Black Hole Caught Burping a Rare X-ray Flare

A NASA satellite controlled by Penn State University has detected a brief, super-bright, high-energy flare — an X-ray nova — erupting from a star system 8,000 light-years away from Earth named V404 Cygni.
This system is in the constellation Cygnus and includes a black hole and a star just slightly smaller than the Sun. This black hole has been known to burp up an X-ray nova occasionally, but it had been slumbering since 1989 until the detection by NASA’s Swift Gamma-ray Burst Explorer on June 15, just before 2:32 p.m. EDT. About 10 minutes after the detection by Swift, the Japanese ” MAXI” experiment (Monitor of All-sky X-ray Image) on the International Space Station also picked up the flare.

“Relative to the lifetime of space observatories, these black-hole eruptions are quite rare,” said Neil Gehrels, Swift’s principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “So when we see one of them flare up, we try to throw everything we have at it, monitoring across the spectrum, from radio waves to gamma rays.”

The Swift satellite has the special capability to rapidly rotate in order to observe gamma-ray bursts (GRBs), which typically last less than a minute, as well as other kinds of high-energy eruptions, including X-ray novas like the one that Swift just detected. X-ray novas are bright, short-lived X-ray sources that reach peak intensity in a few days and then fade out over a period of weeks or months. The outburst occurs when gas abruptly rushes toward a neutron star or black hole. By studying the patterns of the X-rays produced, astronomers can determine the kind of object at the heart of the eruption.

At the heart of this eruption is the V404 Cygni system, which astronomers classify as a low-mass X-ray binary. In V404 Cygni, a star slightly smaller than the Sun orbits a black hole 10 times its mass in only 6.5 days. The close orbit and strong gravity of the black hole produce tidal forces that pull a stream of gas from its Sun-size companion star. The gas travels to a storage disk around the black hole and heats up to millions of degrees, producing a steady stream of X-rays as it falls inward. Flares erupt because this disk flips between two dramatically different conditions. In its cooler state, the gas resists inward flow and just collects in the outer part of the disk like water behind a dam — but inevitably, the build-up of gas overwhelms the dam, and a tsunami of hot bright gas rushes toward the black hole.

When Swift observes an eruption, it automatically determines the blast’s location, broadcasts the position to the astronomical community, and turns toward the site to investigate with its own sensitive telescopes. “Because Swift autonomously responds to sudden bursts of high-energy light, it also provides us with data on a wide range of short-lived events, such as X-ray flares from stars and other objects,” said John Nousek, Swift’s director of mission operations and a professor of astronomy and astrophysics at Penn State. Penn State controls the science and flight operations of the Swift observatory from the Mission Operations Center on the University Park campus.

Astronomers relish the opportunity to collect simultaneous data in a range of wavelengths on black-hole binaries, especially one as comparatively close to Earth as V404 Cygni. Swift carries three telescopes, two of which are led by Penn State astronomers. The lead scientist for Swift’s X-ray Telescope, which detected the flare is David Burrows, a Penn State professor of astronomy and astrophysics. The lead scientist for Swift’s Ultraviolet/Optical Telescope is Michael Siegel, a Penn State senior research scientist. “Right now, V404 Cygni shows exceptional variation at all wavelengths, offering us a rare chance to add to this unique data set,” said Eleonora Troja, a Swift team member at Goddard.

Ongoing or planned satellite observations of the outburst involve NASA’s Swift satellite, Chandra X-ray Observatory, and Fermi Gamma-ray Space Telescope; as well as Japan’s MAXI, the European Space Agency’s INTEGRAL satellite, and the Italian Space Agency’s AGILE gamma-ray mission. Ground-based facilities following the eruption include the 10.4-meter Gran Telescopio Canarias operated by Spain in the Canary Islands, the University of Leicester’s 0.5-meter telescope in Oadby, in the United Kingdom, the Nasu radio telescope at Waseda University in Japan, and amateur observatories.

V404 Cygni has flared many times since the eruption began, with activity ranging from minutes to hours. “It repeatedly becomes the brightest object in the X-ray sky — up to 50 times brighter than the Crab Nebula, which is normally one of the brightest sources,” said Erik Kuulkers, the INTEGRAL project scientist at ESA’s European Space Astronomy Center in Madrid. “It is definitely a ‘once in a professional lifetime’ opportunity.”

In a single week, flares from V404 Cygni generated more than 70 “triggers” of the Gamma-ray Burst Monitor (GBM) aboard Fermi. This is more than five times the number of triggers seen from all objects in the sky in a typical week. The GBM triggers when it detects a gamma-ray flare, then it sends numerous e-mails containing increasingly refined information about the event to scientists on duty.

Every time the GBM recovered from one trigger, V404 Cygni set it off again, resulting in a torrent of emails. The event prompted David Yu, a GBM scientist at the Max Planck Institute of Extraterrestrial Physics in Garching, Germany, to comment on social media: “Achievement Unlocked: Mailbox spammed by a black hole.”