1/3) shape followed by F(nu) \propto nu^(-1/2) up to the peak frequency. This implies that although the emitting particles are in fast cooling, they do not cool completely. This poses a severe challenge to the basic ideas about how and where the emission is produced, because the incomplete cooling requires a small value of the magnetic field, to limit synchrotron cooling, and a large emitting region, to limit the self-Compton cooling, even considering Klein-Nishina scattering effects. Some new and fundamental ingredient is required for understanding the GRBs prompt emission. We propose proton-synchrotron as a promising mechanism to solve the incomplete cooling puzzle., Comment: 7 pages, 3 figures, accepted for publication in A&A

Extragalactic jets are the most powerful persistent sources of the universe. Those pointing at us are called blazars. Their relativistically boosted emission extends from radio frequencies to TeV energies. They are also suspected to be the sources of energetic neutrinos and high energies cosmic rays. The study of their overall spectrum indicates that most of the emission of powerful blazars is in hard X-rays or in soft gamma-rays. In this band we can find the most powerful jets, visible also at high redshifts. It is found that the jet power is linked to the accretion luminosity, and exceeds it, especially if they produce energetic neutrinos, that require the presence of ultrarelativistic protons., Comment: 6 paged, 6 figures, invited paper at the: "12th INTEGRAL conference" - Geneve, Feb. 2019

GrailQuest (Gamma Ray Astronomy International Laboratory for QUantum Exploration of Space-Time) is a mission concept based on a constellation (hundreds/thousands) of nano/micro/small-satellites in low (or near) Earth orbits. Each satellite hosts a non-collimated array of scintillator crystals coupled with Silicon Drift Detectors with broad energy band coverage (keV-MeV range) and excellent temporal resolution ( below or equal 100 nanoseconds) each with effective area around 100 cm2. This simple and robust design allows for mass-production of the satellites of the fleet. This revolutionary approach implies a huge reduction of costs, flexibility in the segmented launching strategy, and an incremental long-term plan to increase the number of detectors and their performance: a living observatory for next-generation, space-based astronomical facilities. GrailQuest is conceived as an all-sky monitor for fast localisation of high signal-to-noise ratio transients in the X/gamma-ray band, e.g. the elusive electromagnetic counterparts of gravitational wave events. Robust temporal triangulation techniques will allow unprecedented localisation capabilities, in the keV-MeV band, of a few arcseconds or below, depending on the temporal structure of the transient event. The ambitious ultimate goal of this mission is to perform the first experiment, in quantum gravity, to directly probe space-time structure down to the minuscule Planck scale, by constraining or measuring a first order dispersion relation for light in vacuo. This is obtained by detecting delays between photons of different energies in the prompt emission of Gamma-ray Bursts., Comment: White paper submitted in response to the ESA call Voyage 2050 - Submitted to Experimental Astronomy

The Gamma Ray Burst (GRB) 180720B is one of the brightest events detected by the Fermi satellite and the first GRB detected by the H.E.S.S. telescope above 100 GeV. We analyse the Fermi (GBM and LAT) and Swift (XRT and BAT) data and describe the evolution of the burst spectral energy distribution in the 0.5 keV - 10 GeV energy range over the first 500 seconds of emission. We reveal a smooth transition from the prompt phase, dominated by synchrotron emission in a moderately fast cooling regime, to the afterglow phase whose emission has been observed from the radio to the GeV energy range. The LAT (0.1 - 100 GeV) light curve initially rises ($F_{\rm LAT}\propto t^{2.4}$), peaks at $\sim$78 s, and falls steeply ($F_{\rm LAT}\propto t^{-2.2}$) afterwards. The peak, which we interpret as the onset of the fireball deceleration, allows us to estimate the bulk Lorentz factor $\Gamma_{0}\sim 150 \ (300)$ under the assumption of a wind-like (homogeneous) circum-burst medium density. We derive a flux upper limit in the LAT energy range at the time of H.E.S.S. detection, but this does not allow us to unveil the nature of the high energy component observed by H.E.S.S. We fit the prompt spectrum with a physical model of synchrotron emission from a non-thermal population of electrons. The 0 - 35 s spectrum after its $E F(E)$ peak (at 1 - 2 MeV) is a steep power law extending to hundreds of MeV. We derive a steep slope of the injected electron energy distribution $N(\gamma)\propto \gamma^{-5}$. Our fit parameters point towards a very low magnetic field ($B'\sim 1 $ G) in the emission region., Comment: 10 pages, 6 figures, submitted to A&A

We present the discovery and properties of DESJ014132.4-542749.9 (DES0141-54), a new powerful radio-loud active galactic nucleus (AGN) in the early Universe (z=5.0). It was discovered by cross-matching the first data release of the Dark Energy Survey (DES DR1) with the Sidney University Molonglo Survey (SUMSS) radio catalog at 0.843 GHz. This object is the first radio-loud AGN at high redshift discovered in the DES. The radio properties of DES0141-54, namely its very large radio-loudness (R>10$^{4}$), the high radio luminosity (L$_{0.8 GHz}$=1.73$\times$10$^{28}$ W Hz$^{-1}$), and the flatness of the radio spectrum ($\alpha$=0.35) up to very high frequencies (120 GHz in the source's rest frame), classify this object as a blazar, meaning, a radio-loud AGN observed along the relativistic jet axis. However, the X--ray luminosity of DESJ0141-54 is much lower compared to those of the high redshift (z$\geq$4.5) blazars discovered so far. Moreover its X-ray-to-radio luminosity ratio (log($\frac{L_{[0.5-10]keV}}{L_{1.4GHz}}$)=9.96$\pm$0.30 Hz) is small also when compared to lower redshift blazars: only 2\% of the low-z population has a similar ratio. By modeling the spectral energy distribution we found that this peculiar X--ray weakness and the powerful radio emission could be related to a particularly high value of the magnetic field. Finally, the mass of the central black hole is relatively small (M$_{BH}$ = 3-8 $\times$10$^8$ M$_{\odot}$) compared to other confirmed blazars at similar redshift, making DES0141-54 the radio-loud AGN that host the smallest supermassive black hole ever discovered at z$\geq$5., Comment: 12 pages, 10 figures, 6 tables. Accepted for publication in Astronomy and Astrophysics

We selected a sample of 28 Type 1 AGNs for which a black hole mass has been inferred using the reverberation mapping technique and single epoch scaling relations. All 28 sources show clear evidence of the "Big Blue Bump" in the optical-UV band whose emission is produced by an accretion disk (AD) around a supermassive black hole. We fitted the spectrum of these sources with the relativistic thin AD model KERRBB in order to infer the black hole masses and compared them with those from Reverberation mapping and Single epoch methods, discussing the possible uncertainties linked to such a model by quantifying their weight on our results. We find that for the majority of the sources, KERRBB is a good description of the AD emission for a wide wavelength range. The overall uncertainty on the black hole mass estimated through the disk fitting procedure is $\sim 0.45$ dex (which includes the uncertainty on fitting parameters such as e.g. spin and viewing angle), comparable to the systematic uncertainty of reverberation mapping and single epoch methods; however, such an uncertainty can be $\leq 0.3$ dex if one of the parameters of the fit is well constrained. Although all of the estimates are affected by large uncertainties, the masses inferred using the three methods are compatible if the dimensionless scale factor $f$ (linked to the unknown kinematics and geometry of the Broad Line Region) is assumed to be larger than one. For the majority of the sources, the comparison between the results coming from the three methods favors small spin values. To check the goodness of the KERRBB results, we compared them with those inferred with other models, such as AGNSED, a model that also accounts for the emission originating from an X-ray corona: using two sources with a good data coverage in the X band, we find that the masses estimated with the two models differ at most by a factor of $\sim 0.2$ dex., Comment: 25 pages, 35 figures, accepted for publication in A&A

In the last few years many exoplanets in the habitable zone (HZ) of M-dwarfs have been discovered, but the X-ray/UV activity of cool stars is very different from that of our Sun. The high-energy radiation environment influences the habitability, plays a crucial role for abiogenesis, and impacts planetary atmospheres. LHS 1140b is a super-Earth-size planet orbiting in the HZ of LHS 1140, an M4.5 dwarf at ~15 parsecs. We present the results of a Swift X-ray/UV observing campaign. We characterize for the first time the X-ray/UV radiation environment of LHS 1140b. We measure the variability of the near ultraviolet (NUV) flux and estimate the far ultraviolet (FUV) flux with a correlation between FUV and NUV flux of a sample of low-mass stars in the GALEX archive. We highlight the presence of a dominating X-ray source close to the J2000 coordinates of LHS 1140, characterize its spectrum, and derive an X-ray flux upper limit for LHS 1140. We find that this contaminant source could have influenced the previously estimated spectral energy distribution. No significant variation of the NUV flux of LHS 1140 is found over 3 months, and we do not observe any flare during the 38 ks on the target. LHS 1140 is in the 25th percentile of least variable M4-M5 dwarfs of the GALEX sample. Analyzing the UV flux experienced by the HZ planet LHS 1140b, we find that outside the atmosphere it receives a NUV flux <2% with respect to that of the present-day Earth, while the FUV/NUV ratio is ~100-200 times higher. This represents a lower limit to the true FUV/NUV ratio since the GALEX FUV band does not include Lyman-alpha, which dominates the FUV output of low-mass stars. This is a warning for future searches for biomarkers, which must take into account this high ratio. The relatively low level and stability of UV flux experienced by LHS 1140b should be favorable for its present-day habitability., Comment: 6 pages, 4 figures, 2 tables. Accepted for pubblication in Astronomy & Astrophysics

We observed with the NuSTAR satellite 3 blazars at z>2, detected in gamma-rays by Fermi/LAT and in the soft X-rays, but not yet observed above 10 keV. The flux and slope of their X-ray continuum, together with Fermi/LAT data allows us to estimate their total electromagnetic output and peak frequency. For some of them we can study the source in different states, and investigate the main cause of the observed different spectral energy distribution. We then collected all blazars at redshift greater than 2 observed by NuSTAR, and confirm that these hard and luminous X-ray blazars are among the most powerful persistent sources in the Universe. We confirm the relation between the jet power and the disk luminosity, extending it at the high energy end., Comment: 10 pages, 13 figures, accepted for publication in A&A

The peculiar short gamma-ray burst GRB 170817A has been firmly associated to the gravitational wave event GW170817, which has been unaninmously interpreted as due to the coalescence of a double neutron star binary. The unprecedented behaviour of the non-thermal afterglow led to a debate about its nature, which was eventually settled by high-resolution VLBI observations, which strongly support the off-axis structured jet scenario. Using information on the jet structure derived from multi-wavelength fitting of the afterglow emission and of the apparent VLBI image centroid motion, we compute the appearance of a GRB 170817A-like jet as seen by an on-axis observer and we compare it to the previously observed population of SGRB afterglows and prompt emission events. We find that the intrinsic properties of the GRB 170817A jet are representative of a typical event in the SGRB population, hinting at a quasi-universal jet structure. The diversity in the SGRB afterglow population could therefore be ascribed in large part to extrinsic (redshift, density of the surrounding medium, viewing angle) rather than intrinsic properties. Although more uncertain, the comparison can be extended to the prompt emission properties, leading to similar conclusions., Comment: 9 pages, 3 figures, submitted to A&A. Comments and suggestions are welcome

We review the physics potential of a next generation search for solar axions: the International Axion Observatory (IAXO). Endowed with a sensitivity to discover axion-like particles (ALPs) with a coupling to photons as small as $g_{a\gamma}\sim 10^{-12}$ GeV$^{-1}$, or to electrons $g_{ae}\sim$10$^{-13}$, IAXO has the potential to find the QCD axion in the 1 meV$\sim$1 eV mass range where it solves the strong CP problem, can account for the cold dark matter of the Universe and be responsible for the anomalous cooling observed in a number of stellar systems. At the same time, IAXO will have enough sensitivity to detect lower mass axions invoked to explain: 1) the origin of the anomalous "transparency" of the Universe to gamma-rays, 2) the observed soft X-ray excess from galaxy clusters or 3) some inflationary models. In addition, we review string theory axions with parameters accessible by IAXO and discuss their potential role in cosmology as Dark Matter and Dark Radiation as well as their connections to the above mentioned conundrums., Comment: Prepared for submission to JCAP; new version with very minor corrections after referee review

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., Comment: 7 pages, 3 figures, 1 table. Accepted for publication on A&A

The long-lasting tension between the observed spectra of gamma ray bursts (GRBs) and the predicted synchrotron emission spectrum might be solved if electrons do not completely cool. Evidence for incomplete cooling was recently found in Swift GRBs with prompt observations down to 0.1 keV and in one bright Fermi burst, GRB 160625B. Here we systematically search for evidence of incomplete cooling in the spectra of the ten brightest short and long GRBs observed by Fermi. We find that in 8/10 long GRBs there is compelling evidence of a low energy break (below the peak energy) and good agreement with the photon indices of the synchrotron spectrum (respectively -2/3 and -3/2 below the break and between the break and the peak energy). Interestingly, none of the ten short GRBs analysed shows a break but the low energy spectral slope is consistent with -2/3. In a standard scenario, these results imply a very low magnetic field in the emission region (B' ~ 10 G in the comoving frame), at odd with expectations., Comment: 14 pages, 15 figures, in press, accepted for publication in A&A

GRB 190114C is the first gamma-ray burst detected at Very High Energies (VHE, i.e. >300 GeV) by the MAGIC Cherenkov telescope. The analysis of the emission detected by the Fermi satellite at lower energies, in the 10 keV -- 100 GeV energy range, up to ~ 50 seconds (i.e. before the MAGIC detection) can hold valuable information. We analyze the spectral evolution of the emission of GRB 190114C as detected by the Fermi Gamma-Ray Burst Monitor (GBM) in the 10 keV -- 40 MeV energy range up to ~60 sec. The first 4 s of the burst feature a typical prompt emission spectrum, which can be fit by a smoothly broken power-law function with typical parameters. Starting on ~4 s post-trigger, we find an additional nonthermal component, which can be fit by a power law. This component rises and decays quickly. The 10 keV -- 40 MeV flux of the power-law component peaks at ~ 6 s; it reaches a value of 1.7e-5 erg cm-2 s-1. The time of the peak coincides with the emission peak detected by the Large Area Telescope (LAT) on board Fermi. The power-law spectral slope that we find in the GBM data is remarkably similar to that of the LAT spectrum, and the GBM+LAT spectral energy distribution seems to be consistent with a single component. This suggests that the LAT emission and the power-law component that we find in the GBM data belong to the same emission component, which we interpret as due to the afterglow of the burst. The onset time allows us to estimate the initial jet bulk Lorentz factor Gamma_0 is about 500, depending on the assumed circum-burst density., Comment: 7 pages, 2 figures, in press, accepted for publication in A&A

We perform the luminosity--volume test, also known as $\langle V/V_{MAX}\rangle$, to Fast Radio Bursts (FRBs). We compare the 23 FRBs, recently discovered by ASKAP, with 20 of the FRBs found by Parkes. These samples have different flux limits and correspond to different explored volumes. We assume that their dispersion measure indicates their redshift and apply the appropriate cosmological corrections to the spectrum and rate in order to compute the $\langle V/V_{MAX}\rangle$ for the ASKAP and Parkes samples. For a radio spectrum of FRBs $F_\nu \propto \nu^{-1.6}$, we find $\langle V/V_{MAX}\rangle=0.66\pm 0.05$ for the ASKAP sample, that includes FRBs up to $z=0.7$, and 0.52$\pm 0.04$ for Parkes, that extends up to $z=2.1$. The ASKAP value suggests that the population of FRB progenitors evolves faster than the star formation rate, while the Parkes value is consistent with it. Even a delayed (as a power law or gaussian) star formation rate cannot reproduce the $\langle V/V_{MAX}\rangle$ of both samples. If FRBs do not evolve in luminosity, the $\langle V/V_{MAX}\rangle$ values of ASKAP and Parkes sample are consistent with a population of progenitors whose density strongly evolves with redshift as $\sim z^{2.8}$ up to $z \sim 0.7$. We discuss possible scenarios accounting for our results., Comment: 10 pages, 5 figures, 3 tables, accepted by A&A on 2019/04/02

We have not identified for sure what is the mechanism launching, accelerating and collimating relativistic jets. The two most likely possibilities are the gravitational energy of the accreting matter or the rotational energy of a spinning black hole. Even the evaluation of the jet power is not trivial, since the radiation from the jet is enhanced by relativistic beaming, and there are fundamental uncertainties concerning the matter content of the jet (electron--proton or electron--positron plasma). However, in recent years, there have been crucial advances mainly driven by the richness of data in the $\gamma$-ray band. This is the band where blazars emit most of their electromagnetic power. Furthermore, there are now large sample of $\gamma$-ray loud blazars covered by optical spectroscopy. For the blazar sub-class of flat spectrum radio quasars (FSRQ) these data provide measurements of the main emission lines and of the underlying continuum. From these data, it is relatively easy to infer the bolometric luminosity of the accretion disk. The relativistic jet emission on one hand, and the disk luminosity on the other hand, allows us to compare the jet power and the accretion luminosity. Although the inferred jet power is subject to a few assumptions and is somewhat model--dependent, it is possible to derive a lower limit to the jet power that is assumption-free and model-independent. Since this lower limit is of the order of the accretion luminosity, we infer that the true jet power is larger., Comment: 12 pages, 4 figures, Invited talk at the meeting: A Decade of Agile: Results, Challenges and Prospects of Gamma-Ray Astrophysics. In memory of Giovanni Fabrizio "Nanni" Bignami. Accademia dei Lincei, December 2017

We investigate the properties of the most distant quasars ULASJ134208.10+092838.61 ($z = 7.54$), ULASJ112001.48+064124.3 ($z = 7.08$) and DELSJ003836.10-152723.6 ($z = 7.02$) studying their Optical-UV emission that shows clear evidence of the presence of an accretion disk. We model such emission applying the relativistic disk models KERRBB and SLIMBH for which we have derived some analytical approximations to describe the observed emission as a function of the black hole mass, accretion rate, spin and the viewing angle. We found that: 1] our black hole mass estimates are compatible with the ones found using the virial argument but with a smaller uncertainty; 2] assuming that the virial argument is a reliable method to have a black hole mass measurement (with no systematic uncertainties involved), we found an upper limit for the black hole spin of the three sources: very high spin values are ruled out; 3] our Eddington ratio estimates are smaller than those found in previous studies by a factor $\sim 2$: all sources are found to be sub-Eddington. Using our results, we explore the parameter space (efficiency, accretion rate) to describe the possible evolution of the black hole assuming a $\sim 10^{2-4} M_{\odot}$ seed: if the black hole in these sources formed at redshift $z = 10 - 20$, we found that the accretion has to proceed at the Eddington rate with a radiative efficiency $\eta \sim 0.1$ in order to reach the observed masses in less than $\sim 0.7$ Gyr., Comment: 11 pages, 5 figures, 3 tables, accepted for publication in A&A

The binary neutron star merger event GW170817 was detected through both electromagnetic radiation and gravitational waves. Its afterglow emission may have been produced by either a narrow relativistic jet or an isotropic outflow. High spatial resolution measurements of the source size and displacement can discriminate between these scenarios. We present Very Long Baseline Interferometry observations, performed 207.4 days after the merger, using a global network of 32 radio telescopes. The apparent source size is constrained to be smaller than 2.5 milliarcseconds at the 90% confidence level. This excludes the isotropic outflow scenario, which would have produced a larger apparent size, indicating that GW170817 produced a structured relativistic jet. Our rate calculations show that at least 10% of neutron star mergers produce such a jet., Comment: 28 pages, 10 figures, 2 tables. This is the author's version of the work. It is posted here by permission of the AAAS for personal use, not for redistribution. The definitive version was published in Science , (2019-02-21), doi: 10.1126/science.aau8815

e-ASTROGAM is a concept for a breakthrough observatory space mission carrying a gamma-ray telescope dedicated to the study of the non-thermal Universe in the photon energy range from 0.15 MeV to 3 GeV. The lower energy limit can be pushed down to energies as low as 30 keV for gamma-ray burst detection with the calorimeter. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with remarkable polarimetric capability. Thanks to its performance in the MeV-GeV domain, substantially improving its predecessors, e-ASTROGAM will open a new window on the non-thermal Universe, making pioneering observations of the most powerful Galactic and extragalactic sources, elucidating the nature of their relativistic outflows and their effects on the surroundings. With a line sensitivity in the MeV energy range one to two orders of magnitude better than previous and current generation instruments, e-ASTROGAM will determine the origin of key isotopes fundamental for the understanding of supernova explosion and the chemical evolution of our Galaxy. The mission will be a major player of the multiwavelength, multimessenger time-domain astronomy of the 2030s, and provide unique data of significant interest to a broad astronomical community, complementary to powerful observatories such as LISA, LIGO, Virgo, KAGRA, the Einstein Telescope and the Cosmic Explorer, IceCube-Gen2 and KM3NeT, SKA, ALMA, JWST, E-ELT, LSST, Athena, and the Cherenkov Telescope Array., Comment: 15 pages, 7 figures. Submitted to the proceedings of the conference SPIE Astronomical Telescopes and Instrumentation 2018: Ultraviolet to Gamma Ray. v2: corrections of authors' affiliations

The role played by the intergalactic medium (IGM) in the X-ray absorption towards high-redshift sources has recently drawn more attention in spectral analysis studies. Here, we study the X-ray absorption towards 15 flat-spectrum radio quasars at $z>2$, relying on high counting statistic ($\gtrsim10\,000$ photons) provided by XMM-Newton, with additional NuSTAR (and simultaneous Swift-XRT) observations when available. Blazars can be confidently considered to have negligible X-ray absorption along the line of sight within the host galaxy, likely swept by the kpc-scale relativistic jet. This makes our sources ideal for testing the absorption component along the IGM. Our new approach is to revisit the origin of the soft X-ray spectral hardening observed in high-z blazars in terms of X-ray absorption occurring along the IGM, with the help of a low-z sample used as comparison. We verify that the presence of absorption in excess of the Galactic value is the preferred explanation to explain the observed hardening, while intrinsic energy breaks, predicted by blazars' emission models, can easily occur out of the observing energy band in most sources. First, we perform an indirect analysis comparing the inferred amount of absorption in excess of the Galactic value with a simulated IGM absorption contribution, that increases with redshift and includes both a minimum component from diffuse IGM metals, and the additional contribution of discrete denser intervening regions. Then, we directly investigate the warm-hot IGM with a spectral model on the best candidates of our sample, obtaining an average IGM density of $n_0=1.01^{+0.53}_{-0.72}\times10^{-7}$ cm$^{-3}$ and temperature of $\log(T/\text{K})=6.45^{+0.51}_{-2.12}$. A more dedicated study is currently beyond reach, but our results can be used as a stepping stone for future more accurate analysis, involving Athena., Comment: 32 pages, 7 figures. Accepted, to be published in A&A

On May 15th, 2017, the \emph{FERMI}/LAT gamma-ray telescope observed a transient source not present in any previous high-energy catalogue: J1544-0649. It was visible for two consecutive weeks, with a flux peak on May 21st. Subsequently observed by a \emph{Swift}/XRT follow-up starting on May 26, the X-ray counterpart position was coincident with the optical transient ASASSN-17gs = AT2017egv, detected on May 25, with a potential host galaxy at $z$=0.171. We conducted a 4-months follow-up in radio (Effelsberg-100m) and optical (San Pedro M\'artir, 2.1m) bands, in order to build the overall Spectral Energy Distribution (SED) of this object. The radio data from 5 to 15 GHz confirmed the flat spectrum of the source, favoring a line of sight close to jet axis, not showing significant variability in the explored post-burst time-window. The Rx ratio, common indicator of radio loudness, gives a value at the border between the radio-loud and radio-quiet AGN populations. The Ca$_{\rm{II}}$ H\&K break value (0.29$\pm$0.05) is compatible with the range expected for the long-sought intermediate population between BL Lacs and FRI radio galaxies. An overall SED fitting from Radio to $\gamma$-ray band shows properties typical of a low-power BL Lac. As a whole, these results suggest that this transient could well be a new example of the recently discovered class of radio-weak BL Lac, showing for the first time a flare in the gamma/X-ray bands., Comment: Accepted for publication in ApJ Letters

The prompt emission of Gamma Ray Bursts extends from the early pulses observed in gamma-rays (>15 keV) to very late flares of X-ray photons (0.3-10 keV). The duration of prompt gamma-ray pulses is rather constant while the width of X-ray flares correlates with their peak time suggesting a possible different origin. However, pulses and flares have similar spectral properties. Considering internal and external shock scenarios, we derive how the energy and duration of pulses scale with their time of occurrence and we compare with observations. The absence of an observed correlation between prompt emission pulse duration and its time of occurrence favours an "internal" origin and confirms the earlier results of Ramirez-Ruiz & Fenimore. We show that also the energetic and temporal properties of X-ray flares are consistent with being produced by internal shocks between slow fireballs with a small contrast between their bulk Lorentz factors. These results relax the requirement of a long lasting central engine to explain the latest X-ray flares., Comment: 8 pages, 5 figures, 1 table, submitted to Astronomy & Astrophysics

We report our observation of the short GRB 170817A, associated to the binary neutron star merger event GW 170817, perfomed in the X-ray band with XMM-Newton 135 d after the event (on the 29th December 2017). We find evidence for a flattening of the X-ray light curve with respect to the previously observed brightening. This is also supported by a nearly simultaneous optical Hubble Space Telescope and successive X-ray Chandra and low-frequency radio observations recently reported in the literature. Since the optical-to-X-ray spectral slope did not change with respect to previous observations, we exclude that the change in the temporal evolution of the light curve is due to the passage of the cooling frequency: its origin must be geometric or dynamical. We interpret all the existing afterglow data with two models: i) a structured jet and ii) a jet-less isotropic fireball with some stratification in its radial velocity structure. Both models fit the data and predict that the radio flux must decrease simultaneously with the optical and the X-ray one, making hard to distinguish between them at the present stage. Polarimetric measures and the rate of short GRB-GW association in future LIGO/Virgo runs will be key to disentangle these two geometrically different scenarios., Comment: Accepted as a Letter in Astronomy & Astrophysics