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NICER / ISS Science Nugget
for September 12, 2024



The TDE-QPE Connection

Observations across the electromagnetic spectrum are uncovering new phenomena around black holes. Among them are tidal disruption events (TDEs), transient celestial flares that occur when a star wanders too close to a supermassive black hole (SMBH) in the center of a galaxy and is ripped apart by gravity. Appearing dim because they typically originate in very distant galaxies, TDEs become luminous in visible and ultraviolet light for several months as the SMBH accretes the remains of the star. Dozens of TDEs have been detected, with star disruptions estimated to happen roughly once every 10,000 to 100,000 years per galaxy. In just the last few years, another new mysterious phenomenon has been discovered: quasi-periodic eruptions (QPEs). These are very energetic bursts of X-rays from the centers of galaxies, repeating roughly (but not exactly) periodically. Only a handful of QPEs are known, but their average recurrence time can be anywhere from a few hours to up to a month. Currently, a leading theory is that they occur because of an orbiting body, such as a star or a smaller black hole, that collides with an accretion disk of gas circling the central SMBH. Alternatively, the eruptions could be caused by unstable gas in this disk, intermittently heating up to very high temperatures. It has been theorized that QPEs and TDEs could be related: TDEs could create the accretion disks that later emit QPEs. To confirm this picture, astronomers have been searching for an unambiguous TDE - with spectroscopic and multi-wavelength confirmation - that is followed by multiple detections of repeating QPEs.

In late 2023, NASA's Chandra X-ray Observatory detected a temporary order-of-magnitude increase in X-rays from AT2019qiz. This was a well-known TDE: because of its close proximity (at discovery, it was the closest known TDE at about 200 million light-years away) it was studied across the entire EM spectrum. After the detection of the highly variable X-ray brightness with Chandra, NICER performed high-cadence observations - nearly once every ISS orbit - over the course of 10 days in Feb-March 2024. Six X-ray bursts were detected, repeating roughly every 48 hours on average. These were luminous in soft X-rays, with a thermal spectrum and a consistent light curve shape. These properties are the hallmarks of QPEs. NASA's Swift satellite and India's AstroSat detected two further eruptions. These results - reported in a manuscript by M. Nicholl (Queen's Univ., Belfast) and collaborators accepted for publication in the prestigious journal Nature - prove that QPEs can occur in the accretion disks left behind by TDEs, a breakthrough in understanding the previously mysterious origin of QPEs.

The galaxy was also observed in the UV using Hubble, contemporaneously with Chandra. Using the UV measurement and assuming the luminosity comes from a spreading disk left behind by the TDE four years earlier, the study authors determined the size of this disk at the time when the QPEs were seen. The model predicts that the disk is now wide enough that if there is another object (such as a star or stellar-mass black hole) orbiting theSMBH with a period of 48 hours, it would have to collide with the spreading disk. In this case the QPEs would only turn on a few years after the TDE, once the disk has become large enough. This discovery has further exciting implications, as it suggests that we could search for QPEs in more TDEs in order to measure the prevalence and distances of objects on close orbits around SMBHs, some of which may soon be detectable by the ESA-NASA Laser Interferometer Space Antenna (LISA) gravitational-wave astrophysics mission, now in development.


Discovery of quasi-periodic X-ray eruptions following the 2019 disruption of a star by a supermassive black hole in the core of a nearby galaxy. a) Three snapshot images obtained by NASA's Chandra X-ray telescope in December 2023, in which a single brightening and dimming episode was seen (panels left to right) at the location of the previously known optical/ultraviolet transient AT2019qiz (adjacent to an unrelated X-ray emitter to the southeast). b) X-ray brightness light curve during March 2024, in photon detections per second as measured initially by NICER (blue dots), demonstrates a repeating pattern of eruptions. When the target became inaccessible to NICER after several days, NASA's Swift observatory's X-ray Telescope (XRT) and India's AstroSat telescope were tasked with following up, and one additional eruption each (note that the XRT and AstroSat count rates are inflated by a factor of 25 to account for their smaller collecting areas relative to NICER). c) The original Chandra flare (open circles) overlayed on three of the NICER detections (other symbols) shows consistency of the fast-rise, slow-decay profile of the long-lived eruption phenomenon across several months. Only NICER provides data quality sufficient for spectroscopy at every stage of this evolution. (Credit: Nicholl et al. 2024)

Discovery of quasi-periodic X-ray eruptions following the 2019 disruption of a star by a supermassive black hole in the core of a nearby galaxy. a) Three snapshot images obtained by NASA's Chandra X-ray telescope in December 2023, in which a single brightening and dimming episode was seen (panels left to right) at the location of the previously known optical/ultraviolet transient AT2019qiz (adjacent to an unrelated X-ray emitter to the southeast). b) X-ray brightness "light curve" during March 2024, in photon detections per second as measured initially by NICER (blue dots), demonstrates a repeating pattern of eruptions. When the target became inaccessible to NICER after several days, NASA's Swift observatory's X-ray Telescope (XRT) and India's AstroSat telescope were tasked with following up, and one additional eruption each (note that the XRT and AstroSat count rates are inflated by a factor of 25 to account for their smaller collecting areas relative to NICER). c) The original Chandra flare (open circles) overlayed on three of the NICER detections (other symbols) shows consistency of the fast-rise, slow-decay profile of the long-lived eruption phenomenon across several months. Only NICER provides data quality sufficient for spectroscopy at every stage of this evolution. (Credit: Nicholl et al. 2024)



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