Hard X-ray lags of Cygnus X-1 on short timescales

Calculating the time lags over different timescales using the cross-correlation technique may lead to a biased estimate of small timescales. Given a timescale for lightcurve binning, we propose to subtract the local average instead of the global average during the cross-correlation, in order to filter variations on timescales larger than the bin size. The new method allows us to make an unbiased estimate of the time lags using RXTE/PCA data on timescales as small as ∼5 ms, where the Fourier technique becomes invalid. We calculate the time lag spectra of Cygnus X-1 at different spectral states with the new method, and find that the source appears to have similar X-ray lags on small timescales independent of its spectral states.

     

The flux modulation of INTEGRAL

We have analyzed International Gamma-Ray Astrophysics Laboratory (INTEGRAL) data corresponding to observations of the high mass X-ray binary (HMXB) V0332+53 during its huge outburst which occurred in 2004–2005, and found that there is a quasi periodic signal with ultra low frequency of roughly 4−5×10⁻⁵ Hz in the observation. This feature is revealed through a series of consistent analyses of imagings and fittings of the energy spectrum for each science window (SCW), and shows up in the corresponding light curves. The analysis of the power spectrum also provides a hint of a power peak located around 4×10⁻⁵ Hz, after subtracting off the linear trend of the light curve, and shows the consistency with the previous analyses. A similar feature shows up in our analysis of the light curve of Crab in hexagonal dithering mode. Therefore, an instrumental effect on the flux modulation might exist in the INTEGRAL data product of different levels: imaging, spectral fitting and light curve analysis, at least for the observational mode of hexagonal dithering. Such an effect is, to our knowledge, not yet available in the current literature.

     

Observation constraints of the hard X-ray modulation telescope HXMT

The hard X-ray modulation telescope HXMT is a low orbit X-ray space telescope whose main science goals are to accomplish a hard X-ray all sky survey and to study timing and spectral characteristics of X-ray sources. HXMT has three kinds of science instruments: the high energy X-ray detector (HE), the medium energy X-ray detector (ME) and the low energy X-ray detector (LE). The observation schedule of HXMT is a key to achieving the science goal of HXMT, and the analysis of the observation constraints is one of the first tasks in making the observation schedule. This paper analyzes how the observation constraints influence the sky visibility and the visible time distribution of X-ray sources and discusses the schedule strategy with regard to the observation constraints.     

Simulation of the in-flight calibration of the collimator alignment and PSF for HXMT

The Hard X-ray Modulation Telescope (HXMT) is an X-ray astronomical satellite in 1–250 keV, consisting of three collimated instruments. We present the in-flight calibration approach of the collimator alignment and Point Spread Function (PSF) for HXMT, using both the direct fitting method and the imaging method. According to observational simulations of the Crab Nebula, we find that these two methods produce almost the same calibration accuracy of the alignment, and with a one-day scanning observation, the alignment can be calibrated to better than 0.45′ and 0.1′ along the wide and narrow directions of the Field of View (FOV) for a detector module, which corresponds to a localization accuracy of better than 0.1′ and meets the scientific requirement.     

The time lag spectrum of Cir X-1 on its normal branch

In the radiation-hydrodynamics model, the time lag and fractional root-mean-squared (rms) amplitude as a function of emitted photon energy should show a minimum on the normal branch oscillations. This has been seen in the Z-sources Cyg X-2 and GX 5-1. Here, using the observations from the Rossi X-ray Timing Explorer (RXTE), we study the energy dependence of normal branch QPOs in the peculiar Z-source Cir X-1. We discuss the results in the context of radiation-hydrodynamics and the Comptonization model.     

The rms-flux relation of Cyg X-2 in the horizontal branch

We have investigated the relation between the root mean square (rms) variability and the X-ray flux (rms-flux relation) of the Z source Cyg X-2, and as well the energy dependence based on the Rossi X-ray Timing Explorer (RXTE) observations. We currently focus on the horizontal branch (HB), due to the negative correlation in flux of the soft and the hard X-rays. The rms-flux correlation has energy dependence as follows: positive at hard X-rays (above 10 keV) but negative at soft X-rays (below 10 keV). This provides a feature different from the previous one, and may be suggestive of different origins of X-rays below and above 10 keV. Nevertheless, the overall spectrum can be well fitted with a model consisting of a blackbody and Comptonization components, but the fitting results do not reveal any features around 10 keV that could account for such a change in the rms-flux relation.