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Astron. Astrophys. 357, L21-L24 (2000)

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3. A long X-ray flux enhancement of 4U 1735-44

In Fig. 1 we show the lightcurve of 4U 1735-44 as observed with the WFC between 21 and 30 August 1996. The persistent countrate varies between 0.2 and 0.3 counts cm-2s-1. Immediately after the earth occultation on MJD 50318.1 a strong enhancement (factor [FORMULA]3) in the X-ray intensity was seen which subsequently decayed exponentially. An expanded lightcurve of this event is also shown in Fig. 1. The position derived for this event is [FORMULA] from the position of 4U 1735-44 as derived from its persistent emission. (Both positions share the same systematic error, and thus their relative position is much more accurate than their absolute positions, which have errors of [FORMULA].) We conclude that the event is from 4U 1735-44.

[FIGURE] Fig. 1. Top: The nine day lightcurve of 4U 1735-44 as observed with the WFC in August 1996. Countrates are for channels 1-31 (energy range 2-28 keV). Each time bin corresponds to 15 minutes. A large enhancement in intensity starts near MJD 50318.1 and ends about [FORMULA]4.0 hours later. The vertical dotted lines indicate the time interval for which the countrate and hardness ratio are shown in the expanded view of the lower frames. The hardness ratio shown is the ratio of the countrate in channels 12-29 (5-20 keV) to that in channels 3-11 (2-5 keV). During the flux enhancement the exponential softening expected for a type-I X-ray burst is clearly visible. The vertical dotted lines indicate the time interval for which we add the data to obtain the burst spectrum.

To the persistent flux we fit the two models discussed in the introduction, i.e. a power law with high energy cutoff, and a sum of bremsstrahlung and black body spectra, in the 2-24 keV range. The spectrum before and after the event are for the intervals MJD 50317.8-50318.1 and MJD 50319.7-50320.8, respectively. The results are given in Table 1. We note that the values of the fit parameters are similar to those for earlier observations. Notwithstanding the different flux levels before and after the flux enhancement, the hardness of the spectrum (also shown in Fig. 1) is similar. The persistent flux corresponds to an X-ray luminosity at 9.2 kpc of [FORMULA] in the 2-28 keV band. In our fits we set the interstellar absorption at a fixed value of [FORMULA]; the hard energy range of the WFC is not much affected by absorption, and fits for different assumed absorption values give results similar to those listed in Table 1.


Table 1. Results of the modeling of the X-ray spectrum. We fit a cutoff photon powerlaw spectrum [FORMULA] and a sum of a bremsstrahlung of temperature [FORMULA] and black body spectrum of temperature [FORMULA] and radius R to the data before and after the burst. For the burst, we fix the parameters of either the cutoff power law or the bremsstrahlung component to the values found after the burst, and fit for a blackbody added to these. The absorption column is fixed at the value of [FORMULA] found by Christian & Swank (1997). For each model we give the total flux in the range observed with the WFC, i.e. 2-28 keV, as well as, for comparison with earlier observations, in the range of 1.4-11 keV.

To describe the flux decline we first fit an exponential [FORMULA] to the observed countrate in the 2-28 keV range. The fit is acceptable (at [FORMULA] for 33 d.o.f.) and [FORMULA] min. Fits to the counts in the 2-5 keV and 5-20 keV ranges give decay times of [FORMULA] and [FORMULA] min, respectively, in accordance with the observed softening of the flux during decline (see Fig. 1). We fit the spectrum during the flux enhancement as follows. First we add all the counts obtained between MJD 50318.10 and 50318.25. We then fit the total spectrum with the sum of a black body and either a cutoff power law spectrum or a thermal bremsstrahlung spectrum. In these fits, the parameters of the power law and bremsstrahlung component are fixed at the values obtained for the fit to the persistent spectrum after the event. The resulting parameters for the black body are also listed in Table 1. At the observed maximum the bolometric flux was [FORMULA] which for a source at 9.2 kpc corresponds to a luminosity of [FORMULA]. The start of the flux enhancement is not observed, but if we assume that its maximum is at the Eddington limit of [FORMULA] (for a neutron star mass of [FORMULA]) and that the decay time is constant, then maximum was reached 23.6 min before the source emerged from earth occultation, leaving at most 12.4 min for the rise to maximum (since the start of the data gap). The decay from maximum was therefore much longer, by a factor [FORMULA]8, than the rise. The fluence in the observed part of the burst is [FORMULA], corresponding to [FORMULA] erg; this is a lower limit to the energy released during the full event.

We have also made fits to the first and second half of the event separately, and find temperatures for the blackbody component of 2.1-2.2 keV and 1.3-1.4 keV for the first and second half respectively, confirming the softening. For the blackbody radius we find 5.7-6.5 km and 8.5-8.8 km, for the first and second half, respectively. This apparent increase in radius is probably due to the difference between the observed colour temperature and the actual effective temperature of the black body; when we apply corrections to the colour temperature as given by van Paradijs et al. (1986) the value for the radius in the first part of the burst increases to 14 km, whereas that for the second half is unchanged.

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Online publication: May 3, 2000