A survey of ultra-compact high-state AM CVn binaries with ZTF and Gaia: New discoveries and observational constraints on Galactic space density
Abstract
Ultra-compact AM CVn binaries in the high-state of stable mass transfer (Ṁ≳10−9M⊙/yr) are expected to be among the loudest persistent sources for upcoming space-based GW observatories. These systems typically have orbital periods Porb≲30 minutes. We present a systematic search for high-state AM CVn binaries in the Milky Way by targeting a region within the Gaia color-magnitude diagram, combined with ZTF time-domain photometry. We focus on variable targets within that sample with 5-30 minute periods. Our survey discovered three new high-state AM CVn binaries (ZTF J1840-1742, ZTF J2007-0527 and ZTF J2111+3158) with orbital periods of 16.86, 18.63, and 16.71 minutes, and recovered three known systems. We obtain high-speed photometry and phase-resolved spectroscopy to confirm their nature. Their spectra show helium emission lines with no detectable hydrogen. In two targets, the lines are double-peaked, and Doppler tomograms confirm an accretion disk. We estimate mass accretion rates of ∼10−9M⊙/yr, and 3σ X-ray luminosity upper limits of ∼1033erg/s. Based on this sample, we infer a local space density of high-state AM CVn population in the Milky Way of ∼(1.0−2.7)×10−8pc−3 (for disk scale heights hz=300−200 pc), representing 2-5% of the total Galactic AM CVn density. Their birth rate, ∼(2.4−4.3)×10−4yr−1 (for hz=300−200 pc), is consistent with the total AM CVn birth rate, implying most systems entering the high-state phase survive and evolve to longer orbital periods. This local space density suggests LISA and TianQin will detect about 36% and 19% of the total Galactic population, respectively, during their nominal 4-yr missions (S/N≥5). The Vera C. Rubin Observatory's LSST will detect about 34-44% of these systems over its 10-yr survey (mr≲26.9 mag).
Versions
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BibTeX
@misc{galiullin2026survey,
title = {{A survey of ultra-compact high-state AM CVn binaries with ZTF and Gaia: New discoveries and observational constraints on Galactic space density}},
author = {Ilkham Galiullin and Antonio C. Rodriguez and Kareem El-Badry and Valery Suleimanov and Vladislav Dodon and Askar Sibgatullin and Warren R. Brown and Kevin Burdge and Jan van Roestel and Edo Berger and V. Ashley Villar and Ilaria Caiazzo},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.04179},
eprint = {2608.04179},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.04179},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A survey of ultra-compact high-state AM CVn binaries with ZTF and Gaia: New discoveries and observational constraints on Galactic space density
AU - Ilkham Galiullin
AU - Antonio C. Rodriguez
AU - Kareem El-Badry
AU - Valery Suleimanov
AU - Vladislav Dodon
AU - Askar Sibgatullin
AU - Warren R. Brown
AU - Kevin Burdge
AU - Jan van Roestel
AU - Edo Berger
AU - V. Ashley Villar
AU - Ilaria Caiazzo
PY - 2026
DA - 2026/08/04
PB - arXiv
DO - 10.48550/arxiv.2608.04179
UR - https://arxiv.org/abs/2608.04179
AB - Ultra-compact AM CVn binaries in the high-state of stable mass transfer (Ṁ≳10^−9M_⊙/yr) are expected to be among the loudest persistent sources for upcoming space-based GW observatories. These systems typically have orbital periods P_orb≲30 minutes. We present a systematic search for high-state AM CVn binaries in the Milky Way by targeting a region within the Gaia color-magnitude diagram, combined with ZTF time-domain photometry. We focus on variable targets within that sample with 5-30 minute periods. Our survey discovered three new high-state AM CVn binaries (ZTF J1840-1742, ZTF J2007-0527 and ZTF J2111+3158) with orbital periods of 16.86, 18.63, and 16.71 minutes, and recovered three known systems. We obtain high-speed photometry and phase-resolved spectroscopy to confirm their nature. Their spectra show helium emission lines with no detectable hydrogen. In two targets, the lines are double-peaked, and Doppler tomograms confirm an accretion disk. We estimate mass accretion rates of ∼10^−9M_⊙/yr, and 3σ X-ray luminosity upper limits of ∼10^33erg/s. Based on this sample, we infer a local space density of high-state AM CVn population in the Milky Way of ∼(1.0−2.7)×10^−8pc^−3 (for disk scale heights h_z=300−200 pc), representing 2-5% of the total Galactic AM CVn density. Their birth rate, ∼(2.4−4.3)×10^−4yr^−1 (for h_z=300−200 pc), is consistent with the total AM CVn birth rate, implying most systems entering the high-state phase survive and evolve to longer orbital periods. This local space density suggests LISA and TianQin will detect about 36% and 19% of the total Galactic population, respectively, during their nominal 4-yr missions (S/N≥5). The Vera C. Rubin Observatory's LSST will detect about 34-44% of these systems over its 10-yr survey (m_r≲26.9 mag).
ER -