arXiv subjects

N. E. White

Publications and source records attributed to N. E. White.

26 records · Page 2Linked to original sources

arXiv2001

Correlations Between Spectral Properties and Spin-Down Rate in Soft Gamma-Ray Repeaters and Anomalous X-ray Pulsars

Anomalous x-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are x-ray sources with unusual properties distinguishing them from both rotation-powered and most accretion-powered pulsars. Using archival ASCA data over the energy range 0.5-10.0 keV, we have studied the spectra of the persistent emission from these sources and their variation with spin-down rate. Using a single power law spectral model, we find that the overall hardness of the spectra increase with increasing spin-down rate, and therefore the spectral and spin-down mechanism are inextricably linked in these objects. In terms of the two-component blackbody plus power law spectral models, this correlation is seen as an increasing hardness of the high energy component with increasing spin-down rate, with the temperature of the low energy blackbody component remaining essentially constant. Also for the two component spectral model: the ratio of the 2-10 keV power law and bolometric blackbody luminosities gradually increases with the spin-down rate. We discuss these results in terms of the various theoretical models for SGRs and AXPs.

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arXiv1999

BeppoSAX monitoring of the ``anomalous'' X-ray pulsar 4U 0142+61

The 8.7s X-ray pulsar 4U 0142+61 was monitored by BeppoSAX between January 1997 and February 1998. This source belongs to the rapidly growing class of "anomalous" X-ray pulsars (AXPs) which have pulse periods in the 6-12s range and no plausible optical, infrared or radio counterparts. The BeppoSAX periods measurements show that 4U 0142+61 continues its spin-down at a nearly constant rate of about 2 times 10^{-12}s s^{-1}. The 0.5-10 keV pulse shape is double peaked. The phase-averaged spectrum can be described by a steep absorbed power-law (Gamma about 4) plus a blackbody with kT = 0.4 keV. 4U 0142+61 was also detected serendipitously in a March 1996 RossiXTE observation pointed towards the nearby 1455s X-ray pulsar RX J0146.9+6121. The timing analysis results are reported for this observation and the spin history of 4U 0142+61 since 1979 is discussed.

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arXiv1998

Quasi-simultaneous observations of 3C 273 by BeppoSAX and ASCA

We report the results of quasi-simultaneous observations of the bright quasar 3C 273 using the BeppoSAX Low-Energy Concentrator Spectrometer (LECS) and the ASCA Solid-State Imaging Spectrometer (SIS). These observations are part of an intercalibration programme and demonstrate good agreement between both instruments in the common energy range 0.5-10 keV. In particular, the absorption feature discovered by BeppoSAX/LECS at $\sim$ 0.5 keV is consistently seen by ASCA/SIS. We present the most accurate measurement yet of the spectral shape of this absorption feature and provide constraints on its physical interpretation. A self-consistent warm absorber model in photoionization equilibrium only gives an adequate spectral fit if a very high velocity inflow is allowed for the warm absorber (v $\sim$ 0.36 c). The ionization parameter and the column density of the warm absorber are then $\sim$ 1 and $\sim$ 1.3e21 cm^-2, respectively. If the differences in best-fit neutral absorption column density between the LECS and each SIS are taken as estimates of the systematic spectral uncertainties, then these are < 0.8e20 and < 2.e20 cm^-2 (at 90 % confidence) for SIS0 and SIS1, respectively.

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arXiv1998

The discovery of 12min X-ray pulsations from 1WGA J1958.2+3232

During a systematic search for periodic signals in a sample of ROSAT PSPC (0.1-2.4 keV) light curves, we discovered 12min large amplitude X-ray pulsations in 1WGA J1958.2+3232, an X-ray source which lies close to the galactic plane. The energy spectrum is well fit by a power law with a photon index of 0.8, corresponding to an X-ray flux of about 10E-12 ergs cmE-2 sE-1. The source is probably a long period, low luminosity X-ray pulsar, similar to X Per, or an intermediate polar.

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arXiv1997

X-ray observations of the slowest known Be/X-ray pulsars RX J0146.9+6121 and X Persei

We present ASCA and ROSAT observations of the Be/X-ray binary pulsars RX J0146.9+6121 and X Persei between 1990 and 1996. Measuring the neutron star spin period and X-ray luminosity of X Persei shows that the episode of low X-ray luminosity and monotonic spin-down since 1978 continued to at least August 1995. ASCA and ROSAT HRI observations of RX J0146.9+6121 detected the pulsar with the longest known neutron star spin period at low luminosities after the X-ray outburst seen by EXOSAT in August 1984. The large spin period decrease seen between 1984 and 1993 has nearly stopped at a period of 1407.3 s in February 1996. We discuss that the X-ray outburst behaviour of X Persei with respect to its Be star phase changes observed in the optical can be caused by an asymmetry in the matter distribution around the Be star.

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arXiv1997

The discovery of 8.9s pulsations from the variable X-ray source 2E0050.1-7247 in the small Magellanic cloud

During a systematic search for periodic signals in a sample of ROSAT PSPC light curves, we discovered ~ 8.9s X-ray pulsations in 2E0050.1-7247, a variable X-ray source in the Small Magellanic Cloud. The source was detected several times between 1979 and 1993 at luminosity levels ranging from about 5 times 10^{34} erg/s up to 1.4 times 10^{36} erg/s with both the Einstein IPC and ROSAT PSPC. The X-ray energy spectrum is consistent with a power law spectrum which steepens as the source luminosity decreases. We revealed a pronounced Halpha activity from at least two B stars in the X-ray error circles. These results strongly suggest that the X-ray pulsar 2E0050.1-7247 is in a Be-type massive binary.

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arXiv1997

A Broad Band and Large Area X-Ray Omni Sky Monitor (BLOSM)

We present a conceptual design for a new X-ray all sky monitor (ASM). Compared with previous ASMs, its salient features are: (1) it has a focusing capability that increases the signal to background ratio by a factor of 3; (2) it has a broad-band width: 200 eV to 15 keV; (3) it has a large X-ray collection area: ~100 square cm; (4) it has a duty cycle of nearly 100%, and (5) it can measure the position of a new source with an accuracy of a few minutes of arc. These features combined open up an opportunity for discovering new phenomena as well as monitoring existing phenomena with unprecedented coverage and sensitivity.

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arXiv1996

The discovery of 13s X-ray pulsations from the hydrogen depleted subdwarf O6 star binary HD49798

We discovered strong 13s X-ray pulsations in the public Rosat PSPC light curve of HD49798, a 1.55d single-component spectroscopic binary containing a hydrogen depleted subdwarf O6 star. We find no evidence for period changes during the about 4hr Rosat pointing. The source X-ray spectrum is extremely soft, with an unabsorbed 0.1-2keV luminosity of a few times 10^32 erg s^-1 (distance of 650pc). A higher luminosity might be hidden in the EUV. Our results imply that the unseen companion is an accreting degenerate star, a white dwarf or, more likely, a neutron star. In either case HD49798 corresponds to a previously unobserved evolutionary stage of a massive binary system, after common envelope and spiral-in.

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