A low-level accretion flare during the quiescent state of the neutron-star X-ray transient SAX J1750.8-2900

We report on a series of Swift/X-ray telescope observations, performed between 2012 February and 22 March, 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, depending on the exposure length (which ranged from ∼0.3 to ∼3.8 ks). The upper limits for the non-detections were consistent with the detected luminosities (when fitting a thermal model to the spectrum) of ∼10³⁴ erg s^-1 (0.5-10 keV). This level is consistent with what has been measured previously for this source in quiescence. However, on March 17 the source was found to have an order of magnitude larger count rate. When fitting the flare spectrum with an absorbed power-law model, we obtained a flare luminosity of (3-4) × 10³⁴ erg s^-1 (0.5-10 keV). Follow-up Swift observations showed that this flare lasted <16 d. This event was very likely due to a brief episode of low-level accretion on to the neutron star and provides further evidence that the quiescent state of neutron-star X-ray transients might not be as quiet as is generally assumed. The detection of this low-level accretion flare raises the question whether the quiescent emission of the source (outside the flare) could also be due to residual accretion, albeit continuous instead of episodic. However, we provide arguments which would suggest that the lowest intensity level might instead represent the cooling of the accretion-heated neutron star.