Professor of Mathematics

Paata Ivanisvili

pivanisv@uci.edu · x.com/PI010101

Department of Mathematics, University of California, Irvine

Paata Ivanisvili

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Galaxy hunt

Public sky surveys, including Gaia DR3, have caught fifty-seven stars, and one star-like spot on old photographic plates, behaving in ways that the published record does not already list, with one exception noted below. Seven stars faded or brightened without a clear cause, and the spot, recorded in 1955, is missing from all later images. Four brightened once and briefly, as a distant star does when the gravity of an object passing in front of it focuses its light. Eleven wobble on the sky as if pulled by an unseen companion. Thirty-five more were steady for years and then dimmed for months; for most of them the likeliest cause is a cloud of dust passing in front of the star. Stars like these can reveal dust where none is expected, a foreground star or dark object acting as a gravitational lens, or a heavy companion that gives off no light. The exception is candidate 58, whose dimming turned out to have been published already; it is kept with a note instead of an estimate. The other fifty-seven are unconfirmed candidates. The percentage beside each one is an estimate of the chance that it is a real, previously unrecorded case of the kind described, and none of these estimates has been checked by outside experts. On the light curves, down means fainter. Candidate 1, candidate 4 and most of the stars that dimmed for months also have difference images, and on those a blue spot means fainter.

The candidates are all described in a paper, A catalogue of 58 candidate stellar anomalies from Gaia and public time-domain surveys (PDF, 37 MB), which explains how they were found and sets out the evidence for each one, with light curves and images.

Unexplained changes in brightness

Possible new microlensing events

A microlensing event is an expected kind of event, not an anomaly. When a foreground star or a dark object passes almost exactly in front of a distant star, its gravity focuses the distant star’s light, which brightens and fades once, smoothly and by the same factor in every colour. For these four stars, the percentage measures only the chance that the event is real and has not been recorded before.

Possible hidden companions

For each of these eleven stars, Gaia measured a small orbit on the sky. For most of them the orbit implies a companion so heavy that an ordinary star of that mass should show in the light, so it could be a neutron star or a black hole. But a pair of faint stars, or an orbit that Gaia has measured wrongly, would explain the same numbers, and both are far more common, which is why every estimate in this list is low.

Stars that dimmed for months

These thirty-five stars were each steady for years, then lost between a few per cent and more than half of their light for months, and recovered. The Zwicky Transient Facility (ZTF), a survey telescope at Palomar Observatory in California, recorded every one of them while stars of similar colour and brightness nearby stayed steady. For most, a second ZTF detector or a second instrument, usually ASAS-SN, a worldwide network of small telescopes, saw the dimming too, so the dimmings themselves are certain or nearly so; for candidates 52, 54 and 58 that support is weak or missing. What remains uncertain is their cause and whether they are new. In most of them green light dimmed more than red, as it does behind dust, so the likeliest cause is a cloud of dust passing in front of the star; spotted stars and young stars with dusty discs can dim in similar ways. None has a spectrum taken during or after its dimming, which is the main reason no estimate is higher. Candidate 58’s dimming has already been published, so it carries no estimate.

All the pictures were made from public archives: the Legacy Surveys, Pan-STARRS1, the NOIRLab Astro Data Archive, DECaPS2, SkyMapper, ZTF, ASAS-SN, Gaia, WISE and NEOWISE, TESS, and the STScI Digitized Sky Survey.

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