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Hotspot ages rising with distance point to four jet outbursts from one black hole
Astronomers led by IISER Pune's Pavan Vijay Khadekar report four pairs of radio hotspots, aged 4.5 to 20.5 million years, around one elliptical galaxy. Their preprint makes the case for the first known quadruple-double radio galaxy, a dated record of one black hole's restarts that wider surveys would need to turn into a rate.
The Scientist · Science desk
What happened
- The source, J023721.13-010528.5, sits in a massive elliptical galaxy at redshift 0.372 and was mapped with the upgraded GMRT, MeerKAT and the Very Large Array.
- A statistical symmetry test found the four-episode reading about 3 million times more likely than the idea that the inner structures are knots in one continuous jet.
- Successive hotspot pairs are tilted counterclockwise by -7, -16 and -24 degrees, a sign, the team says, that the black hole's spin axis is slowly wobbling.
- The whole source is S-shaped and roughly 3.9 million light-years across, the second-largest S-shaped radio source known.
- About 192 double-double radio galaxies were known before this, against seven known or candidate triple-doubles and no confirmed case of four episodes.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- capability A double-double yields one restart interval and this source yields three, enough to begin asking whether a single black hole's outbursts arrive at regular spacing.
- constraint One object cannot tighten estimates of how often supermassive black holes reawaken; a rate needs counts of multi-episode sources set against the number of galaxies searched.
- decision Teams hunting for more multi-episode systems have a place to start, since the researchers name giant S-shaped radio galaxies as possibly the best hunting ground.
The case rests on a prediction that could have failed. In FR II radio galaxies, jets end in bright hotspots where they hit surrounding gas, and each restart leaves a new pair closer to the core while older pairs sit farther out [19]. Knots strung along one continuously running jet should instead all show nearly the same age [7].
The team dated each pair two ways. A kinematic age came from the pair's distance to the core, under an assumed typical jet speed [6]. A spectral age came from how much the radio spectrum had steepened over time [6]. The spectral ages climb from the inner pairs to the outer ones, the order that sequential outbursts predict [9].
The symmetry test's odds set one specific reading, that the three inner pairs are genuine hotspots, against the knots-in-the-jet model [11]. A ratio of that kind says which of two named models fits the data better. It does not weigh explanations outside the pair. On that basis the team wrote in its preprint that the source "exhibits four distinct episodes of jet activity, i.e., a quadruple-double radio galaxy (QDRG)" [2][12].
The evidence is lopsided for a claim built on symmetry. Observation quality is noticeably poorer on the fainter southern side, S3 in particular [10]. Uncertainties there are larger, and the age trend is less clean than along the northern jet [10].
Taken at face value, the spectral ages put 16 million years between the oldest and youngest pairs [1]. Four episodes make three gaps, an average of about 5.3 million years each, though the gaps need not be equal [2]. The jet axis also turned in similar steps: the reported offsets of the older pairs grow by 9 degrees and then by 8 [3].
Among catalogued sources, double-doubles outnumber triple-doubles by about 27 to one [4]. I'd expect that ratio to reflect how hard multi-episode sources are to detect as well as how often black holes restart. Even in this source, the fainter southern pairs are the hardest to measure [10].
What to watch
- Whether the arXiv preprint passes peer review with the quadruple-double classification intact.
- Deeper imaging of the southern jet, which would show whether S3 fits the age-with-distance trend seen in the north.
- Whether the kinematic ages, which rest on an assumed jet speed, line up pair by pair with the spectral ages.