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NICER / ISS Science Nugget
for November 28, 2024



When a star swamps a black hole

A flash of visible light detected in June 2022 from the heart of a relatively nearby, otherwise tranquil galaxy signaled the earliest moments of the destruction of a star that passed too close to a massive black hole. Some months later, the fading visible and ultraviolet emission gave way to X-rays, an indication that accretion of the star's remains onto the black hole had begun. NICER observations of this event, dubbed AT2022lri, were triggered in October 2022 and carry on to this day. Results from analysis of the first 485 days of data were recently published by Y. Yao (Univ. of California Berkeley) and collaborators in The Astrophysical Journal.

The detection of tidal disruption events (TDEs) has become relatively common, but they exhibit a rich diversity of observed properties, presumably reflecting different viewing geometries from our Earth-bound perspective as well as differences in the nature of the destroyed star, its approach trajectory, the mass and spin of the black hole, whether a preexisting accretion disk is present, and other factors. AT2022lri was quickly recognized for being unusually bright in low-energy X-rays - ideally suited to NICER - and because the earliest NICER observations showed variability on hour timescales, never before seen in a TDE. To fully probe this variability, and exploit the X-ray brightness with spectroscopic measurements, the NICER team scheduled intensive observations, on consecutive ISS orbits and for long durations where possible.

The X-ray variability of AT2022lri for the first 100 days of NICER's coverage is dominated by dips in brightness relative to an expected trend of gradual dimming. The dips are best characterized, in the analysis of Yao et al., as absorption of the accretion disk's emitted X-rays by intervening material, of rapidly varying density, along our line of sight. Together with the overall luminosity, the implication is that an overwhelming amount of matter is flowing onto the black hole, faster than the black hole can ingest it. This regime of accretion, called "super-Eddington," is rare for most black holes but may be more common for TDEs, where the rapid destruction of an entire star dumps substantially more material in a short time than, for example, the typical siphoning-off of small quantities of matter from a companion star in a binary system. The result in the super-Eddington scenario is a thick disk and a substantial outflow of matter driven away by intense radiation from regions closest to the black hole. With some of the most sensitive NICER spectra, Yao et al. discern both absorption and emission features, and describe two models that fit the data well, in which direct emission from the thick disk is seen, sometimes veiled and absorbed by the outflow, while some of that emission is also detected after it has interacted with the outflowing plasma, imparting "reflection" emission features. AT2022lri has thus offered a unique look at super-Eddington accretion from a novel vantage point - face-on to the disk - that probes both the inward and outward flow properties of accretion after the destruction of a star.


X-ray brightness evolution, beginning 187 days after the tidal disruption of a star in a nearby galaxy and spanning almost 500 days (horizontal axis at top), as measured with NICER (blue points) and the X-ray Telescope onboard NASA's Swift observatory (red points); the times of three observations with ESA's X-ray Multi-Mirror (XMM) Newton telescope are also indicated (yellow vertical lines). The upper panel shows, on a logarithmic scale, the overall exponential decay trend (yellow dashed curve); the data gap centered near 350 days post-disruption is a seasonal Sun-avoidance pointing restriction for all three missions. The lower panels zoom in successively on an early portion in which pronounced dips are seen and extensively sampled with NICER's frequent and long-duration coverage (lower-right panel, with the ISS orbital period indicated by dotted vertical lines). (Credit: Yao et al. 2024) The presence of both absorption and emission features in NICER spectroscopic data during dips in brightness suggests two possible scenarios, sketched here, for the early dipping behavior. In both cases, the extreme rate of mass inflow results in a puffed-up accretion disk and a strong outflow of ionized gas, which we observe from a vantage point far above the disk. The hottest parts of the disk flow, closest to the massive black hole, emit X-rays that we detect directly, some wavelengths of which are absorbed by the outgoing plasma. At other wavelengths, emission either from excited atoms within the plasma (left) or from photons originating in the disk but scattered by the outflow (right) is also detected. Both models successfully describe the data and may contribute simultaneously. (Credit: Yao et al. 2024)

X-ray brightness evolution, beginning 187 days after the tidal disruption of a star in a nearby galaxy and spanning almost 500 days (horizontal axis at top), as measured with NICER (blue points) and the X-ray Telescope onboard NASA's Swift observatory (red points); the times of three observations with ESA's X-ray Multi-Mirror (XMM) Newton telescope are also indicated (yellow vertical lines). The upper panel shows, on a logarithmic scale, the overall exponential decay trend (yellow dashed curve); the data gap centered near 350 days post-disruption is a seasonal Sun-avoidance pointing restriction for all three missions. The lower panels zoom in successively on an early portion in which pronounced dips are seen and extensively sampled with NICER's frequent and long-duration coverage (lower-right panel, with the ISS orbital period indicated by dotted vertical lines). (Credit: Yao et al. 2024) The presence of both absorption and emission features in NICER spectroscopic data during dips in brightness suggests two possible scenarios, sketched here, for the early dipping behavior. In both cases, the extreme rate of mass inflow results in a puffed-up accretion disk and a strong outflow of ionized gas, which we observe from a vantage point far above the disk. The hottest parts of the disk flow, closest to the massive black hole, emit X-rays that we detect directly, some wavelengths of which are absorbed by the outgoing plasma. At other wavelengths, emission either from excited atoms within the plasma (left) or from photons originating in the disk but scattered by the outflow (right) is also detected. Both models successfully describe the data and may contribute simultaneously. (Credit: Yao et al. 2024)



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