536 research outputs found
The faster the narrower: characteristic bulk velocities and jet opening angles of Gamma Ray Bursts
The jet opening angle theta_jet and the bulk Lorentz factor Gamma_0 are
crucial parameters for the computation of the energetics of Gamma Ray Bursts
(GRBs). From the ~30 GRBs with measured theta_jet or Gamma_0 it is known that:
(i) the real energetic E_gamma, obtained by correcting the isotropic equivalent
energy E_iso for the collimation factor ~theta_jet^2, is clustered around
10^50-10^51 erg and it is correlated with the peak energy E_p of the prompt
emission and (ii) the comoving frame E'_p and E'_gamma are clustered around
typical values. Current estimates of Gamma_0 and theta_jet are based on
incomplete data samples and their observed distributions could be subject to
biases. Through a population synthesis code we investigate whether different
assumed intrinsic distributions of Gamma_0 and theta_jet can reproduce a set of
observational constraints. Assuming that all bursts have the same E'_p and
E'_gamma in the comoving frame, we find that Gamma_0 and theta_jet cannot be
distributed as single power-laws. The best agreement between our simulation and
the available data is obtained assuming (a) log-normal distributions for
theta_jet and Gamma_0 and (b) an intrinsic relation between the peak values of
their distributions, i.e theta_jet^2.5*Gamma_0=const. On average, larger values
of Gamma_0 (i.e. the "faster" bursts) correspond to smaller values of theta_jet
(i.e. the "narrower"). We predict that ~6% of the bursts that point to us
should not show any jet break in their afterglow light curve since they have
sin(theta_jet)<1/Gamma_0. Finally, we estimate that the local rate of GRBs is
~0.3% of all local SNIb/c and ~4.3% of local hypernovae, i.e. SNIb/c with
broad-lines.Comment: 15 pages, 8 figures, 1 table. Accepted for publication in MNRA
Multiple tidal disruption flares in the active galaxy IC 3599
Tidal disruption events occur when a star passes too close to a massive black
hole and it is totally ripped apart by tidal forces. It may also happen that
the star is not close enough to the black hole to be totally disrupted and a
less dramatic event might happen. If the stellar orbit is bound and highly
eccentric, just like some stars in the centre of our own Galaxy, repeated
flares should occur. When the star approaches the black hole tidal radius at
periastron, matter might be stripped resulting in lower intensity outbursts
recurring once every orbital period. We report on Swift observations of a
recent bright flare from the galaxy IC 3599 hosting a middle-weight black hole,
where a possible tidal disruption event was observed in the early 1990s. By
light curve modelling and spectral fitting we can consistently account for the
events as the non-disruptive tidal stripping of a star into a highly eccentric
orbit. The recurrence time is 9.5 yr. IC 3599 is also known to host a
low-luminosity active galactic nucleus. Tidal stripping from this star over
several orbital passages might be able to spoon-feed also this activity.Comment: Accepted for publication to Astronomy & Astrophysic
There is a short gamma-ray burst prompt phase at the beginning of each long one
We compare the prompt intrinsic spectral properties of a sample of short
Gamma--ray Burst (GRB) with the first 0.3 seconds (rest frame) of long GRBs
observed by Fermi/GBM. We find that short GRBs and the first part of long GRBs
lie on the same E_p--E_iso correlation, that is parallel to the relation for
the time averaged spectra of long GRBs. Moreover, they are indistinguishable in
the E_p--L_iso plane. This suggests that the emission mechanism is the same for
short and for the beginning of long events, and both short and long GRBs are
very similar phenomena, occurring on different timescales. If the central
engine of a long GRB would stop after ~0.3 * (1+z) seconds the resulting event
would be spectrally indistinguishable from a short GRB.Comment: 14 pages, 6 figures, MNRAS accepte
Effective absorbing column density in the gamma-ray burst afterglow X-ray spectra
We investigate the scaling relation between the observed amount of absorption
in the X-ray spectra of Gamma Ray Burst (GRB) afterglows and the absorber
redshift. Through dedicated numerical simulations of an ideal instrument, we
establish that this dependence has a power law shape with index 2.4. However,
for real instruments, this value depends on their low energy cut-off, spectral
resolution and on the detector spectral response in general. We thus provide
appropriate scaling laws for specific instruments. Finally, we discuss the
possibility to measure the absorber redshift from X-ray data alone. We find
that 10^5-10^6 counts in the 0.3-10 keV band are needed to constrain the
redshift with 10% accuracy. As a test case we discuss the XMM-Newton
observation of GRB 090618 at z=0.54. We are able to recover the correct
redshift of this burst with the expected accuracy.Comment: MNRAS accepted. 6 figures. 3 table
The return to quiescence of Aql X-1 following the 2010 outburst
Aql X-1 is the most prolific low mass X-ray binary transient hosting a
neutron star. In this paper we focus on the return to quiescence following the
2010 outburst of the source. This decay was monitored thanks to 11 pointed
observations taken with XMM-Newton, Chandra and Swift. The decay from outburst
to quiescence is very fast, with an exponential decay characteristic time scale
of ~2 d. Once in quiescence the X-ray flux of Aql X-1 remained constant, with
no further signs of variability or decay. The comparison with the only other
well-monitored outburst from Aql X-1 (1997) is tail-telling. The luminosities
at which the fast decay starts are fully compatible for the two outbursts,
hinting at a mechanism intrinsic to the system and possibly related to the
neutron star rotation and magnetic field (i.e., the propeller effect). In
addition, for both outbursts, the decay profiles are also very similar, likely
resulting from the shut-off of the accretion process onto the neutron star
surface. Finally, the quiescent neutron star temperatures at the end of the
outbursts are well consistent with one another, suggesting a hot neutron star
core dominating the thermal balance. Small differences in the quiescent X-ray
luminosity among the two outbursts can be attributed to a different level of
the power law component.Comment: MNRAS accepted (4 figures and 6 tables
A search for evidence of irradiation in Centaurus X-4 during quiescence
We present a study of the neutron star X-Ray Transient Cen X-4. Our aim is to
look for any evidence of irradiation of the companion with a detailed analysis
of its radial velocity curve, relative contribution of the donor star and
Doppler tomography of the main emission lines. To improve our study all our
data are compared with a set of simulations that consider different physical
parameters of the system, like the disc aperture angle and the mass ratio. We
conclude that neither the radial velocity curve nor the orbital variation of
the relative donor's contribution to the total flux are affected by
irradiation. On the other hand, we do see emission from the donor star at
H and HeI 5876 which we tentatively attribute to irradiation effects.
In particular, the H emission from the companion is clearly
asymmetric and we suggest is produced by irradiation from the hot-spot.
Finally, from the velocity of the HeI 5876 spot we constrain the disc opening
angle to alpha=7-14 deg.Comment: 4 pages, 5 figures, accepted for publication in A&A as a R
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