44,284 research outputs found
A Morphological Approach to the Pulsed Emission from Soft Gamma Repeaters
We present a geometrical methodology to interpret the periodical light curves
of Soft Gamma Repeaters based on the magnetar model and the numerical
arithmetic of the three-dimensional magnetosphere model for the young pulsars.
The hot plasma released by the star quake is trapped in the magnetosphere and
photons are emitted tangent to the local magnetic field lines. The variety of
radiation morphologies in the burst tails and the persistent stages could be
well explained by the trapped fireballs on different sites inside the closed
field lines. Furthermore, our numerical results suggests that the pulse profile
evolution of SGR 1806-20 during the 27 December 2004 giant flare is due to a
lateral drift of the emitting region in the magnetosphere.Comment: 7 figures, accepted by Ap
Dynamical Electron Mass in a Strong Magnetic Field
Motivated by recent interest in understanding properties of strongly
magnetized matter, we study the dynamical electron mass generated through
approximate chiral symmetry breaking in QED in a strong magnetic field. We
reliably calculate the dynamical electron mass by numerically solving the
nonperturbative Schwinger-Dyson equations in a consistent truncation within the
lowest Landau level approximation. It is shown that the generation of dynamical
electron mass in a strong magnetic field is significantly enhanced by the
perturbative electron mass that explicitly breaks chiral symmetry in the
absence of a magnetic field.Comment: 5 pages, 1 figure, published versio
Diagnosing magnetars with transient cooling
Transient X-ray emission, with an approximate t^{-0.7} decay, was observed
from SGR 1900+14 over 40 days following the the giant flare of 27 Aug 1998. We
calculate in detail the diffusion of heat to the surface of a neutron star
through an intense 10^{14}-10^{15} G magnetic field, following the release of
magnetic energy in its outer layers. We show that the power law index, the
fraction of burst energy in the afterglow, and the return to persistent
emission can all be understood if the star is composed of normal baryonic
material.Comment: 9 pages, 1 eps figur
How Common Are Magnetars? The Consequences of Magnetic-Field Decay
Ultramagnetized neutron stars or magnetars have been invoked to explain
several astrophysical phenomena. We examine how the magnetic field of a
magnetar will decay over time and how this decay affects the cooling of the
object. We find that for sufficiently strong nascent fields, field decay alters
the cooling evolution significantly relative to similarly magnetized neutron
stars with constant fields. As a result, old magnetars can be expected to be
bright in the soft X-ray band. The soft X-ray source RXJ~0720.43125 may well
be the nearest such old magnetar.Comment: 7 pages, 1 figure, accepted for publication in Ap. J. Letter
Pressure-tuned First-order Phase Transition and Accompanying Resistivity Anomaly in CeZn_{1-\delta}Sb_{2}
The Kondo lattice system CeZn_{0.66}Sb_{2} is studied by the electrical
resistivity and ac magnetic susceptibility measurements at several pressures.
At P=0 kbar, ferromagnetic and antiferromagnetic transitions appear at 3.6 and
0.8 K, respectively. The electrical resistivity at T_N dramatically changes
from the Fisher-Langer type (ferromagnetic like) to the Suzaki-Mori type near
17 kbar, i.e., from a positive divergence to a negative divergence in the
temperature derivative of the resistivity. The pressure-induced SM type
anomaly, which shows thermal hysteresis, is easily suppressed by small magnetic
field (1.9 kOe for 19.8 kbar), indicating a weakly first-order nature of the
transition. By subtracting a low-pressure data set, we directly compare the
resistivity anomaly with the SM theory without any assumption on backgrounds,
where the negative divergence in d\rho/dT is ascribed to enhanced critical
fluctuations in the presence of superzone gaps.Comment: 5 pages, 4 figures; journal-ref adde
Fermi Large Area Telescope Detection of Two Very-High-Energy (E>100 GeV) Gamma-ray Photons from the z = 1.1 Blazar PKS 0426-380
We report the Fermi Large Area Telescope (LAT) detection of two
very-high-energy (VHE, E>100 GeV) gamma-ray photons from the directional
vicinity of the distant (redshift, z = 1.1) blazar PKS 0426-380. The null
hypothesis that both the 134 and 122 GeV photons originate from unrelated
sources can be rejected at the 5.5 sigma confidence level. We therefore claim
that at least one of the two VHE photons is securely associated with the
blazar, making PKS 0426-380 the most distant VHE emitter known to date. The
results are in agreement with the most recent Fermi-LAT constraints on the
Extragalactic Background Light (EBL) intensity, which imply a
horizon for 100 GeV photons. The LAT detection of the two VHE
gamma-rays coincided roughly with flaring states of the source, although we did
not find an exact correspondence between the VHE photon arrival times and the
flux maxima at lower gamma-ray energies. Modeling the gamma-ray continuum of
PKS 0426-380 with daily bins revealed a significant spectral hardening around
the time of detection of the first VHE event (LAT photon index \Gamma\
1.4) but on the other hand no pronounced spectral changes near the detection
time of the second one. This combination implies a rather complex variability
pattern of the source in gamma rays during the flaring epochs. An additional
flat component is possibly present above several tens of GeV in the
EBL-corrected Fermi-LAT spectrum accumulated over the ~8-month high state.Comment: 5 pages, 1 table, 4 figures. Accepted by ApJ
An energetic blast wave from the December 27 giant flare of the soft gamma-ray repeater 1806-20
Recent follow-up observations of the December 27 giant flare of SGR 1806-20
have detected a multiple-frequency radio afterglow from 240 MHz to 8.46 GHz,
extending in time from a week to about a month after the flare. The angular
size of the source was also measured for the first time. Here we show that this
radio afterglow gives the first piece of clear evidence that an energetic blast
wave sweeps up its surrounding medium and produces a synchrotron afterglow, the
same mechanism as established for gamma-ray burst afterglows. The optical
afterglow is expected to be intrinsically as bright as at t\la
0.1 days after the flare, but very heavy extinction makes the detection
difficult because of the low galactic latitude of the source. Rapid infrared
follow-up observations to giant flares are therefore crucial for the
low-latitude SGRs, while for high-latitude SGRs (e.g. SGR 0526-66), rapid
follow-ups should result in identification of their possible optical
afterglows. Rapid multi-wavelength follow-ups will also provide more detailed
information of the early evolution of a fireball as well as its composition.Comment: Updated version, accepted for publication in ApJ Letter
A Two-Component Explosion Model for the Giant Flare and Radio Afterglow from SGR1806-20
The brightest giant flare from the soft -ray repeater (SGR) 1806-20
was detected on 2004 December 27. The isotropic-equivalent energy release of
this burst is at least one order of magnitude more energetic than those of the
two other SGR giant flares. Starting from about one week after the burst, a
very bright ( mJy), fading radio afterglow was detected. Follow-up
observations revealed the multi-frequency light curves of the afterglow and the
temporal evolution of the source size. Here we show that these observations can
be understood in a two-component explosion model. In this model, one component
is a relativistic collimated outflow responsible for the initial giant flare
and the early afterglow, and another component is a subrelativistic wider
outflow responsible for the late afterglow. We also discuss triggering
mechanisms of these two components within the framework of the magnetar model.Comment: 7 pages including 3 figures, emulateapj5.sty, accepted for
publication in ApJ Letter
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