arXiv subjects

T. P. Li

Publications and source records attributed to T. P. Li.

53 records · Page 3Linked to original sources

arXiv2006

Timing Features of the Accretion--driven Millisecond X-Ray Pulsar XTE J1807--294 in 2003 March Outburst

In order to probe the activity of the inner disk flow and its effect on the neutron star surface emissions, we carried out the timing analysis of the Rossi X-Ray Timing Explorer (RXTE) observations of the millisecond X-ray pulsar XTE J1807--294, focusing on its correlated behaviors in X-ray intensities, hardness ratios, pulse profiles and power density spectra. The source was observed to have a serial of broad "puny" flares on a timescale of hours to days on the top of a decaying outburst in March 2003. In the flares, the spectra are softened and the pulse profiles become more sinusoidal. The frequency of kilohertz quasi-periodic oscillation (kHz QPO) is found to be positively related to the X-ray count rate in the flares. These features observed in the flares could be due to the accreting flow inhomogeneities. It is noticed that the fractional pulse amplitude increases with the flare intensities in a range of $\sim 2%-14%$, comparable to those observed in the thermonuclear bursts of the millisecond X-ray pulsar XTE J1814--338, whereas it remains at about 6.5% in the normal state. Such a significant variation of the pulse profile in the "puny" flares may reflect the changes of physical parameters in the inner disk accretion region. Furthermore, we noticed an overall positive correlation between the kHz QPO frequency and the fractional pulse amplitude, which could be the first evidence representing that the neutron-star surface emission properties are very sensitive to the disk flow inhomogeneities. This effect should be cautiously considered in the burst oscillation studies.

astro-ph↗

arXiv2005

X-ray spectroscopy of PSR B1951+32 and its pulsar wind nebula

We present spatially resolved X-ray spectroscopy of PSR B1951+32 and its pulsar wind nebula (PWN) in supernova remnant (SNR) CTB 80 using a {\sl Chandra} observation. The {\sl Chandra} X-ray map reveals clearly various components of a ram-pressure confined PWN embedded in the SNR ejecta: a point source representing the pulsar, X-ray emission from the bow shock, a luminous X-ray tail, a 30$\arcsec$ diameter plateau whose northwestern part is absent, and the outside more diffuse X-ray emission. The plateau is just surrounded by the radio, [O III], [S II], and [N II] shells, and the outside diffuse emission is mostly within the H$α$ shells. While the spectra of all the features are well fitted with power law models, a power law plus blackbody model can fit the spectrum of the pulsar significantly better than using a power law model alone. Generally the spectra of these components obey the trend of steepening from the inside to the outside. However, the edge of the plateau probably has a harder spectrum than that of the central region of the plateau. The cause of the apparent hard spectrum of the plateau edge is unclear, and we speculate that it might be due to a shock between the PWN and the SNR ejecta. The possible blackbody radiation component from the pulsar has a temperature of 0.13$\pm0.02$ keV and an equivalent emitting radius of 2.2$^{+1.4}_{-0.8}$ (d/2 kpc) km, and is thus probably from the hot spots on the pulsar. We also show in this paper that the blackbody temperature of the entire surface of PSR B1951+32 is much lower than those predicted by the standard neutron star cooling models.

astro-ph↗

arXiv2004

Linear Accelerating Superluminal Motion Model

Accelerating superluminal motions were detected recently by multi-epoch Very Long Baseline Interferometry (VLBI) observations. Here, a Linear Accelerating Superluminal Motion (LASM) model is proposed to interpret the observed phenomena. The model provides a direct and accurate way to estimate the viewing angle of a relativistic jet. It also predicts that both Doppler boosting and deboosting effects may take place in an accelerating forward jet. The LASM model is applied to the data of the quasar 3C 273, and the initial velocity, acceleration and viewing angle of its three components are derived through model fits. The variations of the viewing angle suggest that a supermassive black hole binary system may exist in the center of 3C273. The gap between the inner and outer jet in some radio loud AGNs my be explained in terms of Doppler deboosting effects when the components accelerate to ultra-relativistic speed.

astro-ph↗

arXiv2004

Timescale Analysis of Spectral Lags

A technique for timescale analysis of spectral lags performed directly in the time domain is developed. Simulation studies are made to compare the time domain technique with the Fourier frequency analysis for spectral time lags. The time domain technique is applied to studying rapid variabilities of X-ray binaries and $γ$-ray bursts. The results indicate that in comparison with the Fourier analysis the timescale analysis technique is more powerful for the study of spectral lags in rapid variabilities on short time scales and short duration flaring phenomena.

astro-ph↗

arXiv2004

X-Ray Spectral Variability in Cygnus X-1

Spectral variability in different energy bands of X-rays from Cyg X-1 in different states is studied with RXTE observations and time domain approaches. In the hard tail of energy spectrum above $\sim 10$ keV, average peak aligned shots are softer than the average steady emission and the hardness ratio decreases when the flux increases during a shot for all states. In regard to a soft band lower $\sim 10$ keV, the hardness in the soft state varies in an opposite way: it peaks when the flux of the shot peaks. For the hard and transition states, the hardness ratio in respect to a soft band during a shot is in general lower than that of the steady component and a sharp rise is observed at about the shot peak. For the soft state, the correlation coefficient between the intensity and hardness ratio in the hard tail is negative and decreases monotonically as the timescale increases from 0.01 s to 50 s, which is opposite to that in regard to a soft band. For the hard and transition states, the correlation coefficients are in general negative and have a trend of decrease with increasing timescale.

astro-ph↗

arXiv2004

Gamma-Ray Bursts are Produced Predominately in the Early Universe

It is known that some observed gamma-ray bursts (GRBs) are produced at cosmological distances and that the GRB production rate may follow the star formation rate. We model the BATSE-detected intensity distribution of long GRBs in order to determine their space density distribution and opening angle distribution. Our main results are: the lower and upper distance limits to the GRB production are z 0.24 and >10, respectively; the GRB opening angle follows an exponential distribution and the mean opening angle is about 0.03 radians; and the peak luminosity appears to be a better standard candle than the total energy of a GRB.

astro-ph↗

arXiv2004

Diagnostics of Neutron Star and Black Hole X-Ray Binaries with X-Ray Shot Widths

A statistic w, the differential coefficient of the mean absolute difference of an observed lightcurve, is proposed for timescale analysis of shot widths. The shortest width of random shots can be measured by the position of the lower cut-off in the timescale spectrum of w. We use the statistic to analyze X-ray lightcurves from a sample of neutron star and black hole binaries and the results show that the timescale analysis can help us distinguish between neutron star binaries and black hole binaries. The analysis can further reveal the structure and dynamics of accretion disks around black holes.

astro-ph↗

arXiv2003

Evolution of Hard X-Ray Spectra Along the Branches in Cir X-1

Using the data from the PCA and HEXTE on board the RXTE satellite, we investigate the evolution of the 3-200 keV spectra of the peculiar low mass X-ray binary (LMXB) Cir X-1 along the branches on its hardness-intensity diagram (HID) from the vertical horizontal branch (VHB), through the horizontal horizontal branch (HHB) and normal branch (NB), to the flaring branch (FB). We detect a power-law hard component in the spectra. It is found that the derived photon indices ($Γ$) of the power-law hard component are correlated with the position on the HID. The power-law component dominates the X-ray emission of Cir X-1 in the energy band higher than $\sim 20$ keV. The fluxes of the power-law component are compared with those of the bremsstrahlung component in the spectra. A possible origin of the power-law hard component is discussed.

astro-ph↗

arXiv2002

Distance Determination of Variable Galactic Sources

We have developed a timing analysis method to determine the distances of variable galactic X-ray sources based on the method advanced by Trümper and Schönfelder in 1973. The light-curve of the halo produced by the scattering of X-rays off the interstellar dust is delayed and smeared by the dust grains. This method utilizes the differences between the power density spectra of the point source and the halo. We present the details of this method and our first applications of this method to the Chandra data of X-ray binary Cyg X-3.

astro-ph↗

arXiv2002

Imaging in Hard X-ray Astronomy

The energy range of hard X-rays is a key waveband to the study of high energy processes in celestial objects, but still remains poorly explored. In contrast to direct imaging methods used in the low energy X-ray and high energy gamma-ray bands, currently imaging in the hard X-ray band is mainly achieved through various modulation techniques. A new inversion technique, the direct demodulation method, has been developed since early 90s. With this technique, wide field and high resolution images can be derived from scanning data of a simple collimated detector. The feasibility of this technique has been confirmed by experiment, balloon-borne observation and analyzing simulated and real astronomical data. Based the development of methodology and instrumentation, a high energy astrophysics mission -- Hard X-ray Modulation Telescope (HXMT) has been proposed and selected in China for a four-year Phase-A study. The main scientific objectives are a full-sky hard X-ray (20-200 keV) imaging survey and high signal-to-noise ratio timing studies of high energy sources.

astro-ph↗

arXiv2002

Timescale Spectra in High Energy Astrophysics

A technique of timescale analysis performed directly in the time domain has been developed recently. We have applied the technique to studying rapid variabilities of hard X-rays from neutron star and black hole binaries, gamma-ray bursts and terrestrial gamma-ray flashes. The results indicate that the time domain method of spectral analysis is a powerful tool in revealing the underlying physics in high-energy processes in objects.

astro-ph↗

arXiv2002

Power Spectra of X-ray Binaries

The interpretation of Fourier spectra in the time domain is critically examined. Power density spectra defined and calculated in the time domain are compared with Fourier spectra in the frequency domain for three different types of variability: periodic signals, Markov processes and random shots. The power density spectra for a sample of neutron stars and black hole binaries are analyzed in both the time and the frequency domains. For broadband noise, the two kinds of power spectrum in accreting neutron stars are usually consistent with each other, but the time domain power spectra for black hole candidates are significantly higher than corresponding Fourier spectra in the high frequency range (10--1000 Hz). Comparing the two kinds of power density spectra may help to probe the intrinsic nature of timing phenomena in compact objects.

astro-ph↗

arXiv2002

Temporal and Spectral Properties of Gamma-Ray Flashes

The temporal and spectral properties of terrestrial gamma-ray flashes (TGFs) are studied. The delay of low energy photons relative to high energy ones in the gamma-ray variations of the TGFs with high signal to noise ratio has been revealed by an approach of correlation analysis in the time domain on different time scales. The temporal structures of the TGFs in high energy band are usually narrower than that in low energy band. The spectral hardness has a general trend of decreasing with time in a flash. The observed temporal and spectral characteristics give constraints and valuable hints on the flash production mechanism.

astro-ph↗

arXiv2001

The Cross-Spectra of Cir X-1: Evolution of Time Lags

Earlier work showed that the track in the X-ray hardness-intensity diagram of Cir X-1 corresponds to a Z track in its color-color diagram. In this paper, we study the cross spectrum of Cir X-1 in different regions of the hardness-intensity diagram with $RXTE$/PCA data. Comparing the light curves of Cir X-1 for the energy band 1.8-5.1 keV to those for 5.1-13.1 keV, we find that Cir X-1 exhibits a hard time lag on the horizontal branch, and a soft time lag on both the normal and the flaring branch. This indicates that Cir X-1 is similar to GX 5-1 and Cyg X-2 on the horizontal branch, but is different from them on the normal branch. We briefly discuss the mechanism of the time lags in the context of Comptonization models.

astro-ph↗

arXiv1999

Temporal and Spectral Correlations of Cyg X-1

Temporal and spectral properties of X-ray rapid variability of Cyg X-1 are studied by an approach of correlation analysis in the time domain on different time scales. The correlation coefficients between the total intensity in 2-60 keV and the hardness ratio of 13-60 keV to 2-6 keV band on the time scale of about 1 ms are always negative in all states. For soft states, the correlation coefficients are positive on all the time scales from about 0.01 s to 100 s, which is significantly different with that for transition and low states. Temporal structures in high energy band are narrower than that in low energy band in quite a few cases. The delay of high energy photons relative to low energy ones in the X-ray variations has also been revealed by the correlation analysis. The implication of observed temporal and spectral characteristics to the production region and mechanism of Cyg X-1 X-ray variations is discussed.

astro-ph↗

arXiv1998

COMPTEL Observation of the Flaring Quasar PKS0528+134

With a direct demodulation method, we have reanalyzed the data from COMPTEL/CGRO observation of PKS0528+134 during the 1993 March flare in gamma-rays. Our results show that during the flare gamma-rays were detected at a level approximately 2.4-3.8 times greater than the observed intensity in two earlier COMPTEL observations VP 0 and VP 1 in the energy range 3 MeV to 30 MeV. The 3-30 MeV time variability of the flux follows well the trend as observed by EGRET/CGRO at higher energies. No convincing excess can be found around the position of PKS0528+134 in the energy range 0.75 MeV to 3 MeV, which indicates a spectral break around 3 MeV. The detections and non-detections in the four standard COMPTEL energy bands are consistent with the earlier reports given by Collmar et al., while the feature that gamma-rays of the quasar still kept on flaring at energies down to 3 MeV is clearly found.

astro-ph↗

arXiv1998

X-ray Shots of Cyg X-1

X-ray shots of Cyg X-1 in different energy bands and spectral states have been studied with PCA/RXTE observations. The detailed shot structure is obtained by superposing many shots with one millisecond time bin through aligning their peaks with an improved algorithm. In general, the shots are composed of a slow rise and fast decay. The shot structures in the different states are different. The duration of shot in the high state is shorter than that in the low and transition states. The shot profile in the high energy band is more asymmetric and narrower than that in the low energy band. The average hardness of shot is lower than that of steady emission in the transition and low states but higher than that in the high state. The time lags between the shots in higher and lower energy bands have been found in the different states. In transition states, the time lag is the largest among the different states of Cyg X-1, and it is the smallest in the low state. The implications of the observed shot features for shot models are discussed.

astro-ph↗