The challenge in searching for non-radio-pulsing isolated neutron stars (INSs) is in excluding association with objects in the very large error boxes ($13 00 , 1 radius) typical of sources from the largest X-ray all-sky survey, the ROSAT All-Sky Survey Bright Source Catalogue (RASS BSC). We search for candidate INSs using statistical analysis of optical (USNO-A2), infrared (IRAS), and radio (NVSS) sources near the ROSAT X-ray localization and show that this selection would find 20% of the INSs in the RASS BSC. This selection finds 32 candidates at declinations > À39 , among which are two previously known INSs, 17 sources that we show are not INSs, and 13 the classification of which are as yet undetermined. These results require a limit of less than 67 INSs (90% confidence, full sky, assuming isotropy) in the RASS BSC. This limit modestly constrains a naive and optimistic model for cooling neutron stars in the Galaxy.
We report on multi‐epoch X‐ray observations of the Type IIn (narrow emission line) Supernova (SN) 1995N with the ROSAT and ASCA satellites. The 1998 January ASCA X‐ray spectrum is well fitted by a thermal bremsstrahlung
or power‐law
model. The X‐ray light curve shows evidence for significant flux evolution between 1996 August and 1998 January: the count rate from the source decreased by 30 per cent between our 1996 August and 1997 August ROSAT observations, and the X‐ray luminosity most likely increased by a factor of ∼2 between our 1997 August ROSAT and 1998 January ASCA observations, although evolution of the spectral shape over this interval is not ruled out. The high X‐ray luminosity,
places SN 1995N in a small group of Type IIn supernovae with strong circumstellar interaction, and the evolving X‐ray luminosity suggests that the circumstellar medium is distributed inhomogeneously.
The X‐ray emission from X‐ray binaries may originate in flares occurring when magnetic loops anchored in the disc reconnect. In analogy with our Sun, Hα emission should arise as the accelerated electrons thermalize in the optically emitting disc, perhaps leading to correlated variability between X‐rays, Hα and the optical continuum. We present simultaneous X‐ray and optical high‐speed photometry of the neutron star low‐mass X‐ray binary Cyg X‐2 to search for such correlations. The highest time resolution achieved is 5 ms in white light and 100 ms with a 3‐nm filter centred on Hα. We find power on time‐scales ≳100 s (flickering) in optical with a total rms of a few per cent, about an order of magnitude less than that seen in X‐rays. We do not find significant correlations between the X‐ray and optical fluxes on short time‐scales, and hence cannot conclude whether magnetic flares contribute significantly to the optical emission.
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