163 research outputs found
Modeling the broadband persistent emission of magnetars
In this paper, we discuss our first attempts to model the broadband
persistent emission of magnetars within a self consistent, physical scenario.
We present the predictions of a synthetic model that we calculated with a new
Monte Carlo 3-D radiative code. The basic idea is that soft thermal photons
(e.g. emitted by the star surface) can experience resonant cyclotron
upscattering by a population of relativistic electrons threated in the twisted
magnetosphere. Our code is specifically tailored to work in the
ultra-magnetized regime; polarization and QED effects are consistently
accounted for, as well different configurations for the magnetosphere. We
discuss the predicted spectral properties in the 0.1-1000 keV range, the
polarization properties, and we present the model application to a sample of
magnetars soft X-ray spectra.Comment: 14 pages, 7 figures, to be published in Advances in Space Research.
Proceedings of the conference "Frontieres of Space Astrophysics, Neutron
Stars & Gamma Ray Bursts", Cairo/Alexandria, 30 March- 4 April 200
Spectral features in isolated neutron stars induced by inhomogeneous surface temperatures
The thermal X-ray spectra of several isolated neutron stars display
deviations from a pure blackbody. The accurate physical interpretation of these
spectral features bears profound implications for our understanding of the
atmospheric composition, magnetic field strength and topology, and equation of
state of dense matter. With specific details varying from source to source,
common explanations for the features have ranged from atomic transitions in the
magnetized atmospheres or condensed surface, to cyclotron lines generated in a
hot ionized layer near the surface. Here we quantitatively evaluate the X-ray
spectral distortions induced by inhomogeneous temperature distributions of the
neutron star surface. To this aim, we explore several surface temperature
distributions, we simulate their corresponding general relativistic X-ray
spectra (assuming an isotropic, blackbody emission), and fit the latter with a
single blackbody model. We find that, in some cases, the presence of a spurious
'spectral line' is required at a high significance level in order to obtain
statistically acceptable fits, with central energy and equivalent width similar
to the values typically observed. We also perform a fit to a specific object,
RX J0806.4-4123, finding several surface temperature distributions able to
model the observed spectrum. The explored effect is unlikely to work in all
sources with detected lines, but in some cases it can indeed be responsible for
the appearance of such lines. Our results enforce the idea that surface
temperature anisotropy can be an important factor that should be considered and
explored also in combination with more sophisticated emission models like
atmospheres.Comment: 11 pages, 7 figures; accepted for publication in MNRA
Modelling of the Surface Emission of the Low-Magnetic Field Magnetar SGR 0418+5729
We perform a detailed modelling of the post-outburst surface emission of the
low magnetic field magnetar SGR 0418+5729. The dipolar magnetic field of this
source, B=6x10^12 G estimated from its spin-down rate, is in the observed range
of magnetic fields for normal pulsars. The source is further characterized by a
high pulse fraction and a single-peak profile. Using synthetic temperature
distribution profiles, and fully accounting for the general-relativistic
effects of light deflection and gravitational redshift, we generate synthetic
X-ray spectra and pulse profiles that we fit to the observations. We find that
asymmetric and symmetric surface temperature distributions can reproduce
equally well the observed pulse profiles and spectra of SGR 0418. Nonetheless,
the modelling allows us to place constraints on the system geometry (i.e. the
angles and that the rotation axis makes with the line of sight and
the dipolar axis, respectively), as well as on the spot size and temperature
contrast on the neutron star surface. After performing an analysis iterating
between the pulse profile and spectra, as done in similar previous works, we
further employed, for the first time in this context, a Markov-Chain
Monte-Carlo approach to extract constraints on the model parameters from the
pulse profiles and spectra, simultaneously. We find that, to reproduce the
observed spectrum and flux modulation: (a) the angles must be restricted to
or ; (b) the
temperature contrast between the poles and the equator must be at least a
factor of , and (c) the size of the hottest region ranges between
0.2-0.7 km (including uncertainties on the source distance). Last, we interpret
our findings within the context of internal and external heating models.Comment: 13 pages, 10 figures. Accepted for publication in MNRA
Gamma-ray upper limits on magnetars with 6 years of Fermi-LAT observations
We report on the search for gamma-ray emission from 20 magnetars using 6
years of Fermi, Large Area Telescope (LAT) observations. No significant
evidence for gamma-ray emission from any of the currently-known magnetars is
found. We derived the most stringent upper limits to date on the 0.1--10 GeV
emission of Galactic magnetars, which are estimated between
erg s cm. Gamma-ray pulsations were
searched for the four magnetars having reliable ephemerides over the observing
period, but none were detected. On the other hand, we also studied the
gamma-ray morphology and spectra of seven Supernova Remnants associated or
adjacent to the magnetars.Comment: 22 pages, 4 figures, 2 tables, submitted to Ap
Discovery of a strongly phase-variable spectral feature in the isolated neutron star RX J0720.4-3125
We present the discovery of a strongly phase-variable absorption feature in
the X-ray spectrum of the nearby, thermally-emitting, isolated neutron star RX
J0720.4-3125. The absorption line was detected performing detailed
phase-resolved spectroscopy in 20 XMM-Newton observations, covering the period
May 2000 - September 2012. The feature has an energy of ~750eV, an equivalent
width of ~30eV, and it is significantly detected for only ~20% of the pulsar
rotation. The absorption feature appears to be stable over the timespan covered
by the observations. Given its strong dependence on the pulsar rotational phase
and its narrow width, a plausible interpretation is in terms of resonant proton
cyclotron absorption/scattering in a confined magnetic structure very close to
the neutron star surface. The inferred field in such a magnetic loop is B_loop
~ 2 x 10^{14} G, a factor of ~7 higher than the surface dipolar magnetic field.Comment: 6 pages, 4 figures; ApJ Letters accepte
Neutron-star Measurements in the Multi-messenger Era
Neutron stars are compact and dense celestial objects that offer the unique
opportunity to explore matter and its interactions under conditions that cannot
be reproduced elsewhere in the Universe. Their extreme gravitational,
rotational and magnetic energy reservoirs fuel the large variety of their
emission, which encompasses all available multi-messenger tracers:
electromagnetic and gravitational waves, neutrinos, and cosmic rays. However,
accurately measuring global neutron-star properties such as mass, radius, and
moment of inertia poses significant challenges. Probing internal
characteristics such as the crustal composition or superfluid physics is even
more complex. This article provides a comprehensive review of the different
methods employed to measure neutron-star characteristics and the level of
reliance on theoretical models. Understanding these measurement techniques is
crucial for advancing our knowledge of neutron-star physics. We also highlight
the importance of employing independent methods and adopting a multi-messenger
approach to gather complementary data from various observable phenomena as
exemplified by the recent breakthroughs in gravitational-wave astronomy and the
landmark detection of a binary neutron-star merger. Consolidating the current
state of knowledge on neutron-star measurements will enable an accurate
interpretation of the current data and errors, and better planning for future
observations and experiments.Comment: 48 pages (including 11 pages of references), 10 figures, 2 tables;
invited review for special issue of Astroparticle Physics on 'Gravitational
Waves and Multi-messenger Astrophysics
Gamma-ray emission from PSR J0007+7303 using 7 years of Fermi Large Area Telescope observations
Based on more than seven years of Fermi Large Area Telescope (LAT) Pass 8
data, we report on a detailed analysis of the bright gamma-ray pulsar (PSR)
J0007+7303. We confirm that PSR J0007+7303 is significantly detected as a point
source also during the off-peak phases with a TS value of 262 ( 16
). In the description of PSR J0007+7303 off-peak spectrum, a power law
with an exponential cutoff at 2.71.21.3 GeV (the first/second
uncertainties correspond to statistical/systematic errors) is preferred over a
single power law at a level of 3.5 . The possible existence of a cutoff
hints at a magnetospheric origin of the emission. In addition, no extended
gamma-ray emission is detected compatible with either the supernova remnant
(CTA 1) or the very high energy (> 100 GeV) pulsar wind nebula. A flux upper
limit of 6.510 erg cm s in the 10-300 GeV energy
range is reported, for an extended source assuming the morphology of the
VERITAS detection. During on-peak phases, a sub-exponential cutoff is
significantly preferred (11 ) for representing the spectral
energy distribution, both in the phase-averaged and in the phase-resolved
spectra. Three glitches are detected during the observation period and we found
no flux variability at the time of the glitches or in the long-term behavior.
We also report the discovery of a previously unknown gamma-ray source in the
vicinity of PSR J0007+7303, Fermi J0020+7328, which we associate with the z =
1.781 quasar S5 0016+73. A concurrent analysis of this source is needed to
correctly characterize the behavior of CTA 1 and it is also presented in the
paper.Comment: 26 pages, 10 figures, 4 tables; Accepted for publication in Ap
- …