We have tried to reconstruct the approximate X-ray lightcurve of the August 1998 outburst by combining data from various measurements. Detections were made by the Wide Field Cameras and the Narrow Field Instruments on board of BeppoSAX on Aug 22 and 26, respectively; and by the XTE All Sky Monitor in a seven-day period starting on Aug 19. We revise the flux detected with the Wide Field Cameras to mCrab, slightly upwards from the values given in In 't Zand et al. (1999), on the basis of a better calibration. Upper limits were obtained with the BeppoSAX Wide Field Cameras on Sep 1 and with the XTE All Sky Monitor in the seven days periods preceding and following the detection. Fig. 3 shows the resulting lightcurve.
Extrapolating the 5-day exponential decay from the BeppoSAX detections to the time of the ROSAT HRI observation, we predict an X-ray luminosity of in the ROSAT band; this is about an order of magnitude above the observed upper limit. This may imply that the decay accelerated; or alternatively that the spectrum softened, since low-energy photons are much more affected by the heavy absorption towards NGC 6440. There is indeed evidence for other X-ray transients that the spectrum in the low state is much softer than during outburst, e.g. for Aql X-1 (Verbunt et al. 1994).
The X-ray lightcurve shown in Fig. 3 implies that the optical observations were made at an X-ray flux level of about 10 mCrab, corresponding to a source luminosity . Van Paradijs & McClintock (1994) give a semi-empirical relation between the orbital period, X-ray luminosity and absolute visual magnitude of a low-mass X-ray binary. Applying this relation to the transient in NGC 6440 with the estimate of the X-ray luminosity for the time of the optical observation, we obtain for an assumed 1 hr period. At the distance and reddening of NGC 6640 this corresponds to . The intrinsic colour of low-mass X-ray binaries is close to zero; with the reddening to NGC 6440 we thus predict for a one hour period. For a period of 5 hr (0.2 hr) the predicted magnitude is about 1 magnitude brighter (fainter). We conclude that the candidate in the error circle of X2 is viable; the proximity of the predicted B magnitude to the observed one is fortuitous, considering that the spread in the relation given by Van Paradijs & McClintock is about a magnitude, and that our estimate of the X-ray luminosity is uncertain. We therefore dare not estimate an orbital period on the basis of the magnitude of our candidate.
The core of NGC 6440 contains PSR B 1745-20 (Lyne et al. 1996). The total energy loss for the pulsar is about , where I is the moment of inertia of the neutron star, its rotation frequency and the time derivative of . Typical X-ray luminosities for radio pulsars are of order (e.g. Fig. 4 in Verbunt et al. 1996). We conclude that it is very unlikely that the pulsar is responsible for the observed X-ray flux of X1 or X2.
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000