2,073 research outputs found
Swift/BAT measurements of the cyclotron line energy decay in the accreting neutron star Her X-1: indication of an evolution of the magnetic field?
Context: The magnetic field is a crucial ingredient of neutron stars. It
governs the physics of accretion and of the resulting high-energy emission in
accreting pulsars. Studies of the cyclotron resonant scattering features
(CRSFs) seen as absorption lines in the X-ray spectra of the pulsars permit
direct measuremets of the field strength. Aims: From an analysis of a number of
pointed observations with different instruments, the energy of CRSF, Ecyc, has
recently been found to decay in Her X-1, which is one of the best-studied
accreting pulsars. We present our analysis of a homogeneous and almost
uninterrupted monitoring of the line energy with Swift/BAT. Methods: We
analyzed the archival Swift/BAT observations of Her X-1 from 2005 to 2014. The
data were used to measure the CRSF energy averaged over several months.
Results: The analysis confirms the long-term decay of the line energy. The
downward trend is highly significant and consistent with the trend measured
with the pointed observations: dEcyc/dt ~-0.3 keV per year. Conclusions: The
decay of Ecyc either indicates a local evolution of the magnetic field
structure in the polar regions of the neutron star or a geometrical
displacement of the line-forming region due to long-term changes in the
structure of the X-ray emitting region. The shortness of the observed timescale
of the decay, -Ecyc/(dEcyc/dt) ~ 100 yr, suggests that trend reversals and/or
jumps of the line energy might be observed in the future.Comment: Accepted for publication in Astronomy&Astrophysic
An overview of the EXTraS project: Exploring the X-ray Transient and Variable Sky
The EXTraS project (Exploring the X-ray Transient and variable Sky) will
harvest the hitherto unexplored temporal domain information buried in the
serendipitous data collected by the European Photon Imaging Camera (EPIC)
instrument onboard the ESA XMM-Newton X-ray observatory since its launch. This
will include a search for fast transients, as well as a search and
characterization of variability (both periodic and aperiodic) in hundreds of
thousands of sources spanning more than nine orders of magnitude in time scale
and six orders of magnitude in flux. X-ray results will be complemented by
multiwavelength characterization of new discoveries. Phenomenological
classification of variable sources will also be performed. All our results will
be made available to the community. A didactic program in selected High Schools
in Italy, Germany and the UK will also be implemented. The EXTraS project
(2014-2016), funded within the EU/FP7 framework, is carried out by a
collaboration including INAF (Italy), IUSS (Italy), CNR/IMATI (Italy),
University of Leicester (UK), MPE (Germany) and ECAP (Germany).Comment: 6 pages, 1 figure. Proceedings of "Swift: 10 years of Discovery", to
appear in Po
Monitoring the Short-Term Variability of Cyg X-1: Spectra and Timing
We present first results from the spectral and temporal analysis of an RXTE
monitoring campaign of the black hole candidate Cygnus X-1 in 1999. The timing
properties of this hard state black hole show considerable variability, even
though the state does not change. This has previously been noted for the power
spectral density, but is probably even more pronounced for the time lags. From
an analysis of four monitoring observations of Cyg X-1, separated by 2 weeks
from each other, we find that a shortening of the time lags is associated with
a hardening of the X-ray spectrum, as well as with a longer characteristic
``shot time scale''. We briefly discuss possible physical/geometrical reasons
for this variability of the hard state properties.Comment: 5 pages, 2 figures, Proc. of the 5th Compton Symposium, AIP, in pres
Disappearing Pulses in Vela X-1
We present results from a 20 h RXTE observation of Vela X-1, ncluding a
peculiar low state of a few hours duration, during which the pulsation of the
X-ray emission ceased, while significant non-pulsed emission remained. This
``quiescent state'' was preceded by a ``normal state'' without any unusual
signs and followed by a ``high state'' of several hours of increased activity
with strong, flaring pulsations. while there is clear spectral evolution from
the normal state to the low state, the spectra of the following high state are
surprisingly similar to those of the low state.Comment: 5 pages, 5 figures, Proceedings of the 5th Compton Symposium, AIP, in
pres
Multiple Cyclotron Lines in the Spectrum of 4U 0115+63
We report phase resolved spectroscopy of the transient accreting pulsar,
4U0115+63. For the first time, more than two cylotron resonance scattering
features are detected in the spectrum of an X-ray pulsar. The shape of the
fundamental line appears to be complex, and this is in agreement with
predictions of Monte-Carlo models. As in other pulsars, the line energies and
optical depths are strong functions of pulse phase. One possible model for this
is an offset of the dipole of the neutron star magnetic field.Comment: 5 pages. To appear in "Proceedings of the 5th Compton Symposium
Tracking the Orbital and Super-orbital Periods of SMC X-1
The High Mass X-ray Binary (HMXB) SMC X-1 demonstrates an orbital variation
of 3.89 days and a super-orbital variation with an average length of 55 days.
As we show here, however, the length of the super-orbital cycle varies by
almost a factor of two, even across adjacent cycles. To study both the orbital
and super-orbital variation we utilize lightcurves from the Rossi X-ray Timing
Explorer All Sky Monitor (RXTE-ASM). We employ the orbital ephemeris from
Wojdowski et al. (1998) to obtain the average orbital profile, and we show that
this profile exhibits complex modulation during non-eclipse phases.
Additionally, a very interesting ``bounceback'' in X-ray count rate is seen
during mid-orbital eclipse phases, with a softening of the emission during
these periods. This bounceback has not been previously identified in pointed
observations. We then define a super-orbital ephemeris (the phase of the
super-orbital cycle as a function of date) based on the ASM lightcurve and
analyze the trend and distribution of super-orbital cycle lengths. SMC X-1
exhibits a bimodal distribution of these lengths, similar to what has been
observed in other systems (e.g., Her X-1), but with more dramatic changes in
cycle length. There is some hint, but not conclusive evidence, for a dependence
of the super-orbital cycle length upon the underlying orbital period, as has
been observed previously for Her X-1 and Cyg X-2. Using our super-orbital
ephemeris we are also able to create an average super-orbital profile over the
71 observed cycles, for which we witness overall hardening of the spectrum
during low count rate times. We combine the orbital and super-orbital
ephemerides to study the correlation between the orbital and super-orbital
variations in the system.Comment: 10 pages, using emulateapj style. To be published in the
Astrophysical Journa
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