Swift J1745-26, with a scale of the moon as it would appear in the field of view from Earth. This image is from September 18, 2012 when the source peaked in hard X-rays. Credit: NASA/Goddard Space Flight Center/S. Immler and H. Krimm
Back in mid-September, the Swift satellite was going about its multi-wavelength business of watching for bursts of bright gamma-ray, X-ray, ultraviolet, or optical events in the sky, when it detected a rising tide of high-energy X-rays from a source toward the center of our Milky Way galaxy. But this was different from any other burst the satellite had detected, and after observing the event for a few days, astronomers knew this had to be a rare X-ray nova. What it meant was that Swift had detected the presence of a previously unknown stellar-mass black hole.
“Bright X-ray novae are so rare that they’re essentially once-a-mission events and this is the first one Swift has seen,” said Neil Gehrels from Goddard Space Flight Center, the mission’s principal investigator. “This is really something we’ve been waiting for.”
The object was named Swift J1745-26 after the coordinates of its sky position, the nova is located a few degrees from the center of our galaxy toward the constellation Sagittarius. While astronomers do not know its precise distance, they think the object resides about 20,000 to 30,000 light-years away in the galaxy’s inner region.
An X-ray nova is a short-lived X-ray source that appears suddenly in the sky and dramatically increases in strength over a period of a few days and then decreases, fading out over a few months. Unlike a conventional nova, where the compact component is a white dwarf, an X-ray nova is caused by material – usually gas — falling onto a neutron star or a black hole.
The rapidly brightening source triggered Swift’s Burst Alert Telescope twice on the morning of Sept. 16, and once again the next day.
Ground-based observatories detected infrared and radio emissions, but thick clouds of obscuring dust have prevented astronomers from catching Swift J1745-26 in visible light.
The nova peaked in hard X-rays — energies above 10,000 electron volts, or several thousand times that of visible light — on Sept. 18, when it reached an intensity equivalent to that of the famous Crab Nebula, a supernova remnant that serves as a calibration target for high-energy observatories and is considered one of the brightest sources beyond the solar system at these energies.
Even as it dimmed at higher energies, the nova brightened in the lower-energy, or softer, emissions detected by Swift’s X-ray Telescope, a behavior typical of X-ray novae. By Wednesday, Swift J1745-26 was 30 times brighter in soft X-rays than when it was discovered and it continued to brighten.
“The pattern we’re seeing is observed in X-ray novae where the central object is a black hole. Once the X-rays fade away, we hope to measure its mass and confirm its black hole status,” said Boris Sbarufatti, an astrophysicist at Brera Observatory in Milan, Italy, who currently is working with other Swift team members at Penn State in University Park, Pa.
Here’s usually happens in events like this: The black hole is part of a binary system with a normal Sun-like star. A stream of material flows into an accretion disk around the black hole. Usually, the disk of gas spirals in steadily to the black hole, heats up and produces a steady X-ray glow. But sometimes, for reasons unknown, the material is held up in the outer regions, held back by some mechanism, almost like a dam. Once enough gas accumulates, the dam breaks and a flood of gas surges towards the black hole, creating the X-ray nova outburst.
“Each outburst clears out the inner disk, and with little or no matter falling toward the black hole, the system ceases to be a bright source of X-rays,” said John Cannizzo, a Goddard astrophysicist. “Decades later, after enough gas has accumulated in the outer disk, it switches again to its hot state and sends a deluge of gas toward the black hole, resulting in a new X-ray outburst.”
This phenomenon, called the thermal-viscous limit cycle, helps astronomers explain transient outbursts across a wide range of systems, from protoplanetary disks around young stars, to dwarf novae — where the central object is a white dwarf star — and even bright emission from supermassive black holes in the hearts of distant galaxies.
It is estimated that our galaxy must harbor some 100 million stellar-mass black holes. Most of these are invisible to us, and only about a dozen have been identified.
Swift discovers about 100 bursts per year. The Burst Alert Telescope detects GRBs and other events and accurately determines their positions on the sky. Swift then relays a 3 arcminute position estimate to the ground within 20 seconds of the initial detection, enabling ground-based observatories and other space observatories the chance to observe the event as well. The Swift spacecraft itself “swiftly” –in less than approximately 90 seconds — and autonomously repoints itself to bring the burst location within the field of view of the sensitive narrow-field X-ray and UV/optical telescopes to observe the afterglow and gather data.