Population properties, dissipation and radiative processes in GRBs.

Gamma Ray Bursts (GRBs) are short and intense flashes of γ–rays with typical energies between keV and a few MeV. They reach luminosities (assuming isotropy) of 1054 erg/s. The γ–ray emission, called “prompt”, is highly variable (with timescales as short as few milliseconds) and can last a fraction of a second (short GRBs, T90 2 s). The prompt is followed by the “afterglow” emission, at lower frequencies (in the X–ray, Optical and Radio band) which has been detected also up to several months after the trigger and is typically smooth and decaying as a function of time. GRBs are cosmological sources having average redshift ∼ 2.5. The progenitors of long GRBs are thought to be very massive stars that collapse at the end of their life, while the progenitors of short GRBs are thought to be the merging of two neutron stars. Two of the key properties characterizing the population of GRBs are their cosmic formation rate ψ(z) (GRBFR) and their luminosity function φ(L) (LF). Recovering ψ (z) and φ(L) of GRBs allows us to test the nature of their progenitor (e.g. through the comparison with the cosmic star formation rate), to study the possible presence of sub–classes of GRBs and to infer intrinsic properties such as the structure of their jetted outflows. The knowledge of the intrinsic population properties is becoming even more compelling with the recent association of short GRBs with gravitational wave signals produced by the merger of two neutron stars. I also concentrated my work on the prompt emission dissipation and radiation mechanism operating in GRBs

Population properties, dissipation and radiative processes in GRBs.

openGamma Ray Bursts (GRBs) are short and intense flashes of γ–rays with typical energies between keV and a few MeV. They reach luminosities (assuming isotropy) of 1054 erg/s. The γ–ray emission, called “prompt”, is highly variable (with timescales as short as few milliseconds) and can last a fraction of a second (short GRBs, T90 2 s). The prompt is followed by the “afterglow” emission, at lower frequencies (in the X–ray, Optical and Radio band) which has been detected also up to several months after the trigger and is typically smooth and decaying as a function of time. GRBs are cosmological sources having average redshift ∼ 2.5. The progenitors of long GRBs are thought to be very massive stars that collapse at the end of their life, while the progenitors of short GRBs are thought to be the merging of two neutron stars. Two of the key properties characterizing the population of GRBs are their cosmic formation rate ψ(z) (GRBFR) and their luminosity function φ(L) (LF). Recovering ψ (z) and φ(L) of GRBs allows us to test the nature of their progenitor (e.g. through the comparison with the cosmic star formation rate), to study the possible presence of sub–classes of GRBs and to infer intrinsic properties such as the structure of their jetted outflows. The knowledge of the intrinsic population properties is becoming even more compelling with the recent association of short GRBs with gravitational wave signals produced by the merger of two neutron stars. I also concentrated my work on the prompt emission dissipation and radiation mechanism operating in GRBs.openFisica e astrofisicaCACCIA, MASSIMOPescalli, AlessioPescalli, Alessi

Structure of Gamma-Ray Burst jets: intrinsic versus apparent properties

With this paper we introduce the concept of apparent structure of a GRB jet, as opposed to its intrinsic structure. The latter is customarily defined specifying the functions epsilon(theta) (the energy emitted per jet unit solid angle) and Gamma(theta) (the Lorentz factor of the emitting material); the apparent structure is instead defined by us as the isotropic equivalent energy E_iso(theta_v) as a function of the viewing angle theta_v. We show how to predict the apparent structure of a jet given its intrinsic structure. We find that a Gaussian intrinsic structure yields a power law apparent structure: this opens a new viewpoint on the Gaussian (which can be understood as a proxy for a realistic narrow, well collimated jet structure) as a possible candidate for a quasi-universal GRB jet structure. We show that such a model (a) is consistent with recent constraints on the observed luminosity function of GRBs; (b) implies fewer orphan afterglows with respect to the standard uniform model; (c) can break out the progenitor star (in the collapsar scenario) without wasting an unreasonable amount of energy; (d) is compatible with the explanation of the Amati correlation as a viewing angle effect; (e) can be very standard in energy content, and still yield a very wide range of observed isotropic equivalent energies.Comment: 10 pages, 8 figures, 1 table. Accepted by MNRA

Light curves and spectra from off-axis gamma-ray bursts

If gamma-ray burst prompt emission originates at a typical radius, and if material producing the emission moves at relativistic speed, then the variability of the resulting light curve depends on the viewing angle. This is due to the fact that the pulse evolution time scale is Doppler contracted, while the pulse separation is not. For off-axis viewing angles $\theta_{\rm view} \gtrsim \theta_{\rm jet} + \Gamma^{-1}$, the pulse broadening significantly smears out the light curve variability. This is largely independent of geometry and emission processes. To explore a specific case, we set up a simple model of a single pulse under the assumption that the pulse rise and decay are dominated by the shell curvature effect. We show that such a pulse observed off-axis is (i) broader, (ii) softer and (iii) displays a different hardness-intensity correlation with respect to the same pulse seen on-axis. For each of these effects, we provide an intuitive physical explanation. We then show how a synthetic light curve made by a superposition of pulses changes with increasing viewing angle. We find that a highly variable light curve, (as seen on-axis) becomes smooth and apparently single-pulsed (when seen off-axis) because of pulse overlap. To test the relevance of this fact, we estimate the fraction of off-axis gamma-ray bursts detectable by \textit{Swift} as a function of redshift, finding that a sizable fraction (between 10\% and 80\%) of nearby ($z<0.1$) bursts are observed with $\theta_{\rm view} \gtrsim \theta_{\rm jet} + \Gamma^{-1}$. Based on these results, we argue that low luminosity gamma-ray bursts are consistent with being ordinary bursts seen off-axis.Comment: 13 pages, 17 figures, submitted to MNRAS main journal; updated estimate of the fraction of off-axis grbs seen by Swif

Luminosity function and jet structure of Gamma Ray Bursts

The structure of Gamma Ray Burst (GRB) jets impacts on their prompt and afterglow emission properties. The jet of GRBs could be uniform, with constant energy per unit solid angle within the jet aperture, or it could instead be structured, namely with energy and velocity that depend on the angular distance from the axis of the jet. We try to get some insight about the still unknown structure of GRBs by studying their luminosity function. We show that low (1e46-1e48 erg/s) and high (i.e. with L > 1e50 erg/s) luminosity GRBs can be described by a unique luminosity function, which is also consistent with current lower limits in the intermediate luminosity range (1e48-1e50} erg/s). We derive analytical expressions for the luminosity function of GRBs in uniform and structured jet models and compare them with the data. Uniform jets can reproduce the entire luminosity function with reasonable values of the free parameters. A structured jet can also fit adequately the current data, provided that the energy within the jet is relatively strongly structured, i.e. E propto theta^{-k} with k > 4. The classical E propto theta^{-2} structured jet model is excluded by the current data.Comment: 11 pages, 2 tables, 7 figures, submitted to MNRA

Is There Light at the Ends of the Tunnel? Wireless Sensor Networks for Adaptive Lighting in Road Tunnels

Existing deployments of wireless sensor networks (WSNs) are often conceived as stand-alone monitoring tools. In this paper, we report instead on a deployment where the WSN is a key component of a closed-loop control system for adaptive lighting in operational road tunnels. WSN nodes along the tunnel walls report light readings to a control station, which closes the loop by setting the intensity of lamps to match a legislated curve. The ability to match dynamically the lighting levels to the actual environmental conditions improves the tunnel safety and reduces its power consumption. The use of WSNs in a closed-loop system, combined with the real-world, harsh setting of operational road tunnels, induces tighter requirements on the quality and timeliness of sensed data, as well as on the reliability and lifetime of the network. In this work, we test to what extent mainstream WSN technology meets these challenges, using a dedicated design that however relies on wellestablished techniques. The paper describes the hw/sw architecture we devised by focusing on the WSN component, and analyzes its performance through experiments in a real, operational tunnel

Consistency with synchrotron emission in the bright GRB 160625B observed by Fermi

We present time-resolved spectral analysis of prompt emission from GRB 160625B, one of the brightest bursts ever detected by Fermi in its nine years of operations. Standard empirical functions fail to provide an acceptable fit to the GBM spectral data, which instead require the addition of a low-energy break to the fitting function. We introduce a new fitting function, called 2SBPL, consisting of three smoothly connected power laws. Fitting this model to the data, the goodness of the fits significantly improves and the spectral parameters are well constrained. We also test a spectral model that combines non-Thermal and thermal (black body) components, but find that the 2SBPL model is systematically favoured. The spectral evolution shows that the spectral break is located around Ebreak ~100 keV, while the usual νFν peak energy feature Epeak evolves in the 0.5-6 MeV energy range. The slopes below and above Ebreak are consistent with the values-0.67 and-1.5, respectively, expected from synchrotron emission produced by a relativistic electron population with a low-energy cut-off. If Ebreak is interpreted as the synchrotron cooling frequency, the implied magnetic field in the emitting region is ~10 Gauss, i.e. orders of magnitudes smaller than the value expected for a dissipation region located at ~1013-14 cm from the central engine. The low ratio between Epeak and Ebreak implies that the radiative cooling is incomplete, contrary to what is expected in strongly magnetized and compact emitting regions

Gamma-ray burst jets: uniform or structured?

The structure of Gamma-Ray Burst (GRB) jets impacts on their prompt and afterglow emission properties. Insights into the still unknown structure of GRBs can be achieved by studying how different structures impact on the luminosity function (LF): i) we show that low (10^{46} < L_{\rm iso} < 10^{48} erg/s) and high (i.e. with L_{\rm iso} > 10^{50} erg/s) luminosity GRBs can be described by a unique LF; ii) we find that a uniform jet (seen on- and off-axis) as well as a very steep structured jet (i.e. $\epsilon(\theta) \propto \theta^{-s}$ with s > 4) can reproduce the current LF data; iii) taking into account the emission from the whole jet (i.e. including contributions from mildly relativistic, off-axis jet elements) we find that $E_{\rm iso}(\theta_{\rm v})$ (we dub this quantity "apparent structure") can be very different from the intrinsic structure $\epsilon(\theta)$: in particular, a jet with a Gaussian intrinsic structure has an apparent structure which is more similar to a power law. This opens a new viewpoint on the quasi-universal structured jet hypothesis

Unveiling the population of orphan Gamma Ray Bursts

Gamma Ray Bursts are detectable in the gamma-ray band if their jets are oriented towards the observer. However, for each GRB with a typical theta_jet, there should be ~2/theta_jet^2 bursts whose emission cone is oriented elsewhere in space. These off-axis bursts can be eventually detected when, due to the deceleration of their relativistic jets, the beaming angle becomes comparable to the viewing angle. Orphan Afterglows (OA) should outnumber the current population of bursts detected in the gamma-ray band even if they have not been conclusively observed so far at any frequency. We compute the expected flux of the population of orphan afterglows in the mm, optical and X-ray bands through a population synthesis code of GRBs and the standard afterglow emission model. We estimate the detection rate of OA by on-going and forthcoming surveys. The average duration of OA as transients above a given limiting flux is derived and described with analytical expressions: in general OA should appear as daily transients in optical surveys and as monthly/yearly transients in the mm/radio band. We find that ~ 2 OA yr^-1 could already be detected by Gaia and up to 20 OA yr^-1 could be observed by the ZTF survey. A larger number of 50 OA yr^-1 should be detected by LSST in the optical band. For the X-ray band, ~ 26 OA yr^-1 could be detected by the eROSITA. For the large population of OA detectable by LSST, the X-ray and optical follow up of the light curve (for the brightest cases) and/or the extensive follow up of their emission in the mm and radio band could be the key to disentangle their GRB nature from other extragalactic transients of comparable flux density.Comment: 9 pages, 4 figures, 2 tables. Accepted for publication by Astronomy and Astrophysic