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Register a new userStudy of the cyclotron feature in MXB 0656-072
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V.A. McBride
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We have monitored a Type II outburst of the Be/X-ray binary MXB 0656-072 in a series of pointed RXTE observations during October through December 2003. The source spectrum shows a cyclotron resonance scattering feature at 32.8 +/- 0.5 keV, corresponding to a magnetic field strength of (3.67 +/- 0.06) x 10^12 G and is stable through the outburst and over the pulsar spin phase. The pulsar, with an average pulse period of 160.4 +/- 0.4 s, shows a spin-up of 0.45 s over the duration of the outburst. From optical data, the source distance is estimated to be 3.9 +/- 0.1 kpc and this is used to estimate the X-ray luminosity and a theoretical prediction of the pulsar spin-up during the outburst.
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We present and analyze the optical photometric and spectroscopic data of the Be/X-ray binary MXB 0656-072 from 2006 to 2009. A 101.2-day orbital period is found, for the first time, from the present public X-ray data(Swift/BAT and RXTE/ASM). The anti-correlation between the H$alpha$ emission and the $UBV$ brightness of MXB 0656$-$072 during our 2007 observations indicates that a mass ejection event took place in the system. After the mass ejection, a low-density region might develop around the Oe star. With the outward motion of the circumstellar disk, the outer part of the disk interacted with the neutron star around its periastron passage and a series of the X-ray outbursts were triggered between MJD 54350 and MJD 54850. The PCA--HEXTE spectra during the 2007-2008 X-ray outbursts could be well fitted by a cut-off power law with low energy absorption, together with an iron line around 6.4 keV, and a broad cyclotron resonance feature around 30 keV. The same variability of the soft and hard X-ray colors in 2.3-21 keV indicated that there were no overall changes in the spectral shape during the X-ray outbursts, which might be only connected with the changes of the mass-accretion rate onto the neutron star.
Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array (PCA) observations of IGR J19294+1816 covering two outburst episodes are reported. The first outburst happened during MJD 54921-54925 (2009 C.E.) and the second one happened during MJD 55499-55507 (2010 C.E.). In both the cases the PCA observations were made during the decay phase of the outburst, with the source exhibiting temporal and spectral evolution with the change in flux. At the bright flux level an absorption feature at 35.5 keV is detected in the spectra which may be attributed to Cyclotron Resonance Scattering Feature corresponding to a magnetic field of B = 4.13*10^12 Gauss. This is also detected at a lower significance in two other observations. In addition an Fe line emission at 6.4 keV is prominently detected during the highest flux. X-ray pulsations are detected in 9 out of 10 observations; no pulsations were found in the observation with the lowest flux level. During this observation with the lowest flux the pulsation phenomenon becomes detectable only at the soft X-ray bands.
Cyclotron resonant scattering features (CRSFs) are formed by scattering of X-ray photons off quantized plasma electrons in the strong magnetic field (of the order 10^12 G) close to the surface of an accreting X-ray pulsar. The line profiles of CRSFs cannot be described by an analytic expression. Numerical methods such as Monte Carlo (MC) simulations of the scattering processes are required in order to predict precise line shapes for a given physical setup, which can be compared to observations to gain information about the underlying physics in these systems. A versatile simulation code is needed for the generation of synthetic cyclotron lines. Sophisticated geometries should be investigatable by making their simulation possible for the first time. The simulation utilizes the mean free path tables described in the first paper of this series for the fast interpolation of propagation lengths. The code is parallelized to make the very time consuming simulations possible on convenient time scales. Furthermore, it can generate responses to mono-energetic photon injections, producing Greens functions, which can be used later to generate spectra for arbitrary continua. We develop a new simulation code to generate synthetic cyclotron lines for complex scenarios, allowing for unprecedented physical interpretation of the observed data. An associated XSPEC model implementation is used to fit synthetic line profiles to NuSTAR data of Cep X-4. The code has been developed with the main goal of overcoming previous geometrical constraints in MC simulations of CRSFs. By applying this code also to more simple, classic geometries used in previous works, we furthermore address issues of code verification and cross-comparison of various models. The XSPEC model and the Greens function tables are available online at http://www.sternwarte.uni-erlangen.de/research/cyclo .
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.
We report on an observation of the low-mass X-ray binary 4U1626-67 performed during the BeppoSAX Science Verification Phase. An absorption feature at ~37 keV, attributable to electron cyclotron resonance, has been discovered in its pulse averaged spectrum. The inferred neutron star magnetic field strength is 3.2 (1+z) x 10E12 G, where z is the gravitational redshift. The feature is deep and narrow and is resolved in both the broad-band fit and in the ratio of observed counts to those seen from the Crab. The cyclotron resonance energy is in good agreement with the empirical relation between cyclotron energy and high energy cutoff, while its width is in agreement with the expected Doppler broadening of thermal electrons at the cyclotron resonance frequency. The broad-band 0.1-200 keV spectrum is well fit by a two-component model: a 0.27 +/- 0.02 keV blackbody and a power law with a photon index of 0.89 +/- 0.02. This is the first broad-band observation made after the change from spin-up to spin-down that occurred in mid 1990: it confirms the harder spectrum with respect to those observed in the 2-10 keV range.
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