647 research outputs found
Spectral formation in a radiative shock: application to anomalous X-ray pulsars and soft gamma-ray repeaters
In the fallback disk model for the persistent emission of Anomalous X-ray
pulsars (AXPs) and soft gamma-ray repeaters (SGRs), the hard X-ray emission
arises from bulk- and thermal Comptonization of bremsstrahlung photons, which
are generated in the accretion column. The relatively low X-ray luminosity of
these sources implies a moderate transverse optical depth to electron
scattering, with photons executing a small number of shock crossings before
escaping sideways. We explore the range of spectral shapes that can be obtained
with this model and characterize the most important parameter dependencies. We
use a Monte Carlo code to study the crisscrossing of photons in a radiative
shock in an accretion column and compute the resulting spectrum. As expected,
high-energy power-law X-ray spectra are produced in radiative shocks with
photon-number spectral index larger than or about 0.5. We find that the
required transverse optical depth is between 1 and 7. Such spectra are observed
in low-luminosity X-ray pulsars. We demonstrate here with a simple model that
Compton upscattering in the radiative shock in the accretion column can produce
hard X-ray spectra similar to those seen in the persistent and transient
emission of AXPs and SGRs. In particular, one can obtain a high-energy
power-law spectrum, with photon-number spectral index ~ 1 and a cutoff at 100 -
200 keV, with a transverse Thomson optical depth of ~ 5, which is shown to be
typical in AXPs/SGRs.Comment: Accepted for publication in A&
On the evolution of the radio pulsar PSR J1734−3333
Recent measurements showed that the period derivative of the ‘hig
h-B’ radio pulsar PSR J1734−3333 is increasing with time. For neutron stars evolving with fallback disks, this rotational behavior is expected in certain phases of the long-term evolution. Using the same model as employed earlier to explain the evolution of anomalous X-ray pulsars and soft gamma-ray repeaters, we show that the period,the first and second period derivatives and the X-ray luminosity of this source can simultaneously acquire the observed values for a neutron star evolving with a fallback disk. We find that the required strength of the dipole field that can produce the source properties is in the range of 10^12 − 10^13 G on the pole of the neutron star. When the model source
reaches the current state properties of PSR J1734−3333, accretion onto the star has not started yet, allowing the source to operate as a regular radio pulsar. Our results imply that PSR J1734−3333 is at an age of ∼3×10^4 −2×10^5years. Such sources will have properties like the X-ray dim isolated neutron stars or transient AXPs at a later epoch of weak accretion from the diminished fallback disk
The Energy Spectrum of Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters
Assuming that AXPs and SGRs accrete matter from a fallback disk, we attempt
to explain both the soft and the hard X-ray emission as the result of the
accretion process. We also attempt to explain their radio emission or the lack
of it. We test the hypothesis that the power-law, hard X-ray spectra are
produced in the accretion flow mainly by bulk-motion Comptonization of soft
photons emitted at the neutron star surface. Fallback disk models invoke
surface dipole magnetic fields of G, which is what we
assume here. Unlike normal X-ray pulsars, for which the accretion rate is
highly super-Eddington, the accretion rate is approximately Eddington in AXPs
and SGRs and thus the bulk-motion Comptonization operates efficiently. As an
illustrative example we reproduce both the hard and the soft X-ray spectra of
AXP 4U 0142+61 well using the XSPEC package compTB. Our model seems to explain
both the hard and the soft X-ray spectra of AXPs and SGRs, as well as their
radio emission or the lack of it, in a natural way. It might also explain the
short bursts observed in these sources. On the other hand, it cannot explain
the giant X-ray outbursts observed in SGRs, which may result from the
conversion of magnetic energy in local multipole fields.Comment: 6 pages, 2 figures, minor corrections, accepted for publication in
A&
On the X-Ray Light Curve, Pulsed-Radio Emission, and Spin Frequency Evolution of the Transient Anomalous X-Ray Pulsar Xte J1810--197 During its X-Ray Outburst
We show that: (i) the long-term X-ray outburst light curve of the transient
AXP XTE J1810-197 can be accounted for by a fallback disk that is evolving
towards quiescence through a disk instability after having been heated by a
soft gamma-ray burst, (ii) the spin-frequency evolution of this source in the
same period can also be explained by the disk torque acting on the
magnetosphere of the neutron star, (iii) most significantly, recently observed
pulsed-radio emission from this source coincides with the epoch of minimum
X-ray luminosity. This is natural in terms of a fallback disk model, as the
accretion power becomes so low that it is not sufficient to suppress the beamed
radio emission from XTE J1810-197.Comment: 13 pages, 2 Figures, accepted for publication in Ap
X-Ray and Infrared Enhancement of Anomalous X-ray Pulsar 1E 2259+58
The long term (~1.5 years) X-ray enhancement and the accompanying infrared
enhancement light curves of the anomalous X-ray pulsar 1E 2259+58 following the
major bursting epoch can be accounted for by the relaxation of a fall back disk
that has been pushed back by a gamma-ray flare. The required burst energy
estimated from the results of our model fits is low enough for such a burst to
have remained below the detection limits. We find that an irradiated disk model
with a low irradiation efficiency is in good agreement with both X-ray and
infrared data. Non-irradiated disk models also give a good fit to the X-ray
light curve, but are not consistent with the infrared data for the first week
of the enhancement.Comment: 17 pages, 3 figures, accepted for publication in Ap
X-ray enhancement and long-term evolution of swift J1822.3-1606
We investigate the X-ray enhancement and the long-term evolution of the recently discovered second "low-B magnetar" Swift J1822.3-1606 in the frame of the fallback disk model. During a soft gamma burst episode, the inner disk matter is pushed back to larger radii, forming a density gradient at the inner disk. Subsequent relaxation of the inner disk could account for the observed X-ray enhancement light curve of Swift J1822.3-1606. We obtain model fits to the X-ray data with basic disk parameters similar to those employed to explain the X-ray outburst light curves of other anomalous X-ray pulsars and soft gamma repeaters. The long period (8.4 s) of the neutron star can be reached by the effect of the disk torques in the long-term accretion phase ((1-3) x 10(5) yr). The currently ongoing X-ray enhancement could be due to a transient accretion epoch, or the source could still be in the accretion phase in quiescence. Considering these different possibilities, we determine the model curves that could represent the long-term rotational and the X-ray luminosity evolution of Swift J1822.3-1606, which constrain the strength of the magnetic dipole field to the range of (1-2) x 10(12) G on the surface of the neutron star
Simultaneous use of linear and nonlinear gradients for B1 + inhomogeneity correction
The simultaneous use of linear spatial encoding magnetic fields (L-SEMs) and nonlinear spatial encoding magnetic fields (N-SEMs) in B1 + inhomogeneity problems is formulated and demonstrated with both simulations and experiments. Independent excitation k-space variables for N-SEMs are formulated for the simultaneous use of L-SEMs and N-SEMs by assuming a small tip angle. The formulation shows that, when N-SEMs are considered as an independent excitation k-space variable, numerous different k-space trajectories and frequency weightings differing in dimension, length, and energy can be designed for a given target transverse magnetization distribution. The advantage of simultaneous use of L-SEMs and N-SEMs is demonstrated by B1 + inhomogeneity correction with spoke excitation. To fully utilize the independent k-space formulations, global optimizations are performed for 1D, 2D RF power limited, and 2D RF power unlimited simulations and experiments. Three different cases are compared: L-SEMs alone, N-SEMs alone, and both used simultaneously. In all cases, the simultaneous use of L-SEMs and N-SEMs leads to a decreased standard deviation in the ROI compared with using only L-SEMs or N-SEMs. The simultaneous use of L-SEMs and N-SEMs results in better B1 + inhomogeneity correction than using only L-SEMs or N-SEMs due to the increased number of degrees of freedom. Copyright © 2017 John Wiley & Sons, Ltd
Hard X-Ray flux upper limits of central compact objects in supernova remnants
We searched for hard X-ray (20–300 keV) emission from nine central compact
objects (CCOs) 1E 1207.4−5209, 1WGA J1713−3949, J082157.5−430017,
J085201.4−461753, J1601−5133, J1613483−5055, J181852.0−150213,
J185238.6+004020, and J232327.9+584843 with the INTEGRAL observatory.
We applied spectral imaging analysis and did not detect any of the sources
with luminosity upper limits in the range of 1033-1034 ergs/s in the 20-75 keV
band. For nearby CCOs (< 4 kpc) the upper limit luminosities are an order
of magnitude lower than the measured persistent hard X-ray luminosities of
AXPs. This may indicate that the central compact objects are low magnetic
field systems with fallback disks around them
Space--time fluctuations and the spreading of wavepackets
Using a density matrix description in space we study the evolution of
wavepackets in a fluctuating space-time background. We assume that space-time
fluctuations manifest as classical fluctuations of the metric. From the
non-relativistic limit of a non-minimally coupled Klein-Gordon equation we
derive a Schr\"odinger equation with an additive gaussian random potential.
This is transformed into an effective master equation for the density matrix.
The solutions of this master equation allow to study the dynamics of
wavepackets in a fluctuating space-time, depending on the fluctuation scenario.
We show how different scenarios alter the diffusion properties of wavepackets.Comment: 11 page
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