494 research outputs found
The long-term evolution of the X-ray pulsar XTE J1814-338: a receding jet contribution to the quiescent optical emission?
We present a study of the quiescent optical counterpart of the Accreting
Millisecond X-ray Pulsar XTE J1814-338, carrying out multiband (BVR) orbital
phase-resolved photometry using the ESO VLT/FORS2. The optical light curves are
consistent with a sinusoidal variability modulated with the orbital period,
showing evidence for a strongly irradiated companion star, in agreement with
previous findings. The observed colours cannot be accounted for by the
companion star alone, suggesting the presence of an accretion disc during
quiescence. The system is fainter in all analysed bands compared to previous
observations. The R band light curve displays a possible phase offset with
respect to the B and V band. Through a combined fit of the multi-band light
curves we derive constraints on the companion star and disc fluxes, on the
system distance and on the companion star mass. The irradiation luminosity
required to account for the observed day-side temperature of the companion star
is consistent with the spin-down luminosity of a millisecond radio pulsar. The
flux decrease and spectral evolution of the quiescent optical emission observed
comparing our data with previous observations, collected over 5 years, cannot
be well explained with the contribution of an irradiated companion star and an
accretion disc alone. The progressive flux decrease as the system gets bluer
could be due to a continuum component evolving towards a lower, bluer spectrum.
While most of the continuum component is likely due to the disc, we do not
expect it to become bluer in quiescence. Hence we hypothesize that an
additional component, such as synchrotron emission from a jet was contributing
significantly in the earlier data obtained during quiescence and then
progressively fading or moving its break frequency toward longer wavelengths.Comment: 7 pages, 8 figures, accepted for publication in Section 7. Stellar
structure and evolution of Astronomy and Astrophysic
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
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
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
On the offset of Short Gamma-ray Bursts
Short Gamma-Ray Bursts (SGRBs) are expected to form from the coalescence of
compact binaries, either of primordial origin or from dynamical interactions in
globular clusters. In this paper, we investigate the possibility that the
offset and afterglow brightness of a SGRB can help revealing the origin of its
progenitor binary. We find that a SGRB is likely to result from the primordial
channel if it is observed within 10 kpc from the center of a massive galaxy and
shows a detectable afterglow. The same conclusion holds if it is 100 kpc away
from a small, isolated galaxy and shows a weak afterglow. On the other hand, a
dynamical origin is suggested for those SGRBs with observable afterglow either
at a large separation from a massive, isolated galaxy or with an offset of
10-100 kpc from a small, isolated galaxy. We discuss the possibility that SGRBs
from the dynamical channel are hosted in intra-cluster globular clusters and
find that GRB 061201 may fall within this scenario.Comment: 5 pages, 3 figures, MNRAS in pres
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
A complete sample of bright Swift short Gamma-Ray Bursts
We present a carefully selected sample of short gamma-ray bursts (SGRBs)
observed by the Swift satellite up to June 2013. Inspired by the criteria we
used to build a similar sample of bright long GRBs (the BAT6 sample), we
selected SGRBs with favorable observing conditions for the redshift
determination on ground, ending up with a sample of 36 events, almost half of
which with a redshift measure. The redshift completeness increases up to about
70% (with an average redshift value of z = 0.85) by restricting to those events
that are bright in the 15-150 keV Swift Burst Alert Telescope energy band. Such
flux-limited sample minimizes any redshift-related selection effects, and can
provide a robust base for the study of the energetics, redshift distribution
and environment of the Swift bright population of SGRBs. For all the events of
the sample we derived the prompt and afterglow emission in both the observer
and (when possible) rest frame and tested the consistency with the correlations
valid for long GRBs. The redshift and intrinsic X-ray absorbing column density
distributions we obtain are consistent with the scenario of SGRBs originated by
the coalescence of compact objects in primordial binaries, with a possible
minor contribution (~10%-25%) of binaries formed by dynamical capture (or
experiencing large natal kicks). This sample is expected to significantly
increase with further years of Swift activity.Comment: 17 pages, 8 figures, 7 tables. Accepted for publication in MNRA
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