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NICER / ISS Science Nugget
for January 25, 2024



Exploring the extraordinary eruptions of not-so-quiescent galaxies

In the hearts of many distant galaxies lurk monster black holes - with masses millions of times that of our Sun - that power some of the most energetic and luminous processes in the universe; these Active Galactic Nuclei (AGN) have been the subject of much research for decades. AGN are outnumbered, however, by galaxies that appear to be much tamer, quietly hosting their populations of stars and gas without the central "fireworks" of an actively accreting supermassive black hole (SMBH). In the last few years, a handful of these unremarkable galaxies has revealed their own surprises: while dim in X-rays and ultraviolet light most of the time, they briefly and periodically erupt in flashes of high-energy emission. With the discovery of the first so-called quasi-periodic eruptions (QPEs) in 2019, and three more since, the phenomenon has attracted intense interest observationally and in the development of theoretical models to explain the erupting behavior. The four known QPE sources exhibit eruption recurrence times measured in hours, with durations approximately 24% of the recurrence time. Another class of transient emission from otherwise quiescent galaxies is known as Repeating Nuclear Transients (RNTs) - these have recurrence times measured in hundreds (sometimes more than a thousand) days, and their properties differ from those of QPEs in a variety of ways, such as the rise vs. decay behavior of the individual flares and whether they are accompanied by optical/UV or radio emission. The two classes have remained quite distinct... until now.

A peer-reviewed paper recently published in the journal Nature Astronomy, led by Johns Hopkins Univ. graduate student M. Guolo, reports the discovery and careful study of a new transient source, Swift J0230+28, from the nucleus of a normal galaxy. Discovered serendipitously by NASA's Swift observatory while pursuing an unrelated supernova, the source was found to brighten by a factor of up to 100x approximately every 22 days, a recurrence period that falls squarely in the gap between the QPE and RNT phenomena. A NICER monitoring campaign was triggered to study the eruptions in detail. They last, on average, 4.5 days, and NICER's sensitivity enabled time-resolved spectroscopy of multiple eruptions. The team reports that Swift J0230's eruption properties - in terms of rise-and-fall behavior and the present/absence of optical and UV emission - are more similar to QPEs than to RNTs, but some differences remain. The currently prevailing, but still hotly debated, explanations for the two phenomena are that QPEs arise when a compact star (neutron star or stellar-mass black hole) interacts with the pre-existing weak accretion flow around an SMBH, while RNTs represent the partial destruction of a normal star passing too close to an SMBH ("tidal disruption events", or TDEs). Swift J0230 may thus be an extreme example of either phenomenon, or something entirely new. Continued monitoring, and further expanding the available sample of galaxies demonstrating these extraordinary behaviors, is the best way to piece together the puzzle and complete the true picture of "quiescent" galaxies.


Multi-wavelength light curves of Swift J0230 spanning June 2022 through February 2023 (MJD is Modified Julian Day). The X-ray behavior is captured in panels a and b by, respectively, the X-Ray Telescope onboard NASAs Swift observatory and NICER. Panel c shows brightness in two ultraviolet bands, also from Swift, while panel d shows the results of three observations with the Karl Jansky Very Large Array radio telescope in New Mexico. In a through c, circular points represent measurements while inverted triangles in all panels indicate firm upper limits (i.e., non-detections consistent with instrument noise); the filled diamond in d is the lone successful radio detection. The 22-day periodicity suggested by the XRT data is represented by the pink vertical bands, while the green dashed lines mark the times of the three radio observations. The NICER data provide unique spectroscopic information about the flares. (Figure credit: Guolo et al. 2024) NICER X-ray spectra of four well-sampled eruptions of Swift J0230. The upper row shows detailed light curves, with symbol colors indicating the rise (orange), peak (cyan), and decay (gold) parts of each eruption. With the same color-coding, the lower panel shows measured brightness (points) as a function of photon energy, best-fitting models of thermal emission (solid traces), and NICER's energy-dependent background (dotted traces). The general trend is of a slow, soft (dominated by low-energy photons) rise, and a fast, harder decay. (Figure credit: Guolo et al. 2024)

Left: Multi-wavelength "light curves" of Swift J0230 spanning June 2022 through February 2023 ("MJD" is Modified Julian Day). The X-ray behavior is captured in panels a and b by, respectively, the X-Ray Telescope onboard NASA's Swift observatory and NICER. Panel c shows brightness in two ultraviolet bands, also from Swift, while panel d shows the results of three observations with the Karl Jansky Very Large Array radio telescope in New Mexico. In a through c, circular points represent measurements while inverted triangles in all panels indicate firm upper limits (i.e., non-detections consistent with instrument noise); the filled diamond in d is the lone successful radio detection. The 22-day periodicity suggested by the XRT data is represented by the pink vertical bands, while the green dashed lines mark the times of the three radio observations. The NICER data provide unique spectroscopic information about the flares (Figure credit: Guolo et al. 2024). Right: NICER X-ray spectra of four well-sampled eruptions of Swift J0230. The upper row shows detailed light curves, with symbol colors indicating the rise (orange), peak (cyan), and decay (gold) parts of each eruption. With the same color-coding, the lower panel shows measured brightness (points) as a function of photon energy, best-fitting models of thermal emission (solid traces), and NICER's energy-dependent background (dotted traces). The general trend is of a slow, soft (dominated by low-energy photons) rise, and a fast, harder decay (Figure credit: Guolo et al. 2024).

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