Extragalactic X-ray Binary IC 10 X-2 is in X-ray Outburst

ATel #17471; Silas Laycock (UMass Lowell), Breanna Binder (Cal Poly), Andre-Nicholas Chene (NOIRLab), Rigel Cappallo (MIT), Dimitris Christodoulou (DePaul U.), Anthony DeVasto (UMass Lowell)
on 3 Nov 2025; 23:32 UT
Credential Certification: Silas Laycock (silaslaycock@gmail.com)

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The extragalactic high-mass X-ray binary IC 10 X-2 has been detected in X-ray outburst by Chandra on October 30, 2025. Follow-up observations are requested, specifically: (1) Visible-light spectroscopy to capture the Hα line, which may show a broad or double-peaked structure revealing whether the mass donor has a circumstellar disk or other outflow. (2) X-ray observations to measure or constrain the outburst duration and profile. (3) time-series imaging/photometry, especially in narrow-band Hα and mid-infrared.

IC 10 X-2 is a large-amplitude X-ray transient (f_max/f_min > 100) with properties similar to Supergiant Fast X-ray Transients (Laycock et al. 2024, ApJ 798, 64), Luminous Blue Variables (Kwan et al. 2018, ApJ 856, 38), and Be X-ray binaries. Recently, a 26.5-day photometric period was determined (Alnaqbi et al. 2025, ApJ 978, 170), strongly suggestive of orbitally modulated flaring in a Be-HMXB. The hard power-law X-ray spectrum is characteristic of neutron star accretion although pulsations have not been detected.

A 30 ks ACIS-S observation (ObsID 29793, 2025-10-29,18:53:47 UT to 2025-10-30, 03:17:31 UT), centered on the dwarf starburst galaxy IC 10, reveals an X-ray point source at RA=00:20:20.86, Dec=59:18:00.65, consistent with the location of IC 10 X-2. Aperture photometry on a 0.3 - 7 keV filtered image shows 34 net counts, for a rate of approximately 1 ct ks^-1. Assuming the same spectral model as Laycock et al. 2014, ApJ 789, 64 (N_H = 6 x 10²¹ cm ^-2, spectral index Γ= 0.3, distance D = 660 kpc), the inferred X-ray luminosity is 1.5 x 10³⁶ erg s^-1.

The optical counterpart's brightness (summarizing the above literature) is typically V, g = 20; R, r = 19; K = 15, with variations of ±1 mag at all wavelengths.

IC 10 X-2 has now been observed by Chandra 12 times and once by XMM-Newton. The current outburst is only the third positive detection (Prior outbursts were detected in 2003 and 2010), the other observations occurring during quiescence. The outburst duration is unknown, but would be several days if driven by the binary period (type I outburst) or longer if driven by a large stellar outflow (type II, giant outburst).

This rare X-ray outburst presents an opportunity to test the various hypotheses for the mass donor in this unusual X-ray binary. No previous spectroscopic data has been obtained during an outburst.