Thar She Blows! U Scorpii Erupts As Predicted

Update, February 5:

As of this morning U Sco was down to about magnitude 12.9. It has declined an average of 0.6 magnitude per day — faster at first, a little less rapidly now. More changes in the fading rate are expected.

In the last few days the first X-ray emission was detected by NASA's Swift satellite. No one knows why novae wait until they've faded substantially before they turn on in X-rays.

Less than a day ago U Sco also started the "flickering" characteristic of a fading nova; it's brightening and fading by about 0.2 magnitude over the course of an hour. This indicates that the hot white dwarf star has again begun accreting matter from its normal-star companion, as was happening before the outburst; an accretion disk around the white dwarf has evidently been re-established.

Now that the debris shell is expanding and thinning, the normal star's total eclipses of the white dwarf and the accretion disk should also again be visible.

Update, February 1:

As of this morning North American time, four days after its eruption, U Sco had dropped to 11th magnitude, consistent with its very fast declines in previous outbursts.

Observations continue worldwide and from space. Super-soft X-ray emission is expected to show up soon. As the star fades further in coming days and weeks, CCD-equipped amateurs should be able to start recording the expected eclipses of the newly heated white dwarf star by its larger, less-hot companion. Unlike many novae and recurrent novae, the U Scorpii binary is (luckily for detailed studies) a totally-eclipsing pair.

No additional pre-discovery sightings were reported. The star's rapid rise to peak brightness was completely missed. Apparently the last pre-discovery measurement was the one by Barbara Harris 24 hours before her discovery of the outburst. At that time, the star was still simmering dimly at its normal minimum.

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Original story, January 28, 2010:

One year ago, Bradley Schaefer predicted that the recurrent nova U Scorpii was due to explode again. Now it's happened.

On Thursday morning, January 28th, amateur variable-star observers Barbara G. Harris and Shawn Dvorak in central Florida independently notified the American Association of Variable Star Observers (AAVSO) that U Sco had leapt to magnitude 7.9. Just 24 hours earlier, Harris had imaged it still in its quiescent state at 18th magnitude, about where it has been since its last eruption in 1999.

Schaefer made his prediction based on a thorough search through old photographic sky-patrol plates since 1900. He found three eruptions that had not been known previously and felt confident that he had a good handle on all of them. They come about every 10 ± 2 years, he found. So another seemed due.

A lot of astronomers, professional and amateur, have been waiting. Observatory schedules are now being preempted, and astronomy satellites are being repointed. Schaefer has been hoping that with his advance warning, this will be the more thoroughly studied nova explosion ever.

Where to look

U Sco is located about 9° north of Antares, currently moderately well up in the southeastern sky just before the first light of dawn. The farther south you are the better, but even from as far north as latitude 45° (Oregon, Montreal, Milan), U Sco is at least 20° above your horizon before morning twilight begins. To find when morning twilight begins at your site, you can put your location into our online almanac. (Make sure the Daylight Saving Time box is unchecked).

U Sco both rises and fades fast. In previous outbursts, it has dropped by 2 magnitude in the first day and a half.

Here's the AAVSO's U Sco Campaign page.

Here are recent AAVSO magnitude estimates.

This is from an announcement just put out by the AAVSO:

"The two astronomers, Dr. Barbara Harris, MD, of New Smyrna Beach, and Shawn Dvorak of Clermont, were active participants in a global research campaign to monitor activity of U Scorpii. Both Harris and Dvorak had been conducting long-term monitoring as part of a campaign run by the AAVSO. This campaign, organized by Dr. Bradley Schaefer (Louisiana State University), involved professional and amateur observers from around the world monitoring this star every night.

"Harris was first to detect the outburst shortly before 6 a.m. local time, with Dvorak's independent detection arriving shortly afterwards. The two near-simultaneous observations provided all the proof required to alert observers and observatories around the world and in space that U Sco's outburst had finally occurred. Within an hour, Schaefer set in motion the global network of observatories, and by the end of the morning, two X-ray satellites (the Rossi X-Ray Timing Observatory and the INTEGRAL satellite) had already made observations.

"Over the next several months, astronomers will be monitoring the progress of this outburst at nearly all wavelengths from radio waves to X-rays, using a number of ground-based telescopes and spaceborne observatories."

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