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Astron. Astrophys. 347, L39-L42 (1999)

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3. Photometric evolution

The photometric evolution of the 1999 outburst has followed quite closely that of previous events as Fig. 1 clearly shows. Small differences may be easily accounted for by (a) different comparison sequences, and (b) a mixture of different observing techniques used by B81 and S88 (visual estimates directly at the eyepiece or on the screen of TV telescope guiding systems, photography, etc.)

With a maximum brightness of [FORMULA] on February 25.562, the 1999 outburst has been characterized by a fast decline with [FORMULA] and [FORMULA] days, very close to the 0.67 mag day-1 reported by Payne-Gaposchkin (1957) for the 1863 and 1936 events. The outburst was discovered by P.Schmeer on February 25.194 when he estimated U Sco at [FORMULA]. This suggests a fast rise to maximum of the order of [FORMULA]mag = 5.2 day-1, or even faster if the observation at [FORMULA] on February 25.562 was actually past the true maximum.

There is an important negative detection on Feb 25.040 listed in Table 2, when U Sco was found fainter than V=14.3. This is just 22 minutes before central eclipse according to the SR95 ephemeris and 3.7 hours earlier than the [FORMULA] outburst discovery by P.Schmeer. Adopting the [FORMULA]mag = 5.2 day-1 rise rate just estimated, U Sco should have been at [FORMULA] mag at the time of the Feb 25.040 negative detection. This seems to suggest that the dimensions of the outbursting WD were still smaller than those of the occulting companion 0.522 days before maximum. However, a different explanation is in order if the SR95 ephemeris should turn out to be no more accurate in 1999 and/or the predicted minima are the eclipse of the hot spot and not those of the WD.

Integrating the lightcurve in Fig. 1, the energy radiated in the V band by U Sco during the time covered by the observations in Table 2 can be expressed as


where [FORMULA] is the extinction in magnitudes and D is the distance in kpc. The slope of the continuum in our spectra suggests a color temperature of about 2[FORMULA] oK. Assuming for sake of discussion that U Sco has radiated on the average as a Kurucz's model atmosphere with T=20,000 oK and log g = 3.0, we find that the global radiated energy is [FORMULA] that radiated in the V band, so Eq. (1) can be rewritten for the bolometric energy as


For any reasonable distance inside the Galaxy and the extinction generally adopted ([FORMULA]=0.6, cf. S88), the 1999 outburst of U Sco appears considerably underluminous compared to those of classical novae (cf. W95). The same conclusion was reached by W81 from IUE observations of the 1979 outburst of U Sco.

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© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999