Cygnus X-1 is the most famous high-mass X-ray binary. About 6,100 light years away in the constellation Cygnus, it’s recently been the site of three extremely bright X-ray flares. It was the first time the European Space Agency’s INTEGRAL spacecraft ever detected such extreme flares. It is the first time such flares have been observed for this system. The new results were posted on August 28 on the pre-print server arXiv.
Astrophysicists first found Cygnus X-1 in 1962. It includes a black hole that is actively sucking in matter from a companion OB star. The black hole and its stellar companion enter into a complex tug-of-war. This sizzle accounts for the black hole being one of the brightest persistent sources of X-rays in the sky. Astronomers have largely exhausted their imaginations on Cygnus X-1, having studied it intensively for decades. These most recent flares are yet another important step forward in our understanding of this cosmic giant.
Details of the Flares
Among other things, researchers observed three never-before-seen flares while Cygnus X-1 was in its soft-intermediate spectral state. In the first phase, it’s the riverbed’s turn to harden. Required exposure time for each flare is roughly 400 seconds. Their peak luminosities were between 110 and 260 undecillion erg/s, within the 1–100 keV gallant attention.
The intensity profiles of these flares are quite complex, showing variations in rise and decay times. The first and third flare were both fast-rising, short-decaying events. The second flare slowly ascended over time before quickly decreasing. This variability is a sign that something has changed in the physical processes taking place around the black hole.
In each of the three cases, the normalized root-mean-square (rms) variability increased much considerably. This implies that Cygnus X-1 is currently showing activity on levels never seen before. The researchers then determined that the three flares exhibited some super weird behavior. So far, such a phenomenon has never been observed during the 21-year long monitoring of Cygnus X-1 with INTEGRAL nor in the earlier RXTE observations from 1997 to 2012.
Historical Context
The production of these new flares evokes a previous episode detected in April 2005 by RXTE. That last event didn’t last much longer. In addition, it was characterized by a 15-30 MHz radio flare with a delay of about 400 seconds relative to the timing of the X-ray flare. This history helps to highlight just how exceptional the recently observed flaring activity is.
The identification of such extreme transients illustrates just how unpredictable things can be even from objects that are quite well understood. Even for sources we think might be “well-known”, researchers stressed the importance of continuing to monitor them. Environmental monitoring & ERT’s vigilance is what allows us to catch these dramatic and rare events in these systems.
Implications for Future Research
The recent flares from Cygnus X-1 are unusually bright and distinct. All of this points to the desperate need for long-term monitoring of high-mass X-ray binaries. Scientists are still diving into this fascinating system. If they’re lucky, they’ll soon learn new truths about how black holes operate and interact with companion stars.
Moreover, the results suggest a rethinking of what we know around well-established, well-studied X-ray sources. With each technological leap, astronomers have a more powerful observational toolbox. This greatly increases their potential for finding surprising phenomena on well-known targets.