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 4: a hot star that brightened for five months

about 35%

Why this estimate. The brightening is real: ZTF recorded it on two of its detectors, flagged it 99 times and saw the star alone change while its neighbours stayed steady, and ASAS-SN, a separate network of small telescopes, saw it too. What it was is unclear. The added light was bluer than the star, it came with a 1.24-day cycle that NASA’s TESS space telescope also recorded, and the infrared hardly changed, which rules out a gravitational lens. A faint unseen star very close to it may be what flared.

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 brightening. 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 brightening, 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 bump upwards in 2022, shaded purple. Below them, a colour comparison and a folded curve with a regular rise and fall.
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 are ASAS-SN, and a note marks the years of Gaia, which publishes only averages. Purple marks the brightening. Middle: a closer view of 2020 to 2024, with the star against its comparison stars and ZTF’s alerts. Bottom left: the change in green against that in red. Bottom right: ZTF’s measurements folded on the 1.24-day cycle, strong during the brightening and absent in the quiet years. Up means brighter. The note under the graphs sums up the measurements.

What changed. The star is a hot white star of type A, about 7,700 K and roughly 300 million years old, some 2,400 parsecs away behind a thick layer of dust in Cepheus. ZTF saw it steady, within about 0.02 magnitudes, from 2018 to 2021. In June 2022 it began to brighten, reached 0.19 magnitudes brighter in green light and 0.13 in red light in September, about 19 per cent more light, and was back to normal by December. While it was bright, ZTF also caught it rising and falling every 1.24 days, by about a tenth of a magnitude in green light, which it does in no other year. TESS recorded the same cycle in September and October 2022, and a 1.52-day cycle in November, as the brightening faded.

Why it might matter. Stars of this kind do not usually brighten by a fifth for months. The light that was added is bluer than the star itself, as if it came from something hotter than about 10,000 K, and at 3.4 microns WISE saw almost no change in July 2022, when the star was 0.13 magnitudes brighter in red light. A gravitational lens would have brightened every colour by the same factor, so that is ruled out. A hot companion or a faint neighbour that flared, or a burst of activity on the star itself, could fit, but none is established. An unseen star within 0.4 arcseconds cannot be excluded, and if one is there it may be the source.

No record of the brightening 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 2196222627638729984, a hot A-type star
Location
RA 318.89815°, Dec +63.08011° (J2000), in Cepheus
Distance
about 2,400 parsecs (7,800 light-years)
Brightness
Gaia G = 16.4, 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 cause is unknown. ASAS-SN sees the star near its faint limit, so its support is noisy. Two faint Gaia neighbours lie close to the star, one of them only 1.4 arcseconds away, and an even closer unseen star cannot be excluded. 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 one between the yellow ticks. A faint neighbour (G = 19.9) is the dot just to its upper left, and a second (G = 20.6), only 1.4 arcseconds away, merges with the star at this scale. 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 red 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 brightening. Middle: an image taken during the brightening. Right: the difference between them, in which red means brighter; the star leaves a dark red 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 sectors in 2019, 2020, 2022 and 2024, one panel per sector. The 1.24-day cycle shows in the panels for September and October 2022, during the brightening, and a 1.52-day cycle in November 2022. 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.

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