Short-Term Variability of X-rays from Accreting Neutron Star Vela X-1: II. Monte-Carlo Modeling

We develop a Monte Carlo Comptonization model for the X-ray spectrum of accretion-powered pulsars. Simple, spherical, thermal Comptonization models give harder spectra for higher optical depth, while the observational data from Vela X-1 show that the spectra are harder at higher luminosity. This suggests a physical interpretation where the optical depth of the accreting plasma increases with mass accretion rate. We develop a detailed Monte-Carlo model of the accretion flow, including the effects of the strong magnetic field ($sim 10^{12}$ G) both in geometrically constraining the flow into an accretion column, and in reducing the cross section. We treat bulk-motion Comptonization of the infalling material as well as thermal Comptonization. These model spectra can match the observed broad-band {it Suzaku} data from Vela X-1 over a wide range of mass accretion rates. The model can also explain the so-called low state, in which the uminosity decreases by an order of magnitude. Here, thermal Comptonization should be negligible, so the spectrum instead is dominated by bulk-motion Comptonization.

Short-Term Variability of X-rays from Accreting Neutron Star Vela X-1: I. Suzaku Observations

We have analyzed the time variability of the wide-band X-ray spectrum of Vela X-1, the brightest wind-fed accreting neutron star, on a short timescale of 2 ks by using {it Suzaku} observations with an exposure of 100 ks. During the observation, the object showed strong variability including several flares and so-called low states, in which the X-ray luminosity decreases by an order of magnitude. Although the spectral hardness increases with the X-ray luminosity, the majority of the recorded flares do not show any significant changes of circumstellar absorption. However, a sign of heavy absorption was registered immediately before one short flare that showed a significant spectral hardening. In the low states, the flux level is modulated with the pulsar spin period, indicating that even at this state the accretion flow reaches the close proximity of the neutron star. Phenomenologically, the broad-band X-ray spectra, which are integrated over the entire spin phase, are well represented by the NPEX function (a combination of negative and positive power laws with an exponential cutoff by a common folding energy) with a cyclotron resonance scattering feature at 50 keV. Fitting of the data allowed us to infer a correlation between the photon index and X-ray luminosity. Finally, the circumstellar absorption shows a gradual increase in the orbital phase interval 0.25--0.3, which can be interpreted as an impact of a bow shock imposed by the motion of the compact object in the supersonic stellar wind.

Dense optical and near-infrared monitoring of CTA 102 during high state in 2012 with OISTER: Detection of intra-night orphan polarized flux flare

CTA 102, classified as a flat spectrum radio quasar at z=1.037, produced exceptionally bright optical flare in 2012 September. Following Fermi-LAT detection of enhanced gamma-ray activity, we densely monitored this source in the optical and near-infrared bands for the subsequent ten nights using twelve telescopes in Japan and South-Africa. On MJD 56197 (2012 September 27, 4-5 days after the peak of bright gamma-ray flare), polarized flux showed a transient increase, while total flux and polarization angle remained almost constant during the orphan polarized-flux flare. We also detected an intra-night and prominent flare on MJD 56202. The total and polarized fluxes showed quite similar temporal variations, but PA again remained constant during the flare. Interestingly, the polarization angles during the two flares were significantly different from the jet direction. Emergence of a new emission component with high polarization degree (PD) up to 40% would be responsible for the observed two flares, and such a high PD indicates a presence of highly ordered magnetic field at the emission site. We discuss that the well-ordered magnetic field and even the observed directions of polarization angle which is grossly perpendicular to the jet are reasonably accounted for by transverse shock(s) propagating down the jet.