37 research outputs found
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
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
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
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
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. 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 (we dub this quantity "apparent structure") can be very different from the intrinsic structure : 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
Short GRBs at the dawn of the gravitational wave era
We derive the luminosity function and redshift distribution of short Gamma
Ray Bursts (SGRBs) using (i) all the available observer-frame constraints (i.e.
peak flux, fluence, peak energy and duration distributions) of the large
population of Fermi SGRBs and (ii) the rest-frame properties of a complete
sample of Swift SGRBs. We show that a steep with a>2.0
is excluded if the full set of constraints is considered. We implement a Monte
Carlo Markov Chain method to derive the and functions
assuming intrinsic Ep-Liso and Ep-Eiso correlations or independent
distributions of intrinsic peak energy, luminosity and duration. To make our
results independent from assumptions on the progenitor (NS-NS binary mergers or
other channels) and from uncertainties on the star formation history, we assume
a parametric form for the redshift distribution of SGRBs. We find that a
relatively flat luminosity function with slope ~0.5 below a characteristic
break luminosity ~3 erg/s and a redshift distribution of SGRBs
peaking at z~1.5-2 satisfy all our constraints. These results hold also if no
Ep-Liso and Ep-Eiso correlations are assumed. We estimate that, within ~200 Mpc
(i.e. the design aLIGO range for the detection of GW produced by NS-NS merger
events), 0.007-0.03 SGRBs yr should be detectable as gamma-ray events.
Assuming current estimates of NS-NS merger rates and that all NS-NS mergers
lead to a SGRB event, we derive a conservative estimate of the average opening
angle of SGRBs: ~3-6 deg. Our luminosity function implies an
average luminosity L~1.5 erg/s, nearly two orders of magnitude
higher than previous findings, which greatly enhances the chance of observing
SGRB "orphan" afterglows. Efforts should go in the direction of finding and
identifying such orphan afterglows as counterparts of GW events.Comment: 13 pages, 5 figures, 2 tables. Accepted for publication in Astronomy
& Astrophysics. Figure 5 and angle ranges corrected in revised versio
Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger.
The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical-near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named 'macronovae' or 'kilonovae', are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides
The 999th Swift gamma-ray burst: Some like it thermal: A multiwavelength study of GRB 151027A
We present a multiwavelength study of GRB 151027A. This is the 999th GRB
detected by the Swift satellite and it has a densely sampled emission in the
X-ray and optical band and has been observed and detected in the radio up to
140 days after the prompt. The multiwavelength light curve from 500 s to 140
days can be modelled through a standard forward shock afterglow but requires an
additional component to reproduce the early X-ray and optical emission. We
present TNG and LBT optical observations performed 19.6, 33.9 and 92.3 days
after the trigger which show a bump with respect to a standard afterglow flux
decay and are possibly interpreted as due to the underlying SN and host galaxy
(of 0.4 uJy in the R band). Radio observations, performed with SRT, Medicina,
EVN and VLBA between day 4 and 140, suggest that the burst exploded in an
environment characterised by a density profile scaling with the distance from
the source (wind profile). A remarkable feature of the prompt emission is the
presence of a bright flare 100 s after the trigger, lasting 70 seconds in the
soft X-ray band, which was simultaneously detected from the optical band up to
the MeV energy range. By combining Swift-BAT/XRT and Fermi-GBM data, the
broadband (0.3-1000 keV) time resolved spectral analysis of the flare reveals
the coexistence of a non-thermal (power law) and thermal blackbody components.
The BB component contributes up to 35% of the luminosity in the 0.3-1000 keV
band. The gamma-ray emission observed in Swift-BAT and Fermi-GBM anticipates
and lasts less than the soft X-ray emission as observed by Swift-XRT, arguing
against a Comptonization origin. The BB component could either be produced by
an outflow becoming transparent or by the collision of a fast shell with a
slow, heavy and optically thick fireball ejected during the quiescent time
interval between the initial and later flares of the burst
Multi-messenger observations of a binary neutron star merger
On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta