78 research outputs found
The onset of the GeV afterglow of GRB 090510
We study the emission of the short/hard GRB 090510 at energies > 0.1 GeV as
observed by the Large Area Telescope (LAT) onboard the Fermi satellite. The GeV
flux rises in time as t^2 and decays as t^-1.5 up to 200 s. The peak of the
high energy flux is delayed by 0.2 s with respect to the main ~MeV pulse
detected by the Fermi Gamma Burst Monitor (GBM). Its energy spectrum is
consistent with F(E)=E^-1. The time behavior and the spectrum of the high
energy LAT flux are strong evidences of an afterglow origin. We then interpret
it as synchrotron radiation produced by the forward shock of a fireball having
a bulk Lorentz factor Gamma ~ 2000. The afterglow peak time is independent of
energy in the 0.1-30 GeV range and coincides with the arrival time of the
highest energy photon (~ 30 GeV). Since the flux detected by the GBM and the
LAT have different origins, the delay between these two components is not
entirely due to possible violation of the Lorentz invariance. It is the LAT
component by itself that allows to set a stringent lower limit on the
quantum-gravity mass of 4.7 times the Planck mass.Comment: 4 pages, 3 figures, submitted to ApJ Letter
Lorentz Factor Constraint from the very early external shock of the gamma-ray burst ejecta
While it is generally agreed that the emitting regions in Gamma-Ray Bursts
(GRBs) move ultra relativistically towards the observer, different estimates of
the initial Lorentz factors, , lead to different, at times
conflicting estimates. We show here that the quiet periods in which the signals
goes down below the instrumental thresholds, put strong upper limits on the
values of . According to the standard internal-external shocks model
an external shock should develop during the prompt stage. This external shock
radiates in the hard X-rays to soft gamma-rays bands and this emission should
be seen as a smooth background signal. The observed deep minima indicate that
this contribution is negligible. This limits, in turn, . We obtain
upper limits on for several bursts with typical values around
hundreds. We compare these values with those obtained by the other methods,
which typically yield lower limits. The results are marginally consistent
leaving only a narrow range of allowed values for .Comment: 9 pages, 2 figures. Submitted to MNRA
Diversity of multiwavelength emission bumps in the GRB 100219A afterglow
Context. Multi-wavelength observations of gamma-ray burst (GRB) afterglows
provide important information about the activity of their central engines and
their environments. In particular, the short timescale variability, such as
bumps and/or rebrightening features visible in the multi-wavelength light
curves, is still poorly understood.
Aims. We analyze the multi-wavelength observations of the GRB100219A
afterglow at redshift 4.7. In particular, we attempt to identify the physical
origin of the late achromatic flares/bumps detected in the X-ray and optical
bands.
Methods. We present ground-based optical photometric data and Swift X-ray
observations on GRB100219A. We analyzed the temporal behavior of the X-ray and
optical light curves, as well as the X-ray spectra.
Results. The early flares in the X-ray and optical light curves peak
simultaneously at about 1000 s after the burst trigger, while late achromatic
bumps in the X-ray and optical bands appear at about 20000 s after the burst
trigger. These are uncommon features in the afterglow phenomenology.
Considering the temporal and spectral properties, we argue that both optical
and X-ray emissions come from the same mechanism. The late flares/bumps may be
produced by late internal shocks from long-lasting activity of the central
engine. An off-axis origin for a structured jet model is also discussed to
interpret the bump shapes. The early optical bump can be interpreted as the
afterglow onset, while the early X-ray flare could be caused by the internal
activity. GRB 100219A exploded in a dense environment as revealed by the strong
attenuation of X-ray emission and the optical-to-X-ray spectral energy
distribution.Comment: A&A accepte
A statistical study of gamma-ray burst afterglows measured by the Swift Ultra-violet Optical Telescope
We present the first statistical analysis of 27 UVOT optical/ultra-violet
lightcurves of GRB afterglows. We have found, through analysis of the
lightcurves in the observer's frame, that a significant fraction rise in the
first 500s after the GRB trigger, that all lightcurves decay after 500s,
typically as a power-law with a relatively narrow distribution of decay
indices, and that the brightest optical afterglows tend to decay the quickest.
We find that the rise could either be produced physically by the start of the
forward shock, when the jet begins to plough into the external medium, or
geometrically where an off-axis observer sees a rising lightcurve as an
increasing amount of emission enters the observers line of sight, which occurs
as the jet slows. We find that at 99.8% confidence, there is a correlation, in
the observed frame, between the apparent magnitude of the lightcurves at 400s
and the rate of decay after 500s. However, in the rest frame a Spearman Rank
test shows only a weak correlation of low statistical significance between
luminosity and decay rate. A correlation should be expected if the afterglows
were produced by off-axis jets, suggesting that the jet is viewed from within
the half-opening angle theta or within a core of uniform energy density
theta_c. We also produced logarithmic luminosity distributions for three rest
frame epochs. We find no evidence for bimodality in any of the distributions.
Finally, we compare our sample of UVOT lightcurves with the XRT lightcurve
canonical model. The range in decay indices seen in UVOT lightcurves at any
epoch is most similar to the range in decay of the shallow decay segment of the
XRT canonical model. However, in the XRT canonical model there is no indication
of the rising behaviour observed in the UVOT lightcurves.Comment: 16 pages, 9 figures, accepted MNRA
Swift captures the spectrally evolving prompt emission of GRB 070616
The origins of Gamma-ray Burst prompt emission are currently not well
understood and in this context long, well-observed events are particularly
important to study. We present the case of GRB 070616, analysing the
exceptionally long-duration multipeaked prompt emission, and later afterglow,
captured by all the instruments on-board Swift and by Suzaku WAM. The high
energy light curve remained generally flat for several hundred seconds before
going into a steep decline. Spectral evolution from hard to soft is clearly
taking place throughout the prompt emission, beginning at 285 s after the
trigger and extending to 1200 s. We track the movement of the spectral peak
energy, whilst observing a softening of the low energy spectral slope. The
steep decline in flux may be caused by a combination of this strong spectral
evolution and the curvature effect. We investigate origins for the spectral
evolution, ruling out a superposition of two power laws and considering instead
an additional component dominant during the late prompt emission. We also
discuss origins for the early optical emission and the physics of the
afterglow. The case of GRB 070616 clearly demonstrates that both broadband
coverage and good time resolution are crucial to pin down the origins of the
complex prompt emission in GRBs.Comment: 13 pages, 11 figures (2 in colour), MNRAS accepte
Impact of the COVID-19 pandemic on paediatric otolaryngology: a nationwide study
Objective: The COVID-19 pandemic profoundly modified the work routine in healthcare; however, its impact on the field of paediatric otorhinolaryngology (ORL) has been rarely investigated. The aim of this study was to assess the impact of COVID-19 on paediatric ORL. Methods: A questionnaire was developed by the Young Otolaryngologists of the Italian Society of ORL-Head and Neck Surgery (GOS). The questionnaire consisted of 26 questions related to workplace and personal paediatric ORL activities. The link was advertised on the official social media platforms and sent by e-mail to 469 Italian otolaryngologists. Results: The questionnaire was completed by 118 responders. During the pandemic, the main reduction was observed for surgical activity (78.8%), followed by outpatient service (16.9%). The conditions that were mostly impacted by a delayed diagnosis were respiratory infections in 45.8% of cases and sensorineural hearing loss in 37.3% of cases. Conclusions: Paediatric ORL was highly impacted by the COVID-19 pandemic, with a significant reduction of surgical and outpatient activities and a delay in time-sensitive diagnosis. Therefore, the implementation of new strategies, such as telemedicine, is recommended
Challenging GRB models through the broadband dataset of GRB060908
Context: Multiwavelength observations of gamma-ray burst prompt and afterglow
emission are a key tool to disentangle the various possible emission processes
and scenarios proposed to interpret the complex gamma-ray burst phenomenology.
Aims: We collected a large dataset on GRB060908 in order to carry out a
comprehensive analysis of the prompt emission as well as the early and late
afterglow. Methods: Data from Swift-BAT, -XRT and -UVOT together with data from
a number of different ground-based optical/NIR and millimeter telescopes
allowed us to follow the afterglow evolution from about a minute from the
high-energy event down to the host galaxy limit. We discuss the physical
parameters required to model these emissions. Results: The prompt emission of
GRB060908 was characterized by two main periods of activity, spaced by a few
seconds of low intensity, with a tight correlation between activity and
spectral hardness. Observations of the afterglow began less than one minute
after the high-energy event, when it was already in a decaying phase, and it
was characterized by a rather flat optical/NIR spectrum which can be
interpreted as due to a hard energy-distribution of the emitting electrons. On
the other hand, the X-ray spectrum of the afterglow could be fit by a rather
soft electron distribution. Conclusions: GRB060908 is a good example of a
gamma-ray burst with a rich multi-wavelength set of observations. The
availability of this dataset, built thanks to the joint efforts of many
different teams, allowed us to carry out stringent tests for various
interpretative scenarios showing that a satisfactorily modeling of this event
is challenging. In the future, similar efforts will enable us to obtain
optical/NIR coverage comparable in quality and quantity to the X-ray data for
more events, therefore opening new avenues to progress gamma-ray burst
research.Comment: A&A, in press. 11 pages, 5 figure
Early emission of rising optical afterglows: The case of GRB 060904B and GRB 070420
We present the time-resolved optical emission of gamma-ray bursts GRB 060904B
and GRB 070420 during their prompt and early afterglow phases. We used time
resolved photometry from optical data taken by the TAROT telescope and time
resolved spectroscopy at high energies from the Swift spacecraft instrument.
The optical emissions of both GRBs are found to increase from the end of the
prompt phase, passing to a maximum of brightness at t_{peak}=9.2 min and 3.3
min for GRB 060904B and GRB 070420 respectively and then decrease. GRB 060904B
presents a large optical plateau and a very large X-ray flare. We argue that
the very large X-flare occurring near t_{peak} is produced by an extended
internal engine activity and is only a coincidence with the optical emission.
GRB 070420 observations would support this idea because there was no X-flare
during the optical peak. The nature of the optical plateau of GRB 060904B is
less clear and might be related to the late energy injection.Comment: 11 pages, 5 color figues, 2 b&w figures, accepted for publication by
Astronomy and Astrophysic
Rise and fall of the X-ray flash 080330: an off-axis jet?
Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO). DOI: 10.1051/0004-6361/200911719Context. X-ray flashes (XRFs) are a class of gamma-ray bursts (GRBs) with a peak energy of the time-integrated spectrum, , typically below 30 keV, whereas classical GRBs have of a few hundreds of keV. Apart from and the systematically lower luminosity, the properties of XRFs, such as their duration or spectral indices, are typical of the classical GRBs. Yet, the nature of XRFs and their differences from GRBs are not understood. In addition, there is no consensus on the interpretation of the shallow decay phase observed in most X-ray afterglows of both XRFs and GRBs. Aims. We examine in detail the case of XRF 080330 discovered by Swift at redshift 1.51. This burst is representative of the XRF class and exhibits an X-ray shallow decay. The rich broadband (from NIR to UV) photometric data set we collected during this phase makes it an ideal candidate for testing the off-axis jet interpretation proposed to explain both the softness of XRFs and the shallow decay phase. Methods. We present prompt -ray, early and late NIR/visible/UV and X-ray observations of the XRF 080330. We derive a spectral energy distribution from NIR to X-ray bands across the shallow/plateau phase and describe the temporal evolution of the multi-wavelength afterglow within the context of the standard afterglow model. Results. The multiwavelength evolution of the afterglow is achromatic from ~102 s to ~8104 s. The energy spectrum from NIR to X-ray is reproduced well by a simple power-law, , with = 0.790.01 and negligible rest-frame dust extinction. The light curve can be modelled by either a piecewise power-law or the combination of a smoothly broken power law with an initial rise up to ~600 s, a plateau lasting up to ~2 ks, followed by a gradual steepening to a power-law decay index of ~2 until 82 ks. At this point, a bump appears to be modelled well with a second component, while the corresponding optical energy spectrum, , reddens by = 0.260.06. Conclusions. A single-component jet viewed off-axis can explain the light curve of XRF 080330, the late-time reddening being due to the reverse shock of an energy injection episode and its being an XRF. Other possibilities, such as the optical rise marking the pre-deceleration of the fireball within a wind environment, cannot be excluded definitely, but appear to be contrived. We exclude the possibility of a dust decreasing column density being swept up by the fireball as explaining the rise of the afterglow.Peer reviewe
Dust and Metal Column Densities in Gamma-Ray Burst Host Galaxies
In this paper we present the results from the analysis of a sample of 28
gamma-ray burst (GRB) afterglow spectral energy distributions, spanning the
X-ray through to near-infrared wavelengths. This is the largest sample of GRB
afterglow spectral energy distributions thus far studied, providing a strong
handle on the optical depth distribution of soft X-ray absorption and
dust-extinction systems in GRB host galaxies. We detect an absorption system
within the GRB host galaxy in 79% of the sample, and an extinction system in
71% of the sample, and find the Small Magellanic Cloud (SMC) extinction law to
provide an acceptable fit to the host galaxy extinction profile for the
majority of cases, consistent with previous findings. The range in the soft
X-ray absorption to dust-extinction ratio, N_{H,X}/Av, in GRB host galaxies
spans almost two orders of magnitude, and the typical ratios are significantly
larger than those of the Magellanic Clouds or Milky Way. Although dust
destruction could be a cause, at least in part, for the large N_{H,X}/Av
ratios, the good fit provided by the SMC extinction law for the majority of our
sample suggests that there is an abundance of small dust grains in the GRB
environment, which we would expect to have been destroyed if dust destruction
were responsible for the large N_{H,X}/Av ratios. Instead, our analysis
suggests that the distribution of N_{H,X}/Av in GRB host galaxies may be mostly
intrinsic to these galaxies, and this is further substantiated by evidence for
a strong negative correlation between N_{H,X}/Av and metallicity for a
subsample of GRB hosts with known metallicity. Furthermore, we find the
N_{H,X}/Av ratio and metallicity for this subsample of GRBs to be comparable to
the relation found in other more metal-rich galaxies.Comment: 23 pages, 10 figures, accepted for publication in MNRA
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