arXiv subjects

Hsiang-Kuang Chang

Publications and source records attributed to Hsiang-Kuang Chang.

77 records · Page 5Linked to original sources

arXiv2008

Periodicity Search of Possible X-ray Counterparts to Radio-quiet Gamma-ray Pulsar Candidates

Periodicity search in gamma-ray data is usually difficult because of the small number of detected photons. A periodicity in the timing signal at other energy bands from the counterpart to the gamma-ray source may help to establish the periodicity in the gamma-ray emission and strengthen the identification of the source in different energy bands. It may, however, still be difficult to find the period directly from X-ray data because of limited exposure. We developed a procedure, by cross-checking two X-ray data sets, to find candidate periods for X-ray sources which are possible counterparts to gamma-ray pulsar candidates. Here we report the results of this method obtained with all the currently available X-ray data of 8 X-ray sources. Some tempting periodicity features were found. Those candidate periods can serve as the target periods for future search when new data become available so that a blind search with a huge number of trials can be avoided.

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arXiv2007

Eigenmodes of elastic vibrations of quaking neutron star encoded in QPOs on light curves of SGR flares

The Newtonian solid-mechanical theory of nodeless spheroidal and torsional of elastic seismic vibrations trapped in the crust of a quaking neutron star is outlined and applied to the modal classification of the quasi-periodic oscillations (QPOs) of X-ray luminosity in the aftermath of giant flares in SGR 1806-20 and SGR 1900+14. The presented analysis relies heavily on the Samuelsson-Andersson identification of the QPOs frequency from the range 30-200 Hz with those for torsional nodeless vibrations of multipole degree in the interval 2-12. Based on this identification, which is used to fix the input parameters entering the obtained spectral formulae, we compute frequency spectrum of nodeless spheroidal elastic vibrations. Particular attention is given to the low-frequency QPOs in the data for SGR 1806-20 whose physical origin has been called into question. Our calculations suggest that these unspecified QPOs are due to nodeless dipole torsional (18 Hz) and dipole spheroidal (26 Hz) elastic shear vibrations.

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arXiv2007

Millisecond dips in the RXTE/PCA light curve of Sco X-1 and TNO occultation

Millisecond dips in the RXTE/PCA light curve of Sco X-1 were reported recently (Chang et al. 2006), which were interpreted as the occultation of X-rays from Sco X-1 caused by Trans-Neptunian Objects (TNO) of hundred-meter size. Inconclusive signatures of possible instrumental effects in many of these dip events related to high-energy cosmic rays were later found (Jones et al. 2006) and the TNO interpretation became shaky. Here we report more detailed analysis aiming at distinguishing true occultation events from those related to cosmic rays. Based on some indicative criteria derived from housekeeping data and two-channel spectral information, we suggest that about 10% of the dips are probable events of occultation. The total number of TNOs of size from 60 m to 100 m is estiamted to be about 10^{15} accordingly. Limited by the coarser time resolution of standard data modes of RXTE/PCA, however, definite results cannot be obtained. Adequately configured observations with RXTE or other new instruments in the future are very much desired.

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arXiv1998

RXTE Observation of PSR B0656+14

PSR B0656+14 was observed by the {\it Rossi X-ray Timing Explorer (RXTE)} with the proportional counter array (PCA) and the high-energy x-ray timing experiment (HEXTE) for 160 ksec during August 22 -- September 3, 1997. No pulsation was firmly found in the timing analysis, in which the contemporaneous radio ephemeris and various statistical tests were applied for searching evidence of pulsation. A marginal detection of pulsation at a confidence level of 95.5% based on the $H$-test was found with data in the whole HEXTE energy band. In the energy band of 2-10 keV the {\it RXTE} PCA upper limits are about one order of magnitude lower than that from {\it ASCA} GIS data. If the {\it CGRO} EGRET detection of this pulsar is real, considering the common trait that most EGRET-detected pulsars have a cooling spectrum in hard x-ray and gamma ray energy bands, the estimated {\it RXTE} upper limits indicate a deviation (low-energy turn-over) from a cooling spectrum starting from 20 keV or higher. It in turn suggests an outer-magnetospheric synchrotron-radiation origin for high-energy emissions from PSR B0656+14. The {\it RXTE} PCA upper limits also suggest that a reported power-law component based on {\it ASCA} SIS data in 1-10 keV fitted jointly with {\it ROSAT} data, if real, should be mainly unpulsed.

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arXiv1997

RXTE Observation of PSR B1951+32

We report results of RXTE observations of PSR B1951+32 using the PCA instrument for 19k seconds during 1996 March 24th. We applied the contemporaneous radio ephemeris and various statistical tests to search for evidence of pulsation. These analyses yield intriguing yet inconclusive evidence for the presence of the pulsation in the time series: confidence level for the presence of pulsation is 94.5% in the 2.0-4.8 keV band and 97.6% in the 4.8-6.3 keV band based on the H-test. Under the premise of non-detection of pulsation, we derive estimated 2-sigma upper limits for the pulsed flux to be $3.3\times 10^{-6} cm^{-2} s^{-1} keV^{-1}$ in the 2.0-4.8 keV band, $2.7\times 10^{-6} cm^{-2} s^{-1} keV^{-1}$ in the 4.8-8.5 keV band, and $2.0\times 10^{-6} cm^{-2} s^{-1} keV^{-1}$ in the 8.5-13.0 keV band. These upper limits are consistent with the trend of spectral turn-over from high-energy gamma-ray emission as suggested by the OSSE upper limit. Such turn-over strongly suggests the outer magnetosphere as the emission site for pulsed gamma-rays. These RXTE upper limits for X-ray pulsation are, on the other hand, not consistent with the extrapolation of reported power-law spectra from the point source observed by ROSAT in the 0.1-2.4 keV band, assuming a constant pulse fraction: The pulsed soft X-ray emission detected by ROSAT must follow a much softer spectrum than that of the overall point source.

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