3,007 research outputs found
XMM-Newton view of Swift J1834.9-0846 and its Magnetar Wind Nebula
We report on the analysis of two XMM-Newton observations of the recently
discovered soft gamma repeater Swift J1834.9-0846, taken in September 2005 and
one month after the source went into outburst on 2011 August 7. We performed
timing and spectral analyses on the point source as well as on the extended
emission. We find that the source period is consistent with an extrapolation of
the Chandra ephemeris reported earlier and the spectral properties remained
constant. The source luminosity decreased to a level of 1.6x10^34 erg s^-1
following a decay trend of . Our spatial analysis of the
source environment revealed the presence of two extended emission regions
around the source. The first (Region A) is a symmetric ring around the point
source, starting at 25arcsec and extending to ~50arcsec. We argue that Region A
is a dust scattering halo. The second (Region B) has an asymmetrical shape
extending between 50arcsec and 150arcsec, and is detected both in the pre- and
post-outburst data. We argue that this region is a possible magnetar wind
nebula (MWN). The X-ray efficiency of the MWN with respect to the rotation
energy loss is substantially higher than those of rotation powered pulsars:
. The
higher efficiency points to a different energy source for the MWN of Swift
J1834.9-0846, most likely bursting activity of the magnetar, powered by its
high magnetic field, B=1.4x10^14 G.Comment: 10 pages, 10 figures, accepted for publication in Ap
The Sleeping Monster: NuSTAR observations of SGR 1806-20, 11 years after the Giant Flare
We report the analysis of 5 NuSTAR observations of SGR 1806-20 spread over a
year from April 2015 to April 2016, more than 11 years following its Giant
Flare (GF) of 2004. The source spin frequency during the NuSTAR observations
follows a linear trend with a frequency derivative
Hz s, implying a surface dipole
equatorial magnetic field G. Thus, SGR 1806-20 has
finally returned to its historical minimum torque level measured between 1993
and 1998. The source showed strong timing noise for at least 12 years starting
in 2000, with increasing one order of magnitude between 2005 and
2011, following its 2004 major bursting episode and GF. SGR 1806-20 has not
shown strong transient activity since 2009 and we do not find short bursts in
the NuSTAR data. The pulse profile is complex with a pulsed fraction of
with no indication of energy dependence. The NuSTAR spectra are well
fit with an absorbed blackbody, keV, plus a power-law,
. We find no evidence for variability among the 5
observations, indicating that SGR 1806-20 has reached a persistent and
potentially its quiescent X-ray flux level after its 2004 major bursting
episode. Extrapolating the NuSTAR model to lower energies, we find that the
0.5-10 keV flux decay follows an exponential form with a characteristic
timescale days. Interestingly, the NuSTAR flux in this energy
range is a factor of weaker than the long-term average measured between
1993 and 2003, a behavior also exhibited in SGR . We discuss our
findings in the context of the magnetar model.Comment: 10 pages, 5 figures, accepted for publication in Ap
Should We Learn Probabilistic Models for Model Checking? A New Approach and An Empirical Study
Many automated system analysis techniques (e.g., model checking, model-based
testing) rely on first obtaining a model of the system under analysis. System
modeling is often done manually, which is often considered as a hindrance to
adopt model-based system analysis and development techniques. To overcome this
problem, researchers have proposed to automatically "learn" models based on
sample system executions and shown that the learned models can be useful
sometimes. There are however many questions to be answered. For instance, how
much shall we generalize from the observed samples and how fast would learning
converge? Or, would the analysis result based on the learned model be more
accurate than the estimation we could have obtained by sampling many system
executions within the same amount of time? In this work, we investigate
existing algorithms for learning probabilistic models for model checking,
propose an evolution-based approach for better controlling the degree of
generalization and conduct an empirical study in order to answer the questions.
One of our findings is that the effectiveness of learning may sometimes be
limited.Comment: 15 pages, plus 2 reference pages, accepted by FASE 2017 in ETAP
Polarized Radiation Signals from Highly Magnetized Neutron Star Surfaces
The surfaces of neutron stars are likely sources of strongly polarized soft X
rays due to the presence of strong magnetic fields. Scattering transport in the
surface layers is critical to the determination of the emergent anisotropy of
light intensity, and is strongly influenced by the complicated interplay
between linear and circular polarization information. We have developed a
magnetic Thomson scattering simulation to model the outer layers of
fully-ionized atmospheres in such compact objects. Here we summarize emergent
intensities and polarizations from extended atmospheric simulations, spanning
considerable ranges of magnetic colatitudes. General relativistic propagation
of light from the surface to infinity is fully included. The net polarization
degrees are moderate and not very small when summing over a variety of field
directions. These results provide an important foundation for observations of
magnetars to be acquired by NASA's new IXPE X-ray polarimeter and future X-ray
polarimetry missions.Comment: 4 pages, 3 figures, accepted for publication in the proceedings of
the IAU Symposium 363, Neutron Star Astrophysics at the Crossroads: Magnetars
and the Multimessenger Revolution, eds. E. Troja & M. G. Barin
XMM-Newton and Chandra observations of the candidate Fermi-LAT pulsar 4FGL J1015.5-6030
4FGL J1015.5-6030 is an unidentified Fermi-LAT source hosting a bright,
extended X-ray source whose X-ray spectrum is consistent with that of a young
pulsar, yet no pulsations have been found. Here we report on XMM-Newton timing
and Chandra imaging observations of the X-ray counterpart of 4FGL J1015.5-6030.
We find no significant periodicity from the source and place a 3
upper-limit on its pulsed fraction of 34. The Chandra observations resolve
the point source from the extended emission. We find that the point source's
spectrum is well fit by a blackbody model, with temperature
keV, plus a weak power-law component, which is consistent with a thermally
emitting neutron star with a magnetospheric component. The extended emission
spans angular scales of a few arcseconds up to about 30 from the point
source and its spectrum is well fit by a power-law model with a photon index
. The extended emission's spectrum and 0.5-10 keV
luminosity of 4 erg s (at a plausible distance of 2 kpc)
are consistent with that of a pulsar wind nebula. Based on a comparison to
other GeV and X-ray pulsars, we find that this putative pulsar is likely a
middle-aged (i.e., --1 Myr) radio-quiet pulsar with
erg s.Comment: Accepted for publication in Ap
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