24 research outputs found
INVESTIGATING THE Ep, i âEiso CORRELATION
The correlation between the spectral peak photon energy, Ep, and the radiated energy or luminosity (i.e., the âAmati relationâ and other correlations derived from it) is one of the central and most debated topics in GRB astrophysics, with implications for physics and the geometry of prompt emission, the identification and understanding of various classes of GRBs (short/long, XRFs,sub-energetic), and GRB cosmology. Fermi is exceptionally suited to provide, also in conjunction with Swift observations, a significant step forward in this field of research. Indeed, one of the main goals of Fermi/GBM is to make accurate measurements of Ep, by exploiting its unprecedented broad energy band from ~8 keV to ~30MeV; in addition, for a small fraction of GRBs, the LAT can extend the spectral measurements up to the GeV energy range, thus allowing a reliable estimate of the bolometric radiated energy/luminosity. We provide a review, an update and a discussion of the impact of Fermi observations in the investigation, understanding and testing of the Ep,i âEiso (âAmatiâ) relation
INVESTIGATING THE Ep, i âEiso CORRELATION
The correlation between the spectral peak photon energy, Ep, and the radiated energy or luminosity (i.e., the âAmati relationâ and other correlations derived from it) is one of the central and most debated topics in GRB astrophysics, with implications for physics and the geometry of prompt emission, the identification and understanding of various classes of GRBs (short/long, XRFs,sub-energetic), and GRB cosmology. Fermi is exceptionally suited to provide, also in conjunction with Swift observations, a significant step forward in this field of research. Indeed, one of the main goals of Fermi/GBM is to make accurate measurements of Ep, by exploiting its unprecedented broad energy band from ~8 keV to ~30MeV; in addition, for a small fraction of GRBs, the LAT can extend the spectral measurements up to the GeV energy range, thus allowing a reliable estimate of the bolometric radiated energy/luminosity. We provide a review, an update and a discussion of the impact of Fermi observations in the investigation, understanding and testing of the Ep,i âEiso (âAmatiâ) relation
Time delays between Fermi LAT and GBM light curves of GRBs
Most Gamma-Ray Bursts (GRBs) detected by the Fermi Gamma-ray Space Telescope
exhibit a delay of up to about 10 seconds between the trigger time of the hard
X-ray signal as measured by the Fermi GBM and the onset of the MeV-GeV
counterpart detected by the LAT. This delay may hint at important physics,
whether it is due to the intrinsic variability of the inner engine or it is
related to quantum dispersion effects in the velocity of light propagation from
the sources to the observer. It is critical to have a proper assessment of how
these time delays affect the overall properties of the light curves. We
cross-correlated the 5 brightest GRBs of the 1st Fermi LAT Catalog by means of
the continuous correlation function (CCF) and of the Discrete Correlation
Function (DCF). A maximum in the DCF suggests the presence of a time lag
between the curves, whose value and uncertainty are estimated through a
Gaussian fitting of the DCF profile and light curve simulation via a Monte
Carlo approach. The cross-correlation of the observed LAT and GBM light curves
yields time lags that are mostly similar to those reported in the literature,
but they are formally consistent with zero. The cross-correlation of the
simulated light curves yields smaller errors on the time lags and more than one
time lag for GRBs 090902B and 090926A; for all 5 GRBs, the time lags are
significantly different from zero and consistent with those reported in the
literature, when only the secondary maxima are considered for those two GRBs.
The DCF method evidences the presence of time lags between the LAT and GBM
light curves and underlines their complexity. While this suggests that the
delays should be ascribed to intrinsic physical mechanisms, more sensitivity
and larger statistics are needed to assess whether time lags are universally
present in the early GRB emission and which dynamical time scales they trace.Comment: 9 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Search of extended or delayed TeV emission from GRBs with HAWC
Gamma-ray bursts (GRBs) are among the most luminous sources in the universe
and the nature of their emission up to very high energy is one of the most
important open issue connected with the study of these peculiar events. The
High Altitude Water Cherenkov (HAWC) gamma-ray observatory, installed at an
altitude of 4100 m a. s. l. in the state of Puebla (Mexico), has completed its
second year of full operations. Thanks to its instantaneous field of view of ~2
sr and its high duty cycle ( 95%), HAWC is an ideal instrument for the
study of transient phenomena such as GRBs. We performed a search for TeV
emission delayed with respect to, and of longer duration than the prompt
emission observed by satellites. We present here the results obtained by
observing at the position of a sample of GRBs detected by the Fermi and Swift
satellites from December 2014 to February 2017. The upper limits resulting from
this analysis are presented and theoretical implications are discussed.Comment: Presented at the 35th International Cosmic Ray Conference (ICRC2017),
Bexco, Busan, Korea. See arXiv:1708.02572 for all HAWC contribution
Evidence for a strong 19.5 Hz flux oscillation in Swift BAT and Fermi GBM gamma-ray data from GRB 211211A
The gamma-ray burst (GRB) GRB~211211A is believed to have occurred due to the
merger of two neutron stars or a neutron star and a black hole, despite its
duration of more than a minute. Subsequent analysis has revealed numerous
interesting properties including the possible presence of a ~Hz
quasiperiodic oscillation (QPO) during precursor emission. Here we perform
timing analysis of Fermi and Swift gamma-ray data on GRB~211211A and, although
we do not find a strong QPO during the precursor, we do find an extremely
significant 19.5~Hz flux oscillation, which has higher fractional amplitude at
higher energies, in a ~second segment beginning ~seconds
after the start of the burst. After presenting our analysis we discuss possible
mechanisms for the oscillation.Comment: 16 pages, 7 figures, 2 table
Reverse Shock Emission Revealed in Early Photometry in the Candidate Short GRB 180418A
We present observations of the possible short GRB 180418A in -rays,
X-rays, and in the optical. Early optical photometry with the TAROT and RATIR
instruments show a bright peak ( 14.2 AB mag) between and
seconds that we interpret as the signature of a reversal shock. Later
observations can be modeled by a standard forward shock model and show no
evidence of jet break, allowing us to constrain the jet collimation to
. Using deep late-time optical observations we place an
upper limit of AB mag on any underlying host galaxy. The detection of
the afterglow in the \textit{Swift} UV filters constrains the GRB redshift to
and places an upper bound on the -ray isotropic equivalent
energy erg.
The properties of this GRB (e.g. duration, hardness ratio, energetic, and
environment) lie at the intersection between short and long bursts, and we can
not conclusively identify its type. We estimate that the probability that it is
drawn from the population of short GRBs is 10\%-30\%.Comment: Accepted por publication in Ap
A Study of GRBs with Low-luminosity Afterglows
We present a sample composed of the 41 faintest X-ray afterglows of the population of long gamma-ray bursts (lGRBs) with known redshift. We study their intrinsic properties (spectral index, decay index, distance, luminosity, isotropic radiated energy, and peak energy) and their luminosity distribution functions to assess whether they belong to the same population as the brighter afterglow events. We find that these events belong to a population of nearby ones, different from the general population of lGRBs. In addition, these events are faint during their prompt phase, and include the few possible outliers of the Amati relation
Modeling Gamma-ray burst Afterglow observations with an Off-axis Jet emission
Gamma-ray bursts (GRBs) are fascinating extragalactic objects. They represent
a fantastic opportunity to investigate unique properties not exhibited in other
sources. Multi-wavelength afterglow observations from some short- and
long-duration GRBs reveal an atypical long-lasting emission that evolves
differently from the canonical afterglow light curves favoring the off-axis
emission. We present an analytical synchrotron afterglow scenario, and the
hydrodynamical evolution of an off-axis top-hat jet decelerated in a stratified
surrounding environment. The analytical synchrotron afterglow model is shown
during the coasting, deceleration (off- and on-axis emission), and the
post-jet-break decay phases, and the hydrodynamical evolution is computed by
numerical simulations showing the time evolution of the Doppler factor, the
half-opening angle, the bulk Lorentz factor, and the deceleration radius. We
show that numerical simulations are in good agreement with those derived with
our analytical approach. We apply the current synchrotron model and describe
successfully the delayed non-thermal emission observed in a sample of long and
short GRBs with evidence of off-axis emission. Furthermore, we provide
constraints on the possible afterglow emission by requiring the
multi-wavelength upper limits derived for the closest Swift-detected GRBs and
promising gravitational-wave events.Comment: 36 pages, 16 figures, accepted for publication in Ap