A shredded star exposes a massive black hole about 3,500 light-years from its galaxy's center

Phys.org reports that the team led by Itai Sfaradi of the University of California ^[7] classifies the off-center flare as a tidal disruption on several observations at once. The host-subtracted optical spectrum shows a clear broad hydrogen line, which puts the event in the TDE-H class, where spectra show only hydrogen emission lines ^[14][16]. The off-nuclear position appears in ZTF alerts and in multiple high-resolution optical, UV and radio images ^[16]. Together with the blue colors and the sustained temperature ^[14], the team says, "we conclude that TDE 2023mfm is an off-nuclear TDE-H" ^[15].

The X-ray record is the untidy part. Missing X-rays at the start are common among tidal disruptions, but a nondetection at later times is less so, because about 40% of optically found TDEs become X-ray bright later ^[12]. The team calls the silence "not unprecedented", since some optically selected TDEs stay very faint for years ^[12].

On mass, the team used three estimation methods ^[17]. "This is similar to many of the known TDEs," they write ^[18]. The brightness and timing also resemble common TDEs, and the event closely matches TDE 2024tvd, another off-center flare, in peak brightness, duration and host galaxy size ^[13].

Carrying a black hole 1.08 kiloparsecs from the center takes some mechanism ^[10]. The source lists four routes: a recoil kick from gravitational waves, a slingshot ejection, orbital decay that stalls while galaxies merge, and massive black holes that stay paired or tripled for a long time ^[4]. The first candidate is a remnant of a small galaxy that collided with the host ^[19]. The host shows no sign of a recent major merger, though researchers say a minor one cannot be ruled out ^[19]. Stars totaling 10 million to 100 million solar masses are clumped at the black hole's position, yet the clump is too low in mass to match the black hole, and so the small galaxy must have lost a large share of its stars ^[20]. The second candidate is ejection from a triple system. "A different possible origin for this system is that the offset MBH was ejected from a triple MBH system due to gravitational wave recoil," the researchers explain ^[21].

Two similar flares are too few to estimate how common such black holes are. The discovery paper was posted to the arXiv preprint server on Sept. 28 ^[3]. In our view the identification is the firmer result. The origin has two candidates, and the Phys.org account does not say which one the team favors.