Professor of Mathematics

Paata Ivanisvili

pivanisv@uci.edu · x.com/PI010101

Department of Mathematics, University of California, Irvine

Paata Ivanisvili

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Candidate 53: a red giant that dimmed for a year

about 25%

Why this estimate. The dimming is real: ZTF flagged it 13 times, and ASAS-SN, a separate network of small telescopes, saw the same dip in two of its cameras. Green light dimmed more than red and the infrared did not change, which argues against starspots. But the star is a giant and probably has a companion, and a machine classification calls it a spotted binary.

How to read the pictures. The finder chart shows a patch of sky two arcminutes across, about a fifteenth of the Moon’s width, with north up and east to the left. The star sits between the two yellow ticks, and the white bar is 30 arcseconds long. On a light curve the vertical axis is magnitude, a brightness scale on which larger numbers mean fainter stars, and it is drawn so that down means fainter. Purple shading marks the dimming. Each survey uses slightly different filters, so each is compared with its own normal level. The ZTF pictures show ZTF’s reference image of the star, an image taken during the dimming, and the difference between the two, in which only what changed remains: blue means fainter and red brighter.

Five graphs. In the upper four, of brightness against time, the points stay level apart from a dip, shaded purple. The fifth compares the change in green and red light.
Top three rows: brightness from 2010 to 2025 in green, red and infrared light, each against the star’s normal level, from Pan-STARRS1 (a survey telescope in Hawaii), ZTF, and WISE and NEOWISE (NASA’s infrared space telescope); gold diamonds and orange crosses are ASAS-SN, and a note marks the years of Gaia, the European Space Agency’s star-mapping satellite, which publishes only averages. Purple marks the dimming. Middle: a closer view, with the star against its comparison stars, ZTF’s alerts and ASAS-SN’s monthly medians. Bottom: the change in green against that in red, with lines for dimming equal in all colours, ordinary dust and fine dust; the solid line is this star’s. Down means fainter. The note under the graphs sums up the measurements.

What changed. The star is a red giant of about 4,800 K and about five times the Sun’s radius, 1,400 to 1,800 parsecs away in Andromeda. ZTF saw it steady from 2018 to 2021, about 0.01 magnitudes faint in green light during 2022, and then 0.06 to 0.08 magnitudes fainter in green and 0.03 to 0.05 in red from spring 2023 to early 2024, about 7 per cent less light. It was back to normal by mid-2024. ASAS-SN saw the same dip from June to December 2023.

Why it might matter. Blue light dimmed about 1.7 times as much as red light, more than ordinary interstellar dust (1.4) or starspots (about 1.1 for a star this cool). WISE observed the star in July and December 2023, deep in the dip, and saw no change at 3.4 microns, which excludes a solid body and is less than starspots would give. Gaia’s measurements of the star scatter more than those of a single star (its RUWE is 1.6), which suggests a companion, and dust in such a pair could pass in front of the giant.

No record of the dimming could be found. The star is not in SIMBAD, VSX (the variable-star index), Gaia’s variable-star lists or the other catalogues consulted, nor among the dimming stars of Tzanidakis et al. (2025), and there is no Transient Name Server entry, Gaia alert or paper giving its Gaia number. ZTF’s alert system holds only machine-generated classifications for it. One of them calls it a spotted binary of the RS Canum Venaticorum kind, with a probability of about one half.

Star
Gaia DR3 1943310596493970048, a red giant
Location
RA 353.89101°, Dec +48.43848° (J2000), in Andromeda
Distance
1,400 to 1,800 parsecs (4,600 to 5,900 light-years)
Brightness
Gaia G = 13.7, far too faint for the eye
Probability
This candidate is unconfirmed. The percentage under the title is an estimate of the chance that it is a real, previously unrecorded case of the kind described. The estimate has not been checked by outside experts. See the introduction to all fifty-eight candidates.
Caveat
Only one ZTF field covers the star. Giants with companions are often spotted and active, and a machine classification labels it a spotted binary of the RS Canum Venaticorum kind. There is no spectrum.
A patch of sky with a star at the centre between two yellow ticks.
Pan-STARRS1 colour image in the g, r and i filters, from 2010 to 2014, two arcminutes by two. The star is the bright orange one between the yellow ticks. North is up, east is left, and the white bar is 30 arcseconds.
Three square images. The star, between orange ticks, looks much the same in the left and middle ones and leaves a dark blue spot in the right.
Cutouts from one ZTF alert, one arcminute across; orange ticks mark the star. Left: the reference image, made in 2018 before the dimming. Middle: an image taken during the dimming. Right: the difference between them, in which blue means fainter; the star leaves a dark blue spot, while its neighbours subtract away.

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