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تسجيل مستخدم جديدVariable quiescent state for the neutron-star X-ray transient SAX J1750.8-2900: not such a hot neutron star after all?
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We monitored the neutron star low-mass X-ray binary SAX J1750.8-2900 after the end of its 2015/2016 outburst using the X-ray Telescope (XRT) aboard Swift to detect possible post-outburst rebrightenings, similar to those seen after its 2008 outburst. We did not detect any rebrightening behaviour, suggesting that the physical mechanism behind the rebrightening events is not always active after each outburst of the source. Any model attempting to explain these rebrightenings should thus be able to reproduce the different outburst profiles of the source at different times. Surprisingly, our Swift/XRT observations were unable to detect the source, contrary to previous Swift/XRT observations in quiescence. We determined a temperature upper limit of $leq$ 106 eV, much colder than the post 2008 outburst value of $sim$ 145 eV. We also report on an archival Chandra observation of the source after its 2011 outburst and found a temperature of $sim$ 126 eV. These different temperatures, including the non-detection very close after the end of the 2015/2016 outburst, are difficult to explain in any model assuming we observe the cooling emission from a neutron star core or an accretion-heated crust. We discuss our observations in the context of a change in envelope (the outer $sim$ 100 m of the crust) composition and (possibly in combination with) a cooling crust. Both hypotheses cannot explain our results unless potentially unrealistic assumptions are made. Irrespective of what causes the temperature variability, it is clear that the neutron star in SAX J1750.8-2900 may not be as hot as previously assumed.
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We report on a series of Swift/XRT observations, performed between February and 22 March 2012, during the quiescent state of the neutron-star X-ray binary SAX J1750.8-2900. In these observations, the source was either just detected or undetected, dep
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Tracking the spectral evolution of transiently accreting neutron stars between outburst and quiescence probes relatively poorly understood accretion regimes. Such studies are challenging because they require frequent monitoring of sources with lumino
sities below the thresholds of current all-sky X-ray monitors. We present the analysis of over 30 observations of the neutron star low-mass X-ray binary SAX J1750.8-2900 taken across four years with the X-ray telescope aboard Swift. We find spectral softening with decreasing luminosity both on long ($sim$1 year) and short ($sim$days to week) timescales. As the luminosity decreases from $4times10^{36}$ erg s$^{-1}$ to $ sim1times10^{35} $ erg s$^{-1}$ (0.5-10 keV), the power law photon index increases from from 1.4 to 2.9. Although not statistically required, our spectral fits allow an additional soft component that displays a decreasing temperature as the luminosity decreases from $4 times 10^{36} $ to $6 times 10^{34}$ erg s$^{-1}$. Spectral softening exhibited by SAX J1750.8-2900 is consistent both with accretion emission whose spectral shape steepens with decreasing luminosity and also with being dominated by a changing soft component, possibly associated with accretion onto the neutron star surface, as the luminosity declines.
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