Friday, January 29, 2010

U Scorpii in Outburst

The recurrent nova U Sco has been discovered in outburst at magnitude V=8.05 on January 28.4385 UT by amateur astronomers B. Harris and shortly after by S. Dvorak (who estimated it at V~8.8). According to the AAVSO website prior to outburst, U Scorpii was measured at V=18.2 on Jan 27.4501 (Harris), and estimated at m(vis) less than 16.5 on January 27.6271 by M. Linnolt.

Novae are binary systems where the companion star feeds matter through an accretion disk to accumulate on a white dwarf. After the eruption, the white dwarf returns to a steady state while the accumulation begins anew so that all novae recur. Each white dwarf will undergo many such eruptions, with the typical timescale between eruptions perhaps as long as 100,000 yr, although a subset of the nova population (called recurrent novae [RNe]) has recurrence timescales from 10 to 100 yr.

U Sco is one of the most famous recurrent novae. In quiescence it usually hovers around magnitude 17.6, but in outburst it briefly shoots up by about 8 or 9 magnitudes. In fact, U Sco is the all-time fastest nova known. Its brightening from minimum to maximum takes only about 5 hours, and its decline to two magnitudes below maximum takes about 38 hours. U Sco's last three eruptions, in 1979, 1987, and 1999, were all discovered by amateur astronomers.

One year ago, Bradley Schaefer predicted that the recurrent nova U Scorpii was due to explode again in April 2009 ± 1.0 year. Schaefer searched the old photographic sky-patrol plates stored at the Harvard College Observatory to discover three more eruptions of U Sco. With these, the pattern of eruption since 1900 has been once every 10 ± 2 years. (Eruptions were likely missed around 1927 and 1957 because Scorpius is hidden in conjunction with the Sun every November and December). Moreover, for his prediction, Schaefer used the system's brightness in quiescence (slightly variable) as a measure of the rate at which matter is falling onto the white dwarf.

Using a remote scope located in Australia, we have been able to image U Sco in outburst on Jan. 28.73:

Below you can see a comparison image showing the field of U Sco imaged by us on April 2009 when it was below the magnitude 17 and the same field imaged few hours ago on January 28, 2010:

On Vsnet mailing list T. Kato reports that Imamura-san (Okayam U. of Sci.) and Fujii-san have obtained spectra of U Sco on January 28.8. Kato comments that P Cyg-type components having a velocity of 4300 km/s for Halpha (Fujii). The lines of He I and N II were strong (also associated with P Cyg-type components). The FWHM of Halpha was 6100 km/s (Imamura).

(Credit: Fujii)

Within an hour of the discovery, U Sco had already be observed by two X-ray satellites (the Rossi X-Ray Timing Observatory and the INTEGRAL satellite). In the following weeks the recurrent nova will be observed by many ground-based telescopes and spaceborne observatories.

These observations will help to answer some questions about the suspected link between RNe and Type Ia SNe. Although it seems that for systems like RS Oph and U Sco, the mass of the White Dwarf (WD) is near the Chandrasekhar limit and increasing, there remain some fundamental questions. For example, what is the type of the WD (if ONe, then no SN explosion will occur); can the Hydrogen in systems with red giant secondaries really be “hidden” at the time of any SN outburst; is the population of (appropriate sub-type) RNe sufficient to explain the observed SN Ia rate? A combination of detailed observations of individual Galactic RNe, coupled with surveys of extragalactic novae, will help to answer these very important points. (Bode, 2009).

By Ernesto Guido & Giovanni Sostero

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