1,724 research outputs found
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
Discovery of a flux-related change of the cyclotron line energy in Her X-1
We present the results of ten years of repeated measurements of the Cyclotron
Resonance Scattering Feature (CRSF) in the spectrum of the binary X-ray pulsar
Her X-1 and report the discovery of a positive correlation of the centroid
energy of this absorption feature in pulse phase averaged spectra with source
luminosity.Our results are based on a uniform analysis of observations bythe
RXTE satellite from 1996 to 2005, using sufficiently long observations of 12
individual 35-day Main-On states of the source. The mean centroid energy E_c of
the CRSF in pulse phase averaged spectra of Her X-1 during this time is around
40 keV, with significant variations from one Main-On state to the next. We find
that the centroid energy of the CRSF in Her X-1 changes by ~5% in energy for a
factor of 2 in luminosity. The correlation is positive, contrary to what is
observed in some high luminosity transient pulsars. Our finding is the first
significant measurement of a positive correlation between E_c and luminosity in
any X-ray pulsar. We suggest that this behaviour is expected in the case of
sub-Eddington accretion and present a calculation of a quantitative estimate,
which is very consistent with the effect observed in Her X-1.We urge that Her
X-1 is regularly monitored further and that other X-ray pulsars are
investigated for a similar behaviour.Comment: 4 pages, 2 figures, accepted by A&A Letter
Discovery of a Cyclotron Resonance Scattering Feature in the X-ray Spectrum of XTE J1946+274
Observations of the transient accreting pulsar XTE J1946+274 made with the
Rossi X-ray Timing Explorer during the course of the 1998 September-November
outburst, reveal a cyclotron resonance scattering feature (or "cyclotron line")
in the hard X-ray spectrum near 35 keV. We determine a centroid energy of 36.2
+0.5/-0.7 keV, which implies a magnetic field strength of 3.1(1+z)x10^12 G,
where z is the gravitational redshift of the scattering region. The optical
depth, Tau = 0.33 +0.07/-0.06, and width, sigma = 3.37 +0.92/-0.75 keV, are
typical of known cyclotron lines in other pulsars. This discovery makes XTE
J1946+274 one of thirteen pulsars with securely detected cyclotron lines
resulting in direct magnetic field measurements.Comment: Five pages including four postscript figures and two tables. Uses
emulateapj5. Published in ApJ Letters:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001ApJ...563L..35
Discovery of a Third Harmonic Cyclotron Resonance Scattering Feature in the X-ray Spectrum of 4U 0115+63
We have discovered a third harmonic cyclotron resonance scattering feature
(CRSF) in observations of the recent outburst of 4U 0115+63 with the Rossi
X-ray Timing Explorer (RXTE). The spectrum in a narrow pulse phase range shows
CRSFs at 12.40+0.65/-0.35, 21.45+0.25/-0.38, and 33.56+0.70/-0.90 keV. With
centroid energy ratios to the fundamental of 1.73+/-0.08 and 2.71+/-0.13, the
CRSFs are not harmonically spaced. Strong variability of the continuum and
CRSFs with pulse phase indicate a complex emission geometry near the neutron
star polar cap. In addition, one RXTE observation, which spanned periastron
passage, revealed a strong 2 mHz quasi-periodic oscillation (QPO). This is
slower by two orders of magnitude than the beat-frequency QPO expected in this
system and slower by a factor of more than 5 compared with other QPOs seen in
accreting X-ray pulsars.Comment: To appear in The Astrophysical Journal Letters. 4 pages, 5 figures.
Uses "emulateapj.sty". Revised version includes new figures and additions to
the analysi
Discovery of a Transition to Global Spin-up in EXO 2030+375
EXO 2030+375, a 42-second transient X-ray pulsar with a Be star companion,
has been observed to undergo an outburst at nearly every periastron passage for
the last 13.5 years. From 1994 through 2002, the global trend in the pulsar
spin frequency was spin-down. Using RXTE data from 2003 September, we have
observed a transition to global spin-up in EXO 2030+375. Although the spin
frequency observations are sparse, the relative spin-up between 2002 June and
2003 September observations, along with an overall brightening of the outbursts
since mid 2002 observed with the RXTE ASM, accompanied by an increase in
density of the Be disk, indicated by infrared magnitudes, suggest that the
pattern observed with BATSE of a roughly constant spin frequency, followed by
spin-up, followed by spin-down is repeating. If so this pattern has
approximately an 11 year period, similar to the 15 +/- 3 year period derived by
Wilson et al. (2002) for the precession period of a one-armed oscillation in
the Be disk. If this pattern is indeed repeating, we predict a transition from
spin-up to spin-down in 2005.Comment: Accepted for publication in ApJ Letters, 4 pages, 5 figures, using
emulateapj.cl
Her X-1: the positive cyclotron line energy / luminosity correlation
Studies of some bright, super-Eddington transient pulsars show a negative
correlation between the energy of the cyclotron resonance scattering feature
(CRSF) and the bolometric luminosity. For Her X-1, using repeated RXTE
observations during 1996-2005, the inverse dependence was found: the energy of
the cyclotron line increases as the luminosity increases. The X-ray flux
measured by the RXTE/ASM (2-10 keV) has been assumed to represent the
luminosity - more precisely: the maximum X-ray flux reached during the
respective 35 d Main-On. Here, we question whether the ASM flux is really an
accurate measure of the bolometric luminosity of the source. We redetermined
the energy of the cyclotron line and performed spectral fits using the combined
data from the PCA (3.5-60 keV) and HEXTE (20-75 keV) instruments on RXTE of the
same 35 d cycles as used in the original work to determine the bolometric flux
from those spectra. We confirm the result of the original analysis that the
cyclotron line energy changes by ~7% for a change in flux by a factor of two.Comment: 5 pages, 5 figures, accepted by A&A on 23.06.201
Discovery of a Cyclotron Resonant Scattering Feature in the RXTE Spectrum of 4U 0352+309 (X Per)
We have discovered a ~29 keV Cyclotron Resonant Scattering Feature (CRSF) in
the X-Ray spectrum of 4U 0352+309 (X Per) using observations taken with the
Rossi X-Ray Timing Explorer. 4U 0352+309 is a persistent low luminosity (L_x =
4.2x10^34 ergs/s) X-ray pulsar, with a 837 s period and which accretes material
from the Be star X Per. The X-Ray spectrum, unusual when compared to brighter
accreting pulsars, may be due to the low mass accretion rate and could be
typical of the new class of persistent low luminosity Be/X-Ray binary pulsars.
We attempted spectral fits with continuum models used historically for 4U
0352+309, and found that all were improved by the addition of a CRSF at ~29
keV. The model that best fit the observations is a combination of a 1.45+/-0.02
keV blackbody with a 5.4x10^8 cm^2 area, and a power-law with a 1.83+/-0.03
photon index modified by the CRSF. In these fits the CRSF energy is
28.6+1.5-1.7 keV, implying a magnetic field strength of 2.5(1+z)x10^12 G in the
scattering region (where z is the gravitational redshift). Phase resolved
analysis shows that the blackbody and cyclotron line energies are consistent
with being constant through the pulse.Comment: 34 pages, 6 figures, 3 tables, Accepted by Ap
Implications of the -ray Polarization of GRB 021206
We compare two possible scenarios for the producing of high level of
polarization within the prompt emission of a GRB: synchrotron emission from a
relativistic jet with a uniform (in space and time) magnetic field and
synchrotron emission from a jet with a random magnetic field in the plane of
the shock. Somewhat surprisingly we find that both scenarios can produce a
comparable level of polarization (% for the uniform field and % for a random field). Uniform time independent field most naturally
arises by expansion of the field from the compact object. It requires a
G field at the source and a transport of the field as . It {\it does not} imply Poynting flux domination of the energy of the
wind. There is a serious difficulty however, within this scenario, accounting
for particle acceleration (which requires random magnetic fields) both for
Poynting flux and non-Poynting flux domination. Significant polarization can
also arise from a random field provided that the observer is located within
orientation from a narrow () jet. While most
jets are wider, the jet of GRB 021206 from which strong polarization was
recently observed, was most likely very narrow. GRB 021206 is among the
strongest bursts ever. Adopting the energy-angle relation we find an estimated
angle of rad or even smaller. Thus, for this particular burst the
required geometry is not unusual. We conclude that the RHESSI observations
suggest that the prompt emission results from synchrotron radiation. However,
in view of the comparable levels of polarizations predicted by both the random
field and the homogeneous field scenarios these observations are insufficient
to rule out or confirm either one.Comment: 14 pages, 4 figure
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