44 research outputs found
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
On radiative acceleration in spine-sheath structured blazar jets
It has been proposed that blazar jets are structured, with a fast spine surrounded by a slower sheath or layer. This structured jet model explains some properties of their emission and morphology. Because of their relative motion, the radiation produced by one component is seen amplified by the other, thus enhancing the inverse Compton emission of both. Radiation is emitted anisotropically in the comoving frames and causes the emitting plasma to recoil. As seen in the observer frame, this corresponds to a deceleration of the fastest component (the spine) and an acceleration of the slower one (the layer). While the deceleration of the spine has already been investigated, here we study for the first time the acceleration of the sheath and find self-consistent velocity profile solutions for both the spine and the sheath while accounting for radiative cooling. We find that the sheath can be accelerated to the velocities required by the observations if its leptons remain energetic in the acceleration region, assumed to be of the order of âŒ100 Schwarzschild radii, demanding continuous injection of energetic particles in that region
Spectral index-flux relation for investigating the origins of steep decay in -ray bursts
-ray bursts (GRBs) are short-lived transients releasing a large
amount of energy ( erg) in the keV-MeV energy range. GRBs are
thought to originate from internal dissipation of the energy carried by
ultra-relativistic jets launched by the remnant of a massive star's death or a
compact binary coalescence. While thousands of GRBs have been observed over the
last thirty years, we still have an incomplete understanding of where and how
the radiation is generated in the jet. Here we show a relation between the
spectral index and the flux found by investigating the X-ray tails of bright
GRB pulses via time-resolved spectral analysis. This relation is incompatible
with the long standing scenario which invokes the delayed arrival of photons
from high-latitude parts of the jet. While the alternative scenarios cannot be
firmly excluded, the adiabatic cooling of the emitting particles is the most
plausible explanation for the discovered relation, suggesting a
proton-synchrotron origin of the GRB emission.Comment: Published in Nature Communication
Multi-messenger prospects for black hole - neutron star mergers in the O4 and O5 runs
The existence of merging black hole-neutron star (BHNS) binaries has been
ascertained through the observation of their gravitational wave (GW) signals.
However, to date, no definitive electromagnetic (EM) emission has been
confidently associated with these mergers. Such an association could help
unravel crucial information on these systems, for example, their BH spin
distribution, the equation of state (EoS) of NS and the rate of heavy element
production. We model the multi-messenger (MM) emission from BHNS mergers
detectable during the fourth (O4) and fifth (O5) observing runs of the
LIGO-Virgo-KAGRA GW detector network, in order to provide detailed predictions
that can help enhance the effectiveness of observational efforts and extract
the highest possible scientific information from such remarkable events. Our
methodology is based on a population synthesis-approach, which includes the
modelling of the signal-to-noise ratio of the GW signal in the detectors, the
GW-inferred sky localization of the source, the kilonova (KN) optical and
near-infrared light curves, the relativistic jet gamma-ray burst (GRB) prompt
emission peak photon flux, and the GRB afterglow light curves in the radio,
optical and X-ray bands. The resulting prospects for BHNS MM detections during
O4 are not promising, with a GW detection rate of
yr, but joint MM rates of yr for the KN and yr for the jet-related emission. In O5 we find an overall
increase in expected detection rates by around an order of magnitude, owing to
both the enhanced sensitivity of the GW detector network, and the coming online
of future EM facilities. Finally, we discuss direct searches for the GRB radio
afterglow with large-field-of-view instruments as a new possible follow-up
strategy in the context of ever-dimming prospects for KN detection.Comment: Submitted to A&A. 17 pages, 11 figures, 2 tables. Comments are
welcome
A bright megaelectronvolt emission line in -ray burst GRB 221009A
The highly variable and energetic pulsed emission of a long gamma-ray burst
(GRB) is thought to originate from local, rapid dissipation of kinetic or
magnetic energy within an ultra-relativistic jet launched by a newborn compact
object, formed during the collapse of a massive star. The spectra of GRB pulses
are best modelled by power-law segments, indicating the dominance of
non-thermal radiation processes. Spectral lines in the X-ray and soft
-ray regime for the afterglow have been searched for intensively, but
never confirmed. No line features ever been identified in the high energy
prompt emission. Here we report the discovery of a highly significant () narrow emission feature at around MeV in the brightest ever GRB
221009A. By modelling its profile with a Gaussian, we find a roughly constant
width MeV and temporal evolution both in energy ( MeV
to MeV) and luminosity ( erg/s to erg/s) over 80 seconds. We interpret this feature as a blue-shifted
annihilation line of relatively cold ()
electron-positron pairs, which could have formed within the jet region where
the brightest pulses of the GRB were produced. A detailed understanding of the
conditions that can give rise to such a feature could shed light on the so far
poorly understood GRB jet properties and energy dissipation mechanism.Comment: Submitte
Gravitational-wave Electromagnetic Counterpart Korean Observatory (GECKO): GECKO Follow-up Observation of GW190425
One of the keys to the success of multimessenger astronomy is the rapid
identification of the electromagnetic wave counterpart, kilonova (KN), of the
gravitational-wave (GW) event. Despite its importance, it is hard to find a KN
associated with a GW event, due to a poorly constrained GW localization map and
numerous signals that could be confused as a KN. Here, we present the
Gravitational-wave Electromagnetic wave Counterpart Korean Observatory (GECKO)
project, the GECKO observation of GW190425, and prospects of GECKO in the
fourth observing run (O4) of the GW detectors. We outline our follow-up
observation strategies during O3. In particular, we describe our
galaxy-targeted observation criteria that prioritize based on galaxy
properties. Armed with this strategy, we performed an optical and/or
near-infrared follow-up observation of GW190425, the first binary neutron star
merger event during the O3 run. Despite a vast localization area of 7460 deg^2,
we observed 621 host galaxy candidates, corresponding to 29.5% of the scores we
assigned, with most of them observed within the first 3 days of the GW event.
Ten transients were discovered during this search, including a new transient
with a host galaxy. No plausible KN was found, but we were still able to
constrain the properties of potential KNe using upper limits. The GECKO
observation demonstrates that GECKO can possibly uncover a GW170817-like KN at
a distance less than 200 Mpc if the localization area is of the order of
hundreds of square degrees, providing a bright prospect for the identification
of GW electromagnetic wave counterparts during the O4 run.Comment: 35 pages, 19 figures Accepted for publication in Ap
Quantum Backaction on kg-Scale Mirrors: Observation of Radiation Pressure Noise in the Advanced Virgo Detector
The quantum radiation pressure and the quantum shot noise in laser-interferometric gravitational wave detectors constitute a macroscopic manifestation of the Heisenberg inequality. If quantum shot noise can be easily observed, the observation of quantum radiation pressure noise has been elusive, so far, due to the technical noise competing with quantum effects. Here, we discuss the evidence of quantum radiation pressure noise in the Advanced Virgo gravitational wave detector. In our experiment, we inject squeezed vacuum states of light into the interferometer in order to manipulate the quantum backaction on the 42 kg mirrors and observe the corresponding quantum noise driven displacement at frequencies between 30 and 70 Hz. The experimental data, obtained in various interferometer configurations, is tested against the Advanced Virgo detector quantum noise model which confirmed the measured magnitude of quantum radiation pressure noise