Wednesday was another busy day at the American Astronomical Society (AAS) meeting. I ran into a few contributors to Astronomy magazine (it’s always great to meet the people I talk to on the phone) and my roommate from a summer research internship I did nearly a decade ago.
Wednesday was a big day for the Crab Nebula at the AAS meeting. NASA/ESA photo
OK, back to the science. At the first press conference of the day, scientists announced all sorts of crazy stuff happening within the Crab Nebula (aka M1). Actually, they focused on the Crab’s pulsar — a stellar remnant left over from the supernova explosion that created the beautiful nebula. This pulsar emits radiation over broad wavelengths, and astronomers had long considered the pulsar stable in X-rays. So much so that scientists would calibrate their X-ray detectors using the Crab pulsar. It turns out, however, that’s probably not a good idea.
Colleen Wilson of NASA’s Marshall Space Flight Center announced that her team has seen the Crab’s energy emission decrease by 7 percent since 2008. And this wasn’t just using one detector — they looked at data from the Fermi Gamma-ray Observatory, the Swift satellite, the Rossi X-ray Timing Explorer, and the INTEGRAL gamma-ray observatory. And this wasn’t the only oddity discovered about the Crab.
The Italian high-energy satellite AGILE detected a gamma-ray flare from the nebula in October 2007. After the observatory saw another flash in September 2010, it became apparent there was some sort of repetition to this signal. AGILE researchers informed the rest of the high-energy astronomy community, and Fermi confirmed the flare just a few days later. The scientists imaged the Crab Nebula’s central region with the Hubble Space Telescope and the Chandra X-ray Observatory to see if there were any sources that could have caused the flares. They aren’t sure if the flares originated from the star’s wind, a shock wave around the pulsar, one of the jets, or the gaseous wind structure around the pulsar. The team did see (in both Hubble and Chandra data) several knots of material within one of the pulsar’s jets (and very close to the object itself) that might be related to the gamma-ray signal. They expect another flare within the year, and they hope to have Chandra and Hubble monitor the pulsar when that happens. (You can read even more about the Crab Nebula in the March issue of Astronomy, on newsstands February 1.)
Later in the day, I attended a press conference about black holes. Karl Gebhardt of the University of Texas announced that his team had made the most precise measurement yet of the supermassive black hole within elliptical galaxy M87. This black hole tips the scales at 6.6 billion solar masses. (For comparison, the Milky Way’s central black hole is around 4 million solar masses — more than 1,000 times less massive). This measurement confirms that M87’s black hole is the most massive one in the nearby universe. (Distant quasars are likely closer to 10 billion solar masses, but astronomers can’t directly measure those yet.)
Gebhardt also mentioned that the black hole’s theoretical event horizon (the radius where nothing can escape from) might be 12 billion miles (20 billion kilometers) wide, which is some 3 times the size of Pluto’s orbit. So of the black holes presently known, the one in M87 offers the best chance for astronomers to actually image its event horizon. This is important because, as Gebhardt said, astronomers don’t know if black holes are actually “black holes.” There’s no proof yet of an event horizon. There’s been talk of virtually connecting many sub-millimeter telescopes across the globe in order to essentially create a detector with a diameter of Earth’s radius. This facility could provide the resolution to directly image the event horizon of M87’s supermassive black hole. And that would be awesome.
AAS meeting: January 11, 2011
AAS meeting: January 10, 2011
AAS meeting: January 9, 2011