902 research outputs found
Unveiling Soft Gamma-Ray Repeaters with INTEGRAL
Thanks to INTEGRAL's long exposures of the Galactic Plane, the two brightest
Soft Gamma-Ray Repeaters, SGR 1806-20 and SGR 1900+14, have been monitored and
studied in detail for the first time at hard-X/soft gamma rays.
This has produced a wealth of new scientific results, which we will review
here. Since SGR 1806-20 was particularly active during the last two years, more
than 300 short bursts have been observed with INTEGRAL. and their
characteristics have been studied with unprecedented sensitivity in the 15-200
keV range. A hardness-intensity anticorrelation within the bursts has been
discovered and the overall Number-Intensity distribution of the bursts has been
determined. In addition, a particularly active state, during which ~100 bursts
were emitted in ~10 minutes, has been observed on October 5 2004, indicating
that the source activity was rapidly increasing. This eventually led to the
Giant Flare of December 27th 2004, for which a possible soft gamma-ray (>80
keV) early afterglow has been detected.
The deep observations allowed us to discover the persistent emission in hard
X-rays (20-150 keV) from 1806-20 and 1900+14, the latter being in a quiescent
state, and to directly compare the spectral characteristics of all Magnetars
(two SGRs and three Anomalous X-ray Pulsars) detected with INTEGRAL.Comment: 8 pages, 7 figures, Presented at the conference "Isolated Neutron
Stars: from the Surface to the Interior", London, UK, 24-28 April 200
Are GRB 980425 and GRB 031203 real outliers or twins of GRB 060218?
GRB 980425 and GRB 031203 are apparently two outliers with respect to the
correlation between the isotropic equivalent energy E_iso emitted in the prompt
radiation phase and the peak frequency E_peak of the spectrum in a vF(v)
representation (the so-called Amati relation). We discuss if these two bursts
are really different from the others or if their location in the E_iso-E_peak
plane is the result of other effects, such as viewing them off-axis, or through
a scattering screen, or a misinterpretation of their spectral properties. The
latter case seems particularly interesting after GRB 060218, that, unlike GRB
031203 and GRB 980425, had a prompt emission detected both in hard and soft
X-rays which lasted ~2800 seconds. This allowed to determine its E_peak and
total emitted energy. Although it shares with GRB 031203 the total energetics,
it is not an outlier with respect to the Amati correlation. We then investigate
if a hard-to-soft spectral evolution in GRB 031203 and GRB 980425, consistent
with all the observed properties, can give rise to a time integrated spectrum
with an E_peak consistent with the Amati relation.Comment: 13 pages, 9 figures, 2 tables. Accepted for publication in MNRA
Properties and observability of glitches and anti-glitches in accreting pulsars
Several glitches have been observed in young, isolated radio pulsars, while a
clear detection in accretion-powered X-ray pulsars is still lacking. We use the
Pizzochero snowplow model for pulsar glitches as well as starquake models to
determine for the first time the expected properties of glitches in accreting
pulsars and their observability. Since some accreting pulsars show
accretion-induced long-term spin-up, we also investigate the possibility that
anti-glitches occur in these stars. We find that glitches caused by quakes in a
slow accreting neutron star are very rare and their detection extremely
unlikely. On the contrary, glitches and anti-glitches caused by a transfer of
angular momentum between the superfluid neutron vortices and the non-superfluid
component may take place in accreting pulsars more often. We calculate the
maximum jump in angular velocity of an anti-glitch and we find that it is
expected to be about 1E-5 - 1E-4 rad/s. We also note that since accreting
pulsars usually have rotational angular velocities lower than those of isolated
glitching pulsars, both glitches and anti-glitches are expected to have long
rise and recovery timescales compared to isolated glitching pulsars, with
glitches and anti-glitches appearing as a simple step in angular velocity.
Among accreting pulsars, we find that GX 1+4 is the best candidate for the
detection of glitches with currently operating X-ray instruments and future
missions such as the proposed Large Observatory for X-ray Timing (LOFT).Comment: Accepted for publication in Astronomy & Astrophysics. 6 pages. Minor
changes to match the final A&A versio
Our distorted view of magnetars: application of the Resonant Cyclotron Scattering model
The X-ray spectra of the magnetar candidates are customarily fitted with an
empirical, two component model: an absorbed blackbody and a power-law. However,
the physical interpretation of these two spectral components is rarely
discussed. It has been recently proposed that the presence of a hot plasma in
the magnetosphere of highly magnetized neutron stars might distort, through
efficient resonant cyclotron scattering, the thermal emission from the neutron
star surface, resulting in the production of non-thermal spectra. Here we
discuss the Resonant Cyclotron Scattering (RCS) model, and present its XSPEC
implementation, as well as preliminary results of its application to Anomalous
X-ray Pulsars and Soft Gamma-ray Repeaters.Comment: 5 pages, 5 color figures; Astrophysics & Space Science, in press
("Isolated Neutron Stars"; London, UK
Deuteron Magnetic Quadrupole Moment From Chiral Effective Field Theory
We calculate the magnetic quadrupole moment (MQM) of the deuteron at leading
order in the systematic expansion provided by chiral effective field theory. We
take into account parity and time-reversal violation which, at the quark-gluon
level, results from the QCD vacuum angle and dimension-six operators that
originate from physics beyond the Standard Model. We show that the deuteron MQM
can be expressed in terms of five low-energy constants that appear in the
parity- and time-reversal-violating nuclear potential and electromagnetic
current, four of which also contribute to the electric dipole moments of light
nuclei. We conclude that the deuteron MQM has an enhanced sensitivity to the
QCD vacuum angle and that its measurement would be complementary to the
proposed measurements of light-nuclear EDMs
RXTE Observations of the Anomalous Pulsar 4U 0142+61
We observed the anomalous X-ray pulsar 4U 0142+61 using the Proportional
Counter Array (PCA) aboard the Rossi X-ray Timing Explorer (RXTE) in March
1996. The pulse frequency was measured as f = 0.11510039(3) Hz with an upper
limit of df/dt < 4 * 10^(-13) Hz/s upon the short term change in frequency over
the 4.6 day span of the observations. A compilation of all historical
measurements showed an overall spin-down trend with slope df/dt = (-3.0 +/-
0.1) * 10^(-14) Hz/s. Searches for orbital modulations in pulse arrival times
yielded an upper limit of a_x sin i < 0.26 lt-s (99% confidence) for the period
range 70 s to 2.5 days. These limits combined with previous optical limits and
evolutionary arguments suggest that 4U 0142+61 is probably not a member of a
binary system.Comment: 20 pages (LaTeX) including 7 figures. Accepted for publication in the
Astrophysical Journa
A XMM-Newton View of the Soft Gamma-ray Repeater SGR 1806--20: Long Term Variability in the pre-Super Giant Flare Epoch
The low energy (<10 keV) X-ray emission of the Soft Gamma-ray Repeater
SGR1806-20 has been studied by means of four XMM-Newton observations carried
out in the last two years, the latter performed in response to a strong
sequence of hard X-ray bursts observed on 2004 October 5. The source was caught
in different states of activity: over the 2003-2004 period the 2-10 keV flux
doubled with respect to the historical level observed previously. The long term
raise in luminosity was accompanied by a gradual hardening of the spectrum,
with the power law photon index decreasing from 2.2 to 1.5, and by a growth of
the bursting activity. The pulse period measurements obtained in the four
observations are consistent with an average spin-down rate of 5.5x10e-10 s/s,
higher than the values observed in the previous years. The long-term behavior
of SGR1806-20 exhibits the correlation between spectral hardness and spin-down
rate previously found only by comparing the properties of different sources
(both SGRs and Anomalous X-ray Pulsars). The best quality spectrum (obtained on
6 September 2004) cannot be fitted by a single power law, but it requires an
additional blackbody component (kT=0.79 keV, R_BB = 1.9 (d/15 kpc)^2 km),
similar to the spectra observed in other SGRs and in Anomalous X-ray Pulsars.
No spectral lines were found in the persistent emission, with equivalent width
upper limits in the range 30-110 eV. Marginal evidence for an absorption
feature at 4.2 keV is present in the cumulative spectrum of 69 bursts detected
in September-October 2004.Comment: Revised version (several changes in text, figures and tables).
Accepted for publication on The Astrophysical Journa
Observation of GRB 030131 with the INTEGRAL satellite
A long Gamma-Ray Burst (GRB) was detected with the instruments on board the
INTEGRAL satellite on January 31 2003. Although most of the GRB, which lasted
150 seconds, occurred during a satellite slew, the automatic software of
the INTEGRAL Burst Alert System was able to detect it in near-real time. Here
we report the results obtained with the IBIS instrument, which detected GRB
030131 in the 15 keV - 200 keV energy range, and ESO/VLT observations of its
optical transient. The burst displays a complex time profile with numerous
peaks. The peak spectrum can be described by a single power law with photon
index 1.7 and has a flux of 2 photons cm s in
the 20-200 keV energy band. The high sensitivity of IBIS has made it possible
for the first time to perform detailed time-resolved spectroscopy of a GRB with
a fluence of 7 erg cm (20-200 keV).Comment: Accepted for publication in A&A, 5 pages, 4 figures, late
Iterated uniform finite-state transducers
A deterministic iterated uniform finite-state transducer (for short, iufst) operates the same length-preserving transduction on several left-to-right sweeps. The first sweep occurs on the input string, while any other sweep processes the output of the previous one. We focus on constant sweep bounded iufsts. We study their descriptional power vs. deterministic finite automata, and the state cost of implementing language operations. Then, we focus on non-constant sweep bounded iufsts, showing a nonregular language hierarchy depending on sweep complexity
From Quantum Query Complexity to State Complexity
State complexity of quantum finite automata is one of the interesting topics
in studying the power of quantum finite automata. It is therefore of importance
to develop general methods how to show state succinctness results for quantum
finite automata. One such method is presented and demonstrated in this paper.
In particular, we show that state succinctness results can be derived out of
query complexity results.Comment: Some typos in references were fixed. To appear in Gruska Festschrift
(2014). Comments are welcome. arXiv admin note: substantial text overlap with
arXiv:1402.7254, arXiv:1309.773
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