Professor of Mathematics

Paata Ivanisvili

pivanisv@uci.edu · x.com/PI010101

Department of Mathematics, University of California, Irvine

Paata Ivanisvili

← Galaxy hunt

Candidate 55: a subgiant that dimmed for two months

about 25%

Why this estimate. The dimming is real: ZTF recorded it on two of its detectors, and ASAS-SN, a separate network of small telescopes, saw it in three of its four cameras observing the star. Green light dimmed more than red, as behind dust, and NASA’s TESS space telescope never saw the star spinning. But the dip is shallow and short, ZTF flagged it only twice, and there is no infrared measurement during it.

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. On the TESS light curves the star is black and the comparison stars are coloured.

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. Bottom right: the star and its comparison stars on two separate ZTF fields, recorded by different detectors. Down means fainter. The note under the graphs sums up the measurements.

What changed. The star is a subgiant of about 5,300 K, some 2 billion years old, about 2,200 parsecs away in Draco. ZTF saw it steady from 2018 to mid-2024. From about 26 June 2024 it was 0.05 to 0.07 magnitudes fainter in green light and 0.03 to 0.05 in red, about 6 per cent less light, until mid-August; it recovered over the next two weeks and was normal from early September. ASAS-SN saw it 0.05 magnitudes faint in July and 0.04 in August 2024.

Why it might matter. Blue light dimmed about 1.4 to 1.8 times as much as red light, at or above ordinary interstellar dust (1.4) and more than spots on a star this cool would give (1.1 to 1.2). TESS watched the star in twenty sectors and found no rotation, so it does not look spotted. Alerts in 2025 and early 2026 suggest it may have dimmed again, but those are not confirmed.

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.

Star
Gaia DR3 1706330828131818240, a subgiant
Location
RA 260.88431°, Dec +78.84802° (J2000), in Draco
Distance
about 2,200 parsecs (7,200 light-years)
Brightness
Gaia G = 14.1, 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
The dip is shallow and short, ZTF flagged it only twice, and WISE did not observe during it. The later alerts that hint at a repeat are unconfirmed. 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 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.
Wide panels of dotted light curves, one per TESS sector. In each the star is the black row at the bottom, below the coloured rows of comparison stars.
TESS light curves from twenty sectors, from 2019 to 2024; the last was taken during the recovery. The comparison star with a clear 2.77-day cycle is a different variable star; this star shows no rotation in any panel. In each panel the star is in black at the bottom, below comparison stars on the same images, each divided by the others, averaged over two hours and offset for clarity.

· All candidates ·