arXiv subjects

M. J. Church

Publications and source records attributed to M. J. Church.

41 records · Page 3Linked to original sources

arXiv1998

Progressive Covering of the ADC during Dipping in the LMXB XBT 0748-676

We report results of analysis of the ASCA observation of 1993, May 7th of the dipping LMXB source XBT 0748-676, and propose a new explanation of the spectral evolution in dipping in this source. We show that the non-dip and dip spectra in several intensity bands are well fitted using the same physical model that we have previously shown gives good explanations of several dipping sources, consisting of point source blackbody emission from the neutron star, plus extended Comptonised emission from the accretion disk corona (ADC), with progressive covering of the ADC during dipping. Best fit values of kT_bb = 1.99 +/- 0.16 keV and power law photon index Gamma = 1.70 +/- 0.16 are found. The strong excess below 1 keV was well fitted by a Gaussian line at ~0.65 keV. In dipping, good fits were obtained by allowing it to be covered by the same progressive covering factor as the extended continuum emission, providing strong evidence that the line originates in the ADC. By using our two-component model we show that the unabsorbed component is the uncovered fraction of the Comptonised emission, and in the band 1 - 10 keV, we do not need to invoke substantial electron scattering to explain dipping.

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arXiv1997

Simple Photoelectric Absorption during Dipping in the ASCA Observation of XB1916-053

We report results of analysis of the ASCA observation of the Low Mass X-ray Binary dipping source XB1916-053 made on 1993, May 2nd. We have shown that the spectra can be well fitted by the same two-component model previously used to give good explanations of the very different dip sources X1755-338 and X1624-490, consisting of point-source blackbody emission from the neutron star plus extended Comptonised emission from the accretion disk corona. In XB1916-053, all levels of dipping can be fitted using kT_bb= 2.14+/-0.28 keV and power law photon index = 2.42 +/-0.21. Dipping is shown to be due to large increases of column density for the point-like blackbody, combined with the extended power law component being progressively covered by absorber until in the deepest parts of dips, the partial covering fraction approaches unity. This approach differs radically from the ``absorbed plus unabsorbed'' approach previously used in spectral modelling of XB1916-053 and similar sources, in which the normalisation of the unabsorbed component is allowed to decrease markedly in dipping, behavior generally attributed to the effects of electron scattering. Thus we have shown that spectral evolution in XB1916-053 can be explained simply in terms of photoelectric absorption without the need for substantial electron scattering. This explanation is supported by calculation of the relative importance of photoelectric absorption and electron scattering in the absorbing region which shows that little electron scattering is expected in the ASCA energy band.

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arXiv1997

The Cessation of Flickering during Dips in Cygnus X-1

We report the discovery of the cessation of flickering in dips in the black hole candidate Cygnus X-1, detected for the first time in the ASCA observation of May 9th., 1995. During this observation, particularly deep dipping took place resulting in strong changes in hardness ratio corresponding to absorption of the power law spectral component. The deadtime corrected light curve with high time resolution clearly shows a dramatic decrease in the extent of flickering in the band 0.7 - 4.0 keV during dipping, but in the band 4.0 - 10.0 keV, there is relatively little change. We show that the rms flickering amplitude in the band 0.7 - 4.0 keV is proportional to the X-ray intensity in this band which changes by a factor of almost three. This is direct evidence that the strong Low State flickering is intrinsic to the power law emission; ie takes place as part of the emission process. The rms amplitude is proportional to the intensity in the low energy band, except for a possible deviation from linearity at the lower intensities. If confirmed, this non-linearity could imply a process such as electron scattering of radiation which will tend to smear out the fluctuations, or a process of fluctuation generation which depends on radial position in the source. Thus timing observations during absorption dips can give information about the source region and may place constraints on its size.

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arXiv1997

Attenuation of dipping at low energies in the LMXB source X 1755-338

We report spectral fitting results for Rosat PSPC observations of the dipping source X 1755-338. These results are consistent with the two-component model that we previously proposed consisting of a blackbody point source plus an extended power law component. Remarkably, the low energy cut-off of the spectrum does not change appreciably in dips, and it can be seen that dipping takes place in the higher part of the PSPC band where the blackbody contributes to the spectrum. Thus dipping consists primarily of absorption of this component, whereas the low energy cut-off is determined by the power law. Thus, above 0.5 keV, the dipping is approximately independent of energy as seen in Exosat and 10 +/- 0.4% deep. Below 0.5 keV, there may be a small residual dipping effect of up to 3%; however dipping is certainly substantially reduced compared with the 20% seen by Exosat, and this is the first time that the effective cessation of dipping at low energies in such sources has been seen.

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arXiv1995

Identification of the soft X-ray excess in Cygnus X-1 with disc emission

We present results of a study of the soft X-ray excess in the black hole candidate Cygnus X-1 made with the {\it Rosat PSPC}, using observations taken during persistent emission at orbital phases close to 0.5. The soft excess can be well fitted as a blackbody with temperature $\rm {kT_{bb}}$ = $\rm {0.13\pm 0.02}$ keV. $\rm {kT_{bb}}$ did not vary appreciably with intensity of the source. By assuming that the distance of the source is its lower limit of 2.5 kpc, a luminosity of the soft excess of $\rm {4.7\cdot 10^{36}}$ erg $\rm {s^{-1}}$ was obtained. From this, disc temperatures were calculated as a function of radius, assuming the compact object to be a 10 $\rm{M_{\sun}}$ black hole, in particular, the temperature at 7 Schwarzschild radii expected to be highly representative of the total disc emission. This was found to be 0.13 keV, in very good agreement with the spectral fitting result. This good agreement strongly supports the identification of the soft excess with emission from the disc around a black hole.

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