511 research outputs found
The on-axis view of GRB 170817A
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
welcom
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
Gamma-ray burst jet propagation, development of angular structure, and the luminosity function
The fate and observable properties of gamma-ray burst jets depend crucially
on their interaction with the progenitor material that surrounds the central
engine. We present a semi-analytical model of such interaction, which builds
upon several previous analytical and numerical works, aimed at predicting the
angular distribution of jet and cocoon energy and Lorentz factor after
breakout, given the properties of the ambient material and of the jet at
launch. Using this model, we construct synthetic populations of structured
jets, assuming either a collapsar (for long gamma-ray bursts -- LGRBs) or a
binary neutron star merger (for short gamma-ray bursts -- SGRBs) as progenitor.
We assume all progenitors to be identical, and we allow little variability in
the jet properties at launch: our populations therefore feature a
quasi-universal structure. These populations are able to reproduce the main
features of the observed LGRB and SGRB luminosity functions, although several
uncertainties and caveats remain to be addressed.Comment: 15 pages, 12 figures. Revised version, submitted to A&A (several new
figures and expanded discussion. Conclusions unchanged). Comments and
suggestions are welcome
Resolving the decades-long transient FIRST J141918.9+394036: an orphan long gamma-ray burst or a young magnetar nebula?
Ofek (2017) identified FIRST J141918.9+394036 (hereafter FIRST J1419+3940) as
a radio source sharing similar properties and host galaxy type to the compact,
persistent radio source associated with the first known repeating fast radio
burst, FRB 121102. Law et al. (2018) showed that FIRST J1419+3940 is a
transient source decaying in brightness over the last few decades. One possible
interpretation is that FIRST J1419+3940 is a nearby analogue to FRB 121102 and
that the radio emission represents a young magnetar nebula (as several
scenarios assume for FRB 121102). Another interpretation is that FIRST
J1419+3940 is the afterglow of an `orphan' long gamma-ray burst (GRB). The
environment is similar to where most such events are produced. To distinguish
between these hypotheses, we conducted radio observations using the European
VLBI Network at 1.6 GHz to spatially resolve the emission and to search for
millisecond-duration radio bursts. We detect FIRST J1419+3940 as a compact
radio source with a flux density of (on 2018
September 18) and a source size of (i.e. given the angular diameter distance of ).
These results confirm that the radio emission is non-thermal and imply an
average expansion velocity of . Contemporaneous
high-time-resolution observations using the 100-m Effelsberg telescope detected
no millisecond-duration bursts of astrophysical origin. The source properties
and lack of short-duration bursts are consistent with a GRB jet expansion,
whereas they disfavor a magnetar birth nebula.Comment: 8 pages, 4 figures, accepted for publication in ApJ
Bulk Lorentz factors of Gamma-Ray Bursts
Knowledge of the bulk Lorentz factor of GRBs allows us to
compute their comoving frame properties shedding light on their physics. Upon
collisions with the circumburst matter, the fireball of a GRB starts to
decelerate, producing a peak or a break (depending on the circumburst density
profile) in the light curve of the afterglow. Considering all bursts with known
redshift and with an early coverage of their emission, we find 67 GRBs with a
peak in their optical or GeV light curves at a time . For another
106 GRBs we set an upper limit . We show that
is due to the dynamics of the fireball deceleration and not to the passage of a
characteristic frequency of the synchrotron spectrum across the optical band.
Considering the of 66 long GRBs and the 85 most constraining upper
limits, using censored data analysis methods, we reconstruct the most likely
distribution of . All are larger than the time when the prompt emission peaks, and are much larger than the time when the fireball becomes transparent. The reconstructed distribution of
has median value 300 (150) for a uniform (wind) circumburst
density profile. In the comoving frame, long GRBs have typical isotropic
energy, luminosity, and peak energy erg, erg s ,
and keV in the homogeneous (wind) case. We
confirm that the significant correlations between and the rest frame
isotropic energy (), luminosity () and peak energy
() are not due to selection effects. Assuming a typical opening
angle of 5 degrees, we derive the distribution of the jet baryon loading which
is centered around a few .Comment: 19 pages, 11 figures, 6 tables. Accepted for publication on Astronomy
& Astrophysic
The short gamma-ray burst population in a quasi-universal jet scenario
We describe a model of the short gamma-ray burst (SGRB) population under a
`quasi-universal jet' scenario in which jets can differ in their on-axis peak
prompt emission luminosity , but share a universal angular luminosity
profile as a function of the viewing angle
. The model is fitted, through a Bayesian hierarchical approach
inspired by gravitational wave (GW) population analyses, to 3 observed SGRB
samples simultaneously: the Fermi/GBM sample of SGRBs with spectral information
in the catalogue (367 events); a flux-complete sample of 16 Swift/BAT SGRBs
also detected by GBM, with a measured redshift; and a sample of SGRBs with a
binary neutron star (BNS) merger counterpart, which only includes GRB~170817A
at present. The results favour a narrow jet core with half-opening angle
deg (90\% credible intervals from our fiducial
`full sample' analysis) whose on-axis peak luminosity is distributed as with above a minimum luminosity
erg s. For ,
the luminosity scales as a power law with
, with no evidence for a break. While the model
implies an intrinsic `Yonetoku' correlation between and the peak photon
energy , its slope is somewhat shallower than
the apparent one, and the normalization is offset towards larger , due to
selection effects. The implied local rate density of SGRBs is between about 100
up to several thousands of events per Gpc yr, in line with the BNS merger
rate density inferred from GW observations. Based on the model, we predict 0.2
to 1.3 joint GW+SGRB detections per year by the Advanced GW detector network
and Fermi/GBM during the O4 observing run.Comment: 30 pages, 23 figures, submitted to A&A. Comments are welcome
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
GRB 190114C: from prompt to afterglow?
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&
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