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TCP J07222683$+$6220548: a new AM CVn type system with infrequent outbursts

Alexander Tarasenkov, Kirill Sokolovsky, Alexandr Dodin, Oxana Chernyshenko, Stanislav Korotkiy, Ivan Strakhov, Marina Burlak, Sergey Naroenkov, Franz-Josef Hambsch, Tamás Tordai, Hiroshi Itoh, Yasuo Sano, Yusuke Tampo, Ferdinand

TL;DR

TCP J07222683$+$6220548 is identified as a new AM CVn ultracompact binary following a bright $ m \\Delta V = 7.6$ mag outburst in 2025; spectroscopy reveals helium-dominated features (broad He I absorption and He II 4686 emission) with no Balmer lines, confirming the AM CVn nature. Time-series photometry detects a short, tentative period during the main outburst ($P \approx 0.016209$ d) and a robust positive superhump signal during the rebrightening ($P = 0.032546 \pm 0.000084$ d, 46.87 min), indicating a tidally driven, precessing accretion disk. The outburst morphology—rapid rise, long plateau, and multiple rebrightenings—along with the superhump behavior, places J0722 among AM CVn systems and differentiates it from hydrogen-rich dwarf novae; Gaia-based distance is $575^{+304}_{-153}$ pc with modest extinction. This study demonstrates the importance of dense, multi-wavelength follow-up to correctly classify outbursting AM CVn systems and suggests that some similar events may be misidentified without such data.

Abstract

We present the discovery of TCP J07222683$+$6220548, a new ultracompact binary system of the AM CVn type. This system was first identified displaying a $ΔV = 7.6$ mag outburst on 2025-01-20.9416 UTC by the New Milky Way wide-field survey for transients and later independently detected by ASAS-SN and ZTF. The outburst peaked at $V_{\rm max} = 12.45$ and lasted for seven days, followed by a series of rebrightenings. No previous outbursts are found in archival data. Positive superhumps with a period of $0.032546 \pm 0.000084$ d ($46.87 \pm 0.12$ min), barely detectable during the main outburst, became clearly visible during the first rebrightening that lasted from day 18 to day 24 after the initial outburst. No convincing change in the superhump period was detected. Dense time-series photometry follow-up by a pair of 0.5-m INASAN robotic telescopes, together with VSNET and AAVSO observers, was essential for identifying TCP J07222683$+$6220548 as an AM CVn system and triggering confirmation spectroscopy with the 2.5-m CMO SAI telescope. Some outbursting AM CVn systems lacking such detailed follow-up may remain unrecognized among the newly discovered cataclysmic variable candidates.

TCP J07222683$+$6220548: a new AM CVn type system with infrequent outbursts

TL;DR

TCP J072226836220548 is identified as a new AM CVn ultracompact binary following a bright mag outburst in 2025; spectroscopy reveals helium-dominated features (broad He I absorption and He II 4686 emission) with no Balmer lines, confirming the AM CVn nature. Time-series photometry detects a short, tentative period during the main outburst ( d) and a robust positive superhump signal during the rebrightening ( d, 46.87 min), indicating a tidally driven, precessing accretion disk. The outburst morphology—rapid rise, long plateau, and multiple rebrightenings—along with the superhump behavior, places J0722 among AM CVn systems and differentiates it from hydrogen-rich dwarf novae; Gaia-based distance is pc with modest extinction. This study demonstrates the importance of dense, multi-wavelength follow-up to correctly classify outbursting AM CVn systems and suggests that some similar events may be misidentified without such data.

Abstract

We present the discovery of TCP J072226836220548, a new ultracompact binary system of the AM CVn type. This system was first identified displaying a mag outburst on 2025-01-20.9416 UTC by the New Milky Way wide-field survey for transients and later independently detected by ASAS-SN and ZTF. The outburst peaked at and lasted for seven days, followed by a series of rebrightenings. No previous outbursts are found in archival data. Positive superhumps with a period of d ( min), barely detectable during the main outburst, became clearly visible during the first rebrightening that lasted from day 18 to day 24 after the initial outburst. No convincing change in the superhump period was detected. Dense time-series photometry follow-up by a pair of 0.5-m INASAN robotic telescopes, together with VSNET and AAVSO observers, was essential for identifying TCP J072226836220548 as an AM CVn system and triggering confirmation spectroscopy with the 2.5-m CMO SAI telescope. Some outbursting AM CVn systems lacking such detailed follow-up may remain unrecognized among the newly discovered cataclysmic variable candidates.

Paper Structure

This paper contains 12 sections, 4 equations, 5 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations and analysis
  3. NMW survey discovery
  4. Gaia counterpart, distance and extinction
  5. Spectroscopy with the 2.5-m CMO SAI telescope
  6. Photometry
  7. Results and discussion
  8. Overall shape of the lightcurve
  9. Periodic modulation
  10. Recognizing J0722 as AM CVn
  11. Comparison with other AM CVn binaries
  12. Conclusions

Figures (5)

  • Figure 1: The lightcurve of J0722 combining $V$-band photometry from INASAN Terskol and Kislovodsk telescopes (Section \ref{['sec:photometry']}) with $V$-band and unfiltered $CV$-band measurements collected by VSNET and AAVSO observers (including $CV$ photometry from the two NMW cameras), ASAS-SN $g$, ATLAS $o$ (orange) and $c$ (cyan), and ZTF $g$ and $r$ data. The top panel shows the complete combined lightcurve with the vertical dashed lines indicating the outburst plotted in the middle panel. The vertical dashed lines in the middle panel indicate time range of bottom plot that presents an example section of the lightcurve where superhumps are visible (Section \ref{['sec:period']}). The purple curve approximates the superhump shape as a sum of two sine waves, one with a period from eqn. (\ref{['eq:period']}) and the peak-to-middle amplitude of 0.025 mag and the other with twice that period and half the amplitude (c.f. Fig. \ref{['fig:phasedlcbest']}), it is plotted to guide the eye. The outburst plot (middle panel)
  • Figure 2: The spectra of J0722 obtained with the 2.5-m SAI MSU telescope during the rebrightening. The green and dark blue marks show the location of prominent He I and He II lines, respectively. The red marks indicate the expected location of Balmer lines.
  • Figure 3: Power spectra with their corresponding spectral windows and phased lightcurves of J0722 during the main outburst (top panels; INASAN-Terskol; $V$ filter), the rise to the rebrightening (middle panels; VSNET Hiroshi Itoh; Clear band) and during rebrightening (bottom panels). All lightcurves are phased with the light elements ${\rm HJD(TT)} = 2460718.3052 + 0.032516 \times {\rm E}$ corresponding to the highest peak in the rebrightening power spectrum (bottom left panel; INASAN-Kislovodsk; $V$ filter).
  • Figure 4: Power spectrum with its corresponding spectral window (left panel) and phased lightcurve of J0722 (right panel) during the rebrightening constructed by combining INASAN-Kislovodsk $BVRI$ and VSNET Clear band data. This dataset combines the data presented in the middle and lower panels of Figure \ref{['fig:phasedlc']} with additional Clear band photometry. The lightcurve (right panel) is phased with the light elements ${\rm HJD(TT)} = 2460721.0757 + 0.032546 \times {\rm E}$ corresponding to the highest peak in the rebrightening power spectrum (left panel).
  • Figure 5: Outburst duration as a function of the orbital period for AM CVn stars from 2019AJ....157..130C2021MNRAS.508.3275P2022ApJ...926...10R2024MNRAS.532.4205P. For systems where 2019AJ....157..130C and 2021MNRAS.508.3275P give different outburst durations (KL Dra, CP Eri) both estimates are plotted as they are presumably based on different outbursts. Open triangles represent upper limits on the outburst duration. The top blue curve represents the empirical relation of 2015MNRAS.446..391L. The bottom orange curve is the accretion disk limit cycle model of 2015ApJ...803...19C2019AJ....157..130C.