49 research outputs found
Resonant Cyclotron Scattering and Comptonization in Neutron Star Magnetospheres
Resonant cyclotron scattering of the surface radiation in the magnetospheres
of neutron stars may considerably modify the emergent spectra and impede
efforts to constraint neutron star properties. Resonant cyclotron scattering by
a non-relativistic warm plasma in an inhomogeneous magnetic field has a number
of unusual characteristics: (i) in the limit of high resonant optical depth,
the cyclotron resonant layer is half opaque, in sharp contrast to the case of
non-resonant scattering. (ii) The transmitted flux is on average Compton
up-scattered by ~ , where is the typical thermal
velocity in units of the velocity of light; the reflected flux has on average
the initial frequency. (iii) For both the transmitted and reflected fluxes the
dispersion of intensity decreases with increasing optical depth. (iv) The
emergent spectrum is appreciably non-Plankian while narrow spectral features
produced at the surface may be erased.
We derive semi-analytically modification of the surface Plankian emission due
to multiple scattering between the resonant layers and apply the model to
anomalous X-ray pulsar 1E 1048.1--5937. Our simple model fits just as well as
the ``canonical'' magnetar spectra model of a blackbody plus power-law.Comment: 37 pages, 10 figures, accepted by MNRAS, minor change
The 2006-2007 Active Phase of Anomalous X-Ray Pulsar 4U 0142+61: Radiative and Timing Changes, Bursts,and Burst Spectral Features
After at least 6 years of quiescence, Anomalous X-ray Pulsar (AXP) 4U 0142+61 entered an active phase in 2006 March that lasted several months and included six X-ray bursts as well as many changes in the persistent X-ray emission. The bursts, the first seen from this AXP in > 11 years of Rossi X-ray Timing Explorer monitoring, all occurred in the interval between 2006 April 6 and 2007 February 7. The burst durations ranged from 0.4 - 1.8 x 10(exp 3) s. The first five burst spectra are well modeled by blackbodies, with temperatures kT approx 2 - 9 keV. However, the sixth burst had a complicated spectrum that is well characterized by a blackbody plus two emission features whose amplitude varied throughout the burst. The most prominent feature was at 14.0 keV. Upon entry into the active phase the pulsar showed a significant change in pulse morphology and a likely timing glitch. The glitch had a total frequency jump of (1.9+/-0.4) x 10(exp -7) Hz, which recovered with a decay time of 17+/-2 days by more than the initial jump, implying a net spin-down of the pulsar. Within the framework of the magnetar model, the net spin-down of the star could be explained by regions of the superfluid that rotate. slower than the rest. The bursts, flux enhancements, and pulse morphology changes can be explained as arising from crustal deformations due to stresses imposed by the highly twisted internal magnetic field. However, unlike other AXP outbursts, we cannot account for a major twist being implanted in the magnetosphere
RXTE Monitoring of the Anomalous X-ray Pulsar 1E 1048.1-5937: Long-Term Variability and the 2007 March Event
After three years of no unusual activity, Anomalous X-ray Pulsar 1E
1048.1-5937 reactivated in 2007 March. We report on the detection of a large
glitch (Delta(nu)/nu =1.63(2)X~10^{-5}) on 2007 March 26 (MJD 54185.9),
contemporaneous with the onset of a pulsed-flux flare, the third flare observed
from this source in 10 years of monitoring with the Rossi X-ray Timing
Explorer. Additionally, we report on a detailed study of the evolution of the
timing properties, the pulsed flux, and the pulse profile of this source as
measured by RXTE from 1996 July to 2008 January. In our timing study, we
attempted phase coherent timing of all available observations. We show that in
2001, a timing anomaly of uncertain nature occurred near the rise of the first
pulsed flux flare; we show that a likely glitch (Delta(nu)/nu =2.91(9)X10^{-6})
occurred in 2002, near the rise of the second flare, and we present a detailed
description of the variations in the spin-down. In our pulsed flux study, we
compare the decays of the three flares and discuss changes in the hardness
ratio. In our pulse profile study, we show that the profile exhibited large
variations near the peak of the first two flares, and several small short-term
profile variations during the most recent flare. Finally, we report on the
discovery of a small burst 27 days after the peak of the last flare, the fourth
burst discovered from this source. We discuss the relationships between the
observed properties in the framework of the magnetar model.Comment: 39 pages, 15 figures, accepted for publication in the astrophysical
journa
Anomalous X-ray Pulsar 1E 1048.1-5937: Pulsed Flux Flares and Large Torque Variations
We report on continued monitoring of the Anomalous X-ray pulsar (AXP) 1E
1048.1-5937 using the Rossi X-ray Timing Explorer. We confirm that this pulsar
has exhibited significant pulsed flux variability. The principal features of
this variability are two pulsed X-ray flares. Both flares lasted several months
and had well-resolved few-week-long rises. The long rise times of the flares
are a phenomenon not previously reported for this class of object. The epochs
of the flare peaks were MJD 52,218.8+/-4.5 and 52,444.4+/-7.0. Both flares had
shorter rise than fall times. The flares had peak fluxes of 2.21+/-0.16 and
3.00+/-0.13 times the quiescent value. We estimate a total 2-10 keV energy
release of ~2.7x10^40 ergs and ~2.8x10^41 ergs for the flares, assuming a
distance of 5 kpc. We also report large (factor of ~12) changes to the pulsar's
spin-down rate on time scales of weeks to months, shorter than has been
reported previously. We find marginal evidence for correlation between the flux
and spin-down rate variability, with probability of nonrandom correlation 6%.
We discuss the implications of our findings for AXP models.Comment: 11 pages, 2 figures, ApJ Letters accepted. Modifications to the text,
calculations and figures. We corrected a problem which resulted in reducing
the scatter in the earlier flux points in Figure 1. Conclusions unchange
An Fe XXVI Absorption Line in the Persistent Spectrum of the Dipping Low Mass X-ray Binary 1A 1744-361
We report on Chandra X-ray Observatory (CXO) High-Energy Transmission Grating
(HETG) spectra of the dipping Low Mass X-ray Binary (LMXB) 1A 1744-361 during
its July 2008 outburst. We find that its persistent emission is well modeled by
a blackbody (kT ~ 1.0 keV) plus power-law ( ~ 1.7) with an absorption
edge at 7.6 keV. In the residuals of the combined spectrum we find a
significant absorption line at 6.961+/-0.002 keV, consistent with the Fe XXVI
(hydrogen-like Fe) 2 - 1 transition. We place an upper limit on the velocity of
a redshifted flow of v < 221 km/s. We find an equivalent width for the line of
27^+2_-3 eV, from which we determine a column density of 7+/-1x10^17 cm^-2 via
a curve-of-growth analysis. Using XSTAR simulations, we place a lower limit on
the ionization parameter of > 10^3.6 erg cm/s. The properties of this line are
consistent with those observed in other dipping LMXBs. Using Rossi X-ray Timing
Explorer (RXTE) data accumulated during this latest outburst we present an
updated color-color diagram which clearly shows that 1A 1744-361 is an "atoll"
source. Finally, using additional dips found in the RXTE and CXO data we
provide an updated orbital period estimate of 52+/-5 minutes.Comment: 8 pages, 7 figures, submitted to Ap
An Fe XXIV Absorption Line in the Persistent Spectrum of the Dipping Low-Mass X-Ray Binary 1A 1744-361
We report on Chandra X-ray Observatory (Chandra) High Energy Transmission Grating spectra of the dipping low-mass X-ray binary 1A 1744361 during its 2008 July outburst. We find that its persistent emission is well modeled by a blackbody (kT approx. 1.0 keV) plus power law (Gamma approx. 1.7) with an absorption edge. In the residuals of the combined spectrum, we find a significant absorption line at 6.961 +/- 0.002 keV, consistent with the Fe xxvi (hydrogen-like Fe) 2-1 transition.We place an upper limit on the velocity of a redshifted flow of nu 103.6 erg cm/s. We discuss what implications the feature has on the system and its geometry. We also present Rossi X-ray Timing Explorer data accumulated during this latest outburst and, via an updated color-color diagram, clearly show that 1A 1744361 is an "atoll" sourc