An Optically Discovered Outburst from XTE J1859+226

Abstract

Using the Zwicky Transient Facility, in 2021 February we identified the first known outburst of the black hole X-ray transient XTE J1859+226 since its discovery in 1999. The outburst was visible at X-ray, UV, and optical wavelengths for less than 20 days, substantially shorter than its full outburst of 320 days in 1999, and the observed peak luminosity was 2 orders of magnitude lower. Its peak bolometric luminosity was only 2 × 1035 erg s−1, implying an Eddington fraction of about 3 × 10−4. The source remained in the hard spectral state throughout the outburst. From optical spectroscopy measurements we estimate an outer disk radius of 1011 cm. The low observed X-ray luminosity is not sufficient to irradiate the entire disk, but we observe a surprising exponential decline in the X-ray light curve. These observations highlight the potential of optical and infrared synoptic surveys to discover low-luminosity activity from X-ray transients.

An Optical Search for New Outbursting Low Mass X-Ray Binaries

“…We assume the X-ray outburst parameters are representative of the optical outbursts. However, there is evidence that short optical outbursts can happen without a state change or full X-ray brightening, such as in misfired or failed outbursts (e.g., Alabarta et al 2021;Baglio et al 2022;Bellm et al 2023) and possibly the outburst of Swift J1943+0228 reported in this paper. As a result, our cut here may be too stringent, as systems may undergo optical outbursts more frequently, particularly as activity without the substantial X-ray brightening needed to trigger X-ray monitors.…”

“…Examples of lightcurves passing all cuts are given in Figure 5. After visual inspections and literature searches for lightcurves displaying characteristics of LMXB outbursting, we recovered three known LMXBs in the alert archives: XTE J1859+226 (ZTF21aagyzqr; Bellm 2021a; Bellm et al 2023), Swift J1911.2 +0036 (ZTF18accedau), and MAXI J1807+132 (ZTF19abvioim; Shidatsu et al 2019). One source from the live search, Swift J1911.2+0036, was also recovered in the archival search.…”

“…Because almost all XRB outbursts, even failed transition outbursts, are discovered due to X-ray brightening, untargeted searches at optical wavelengths of outbursts can provide a powerful complementary pathway to the discovery and characterization of LMXBs and their outbursts (e.g., Bellm et al 2023). Due to their increased sensitivity to low luminosity and earlier activity, optical searches will offer a unique window into a population of outbursts that are not well understood.…”

“…For identification of outbursts from known LMXBs in ZTF, we make use of a watchlist to notify us of any activity from positions of sources in the Ritter & Kolb (2003), BlackCAT (Corral-Santana et al 2016), and WATCHDOG (Tetarenko et al 2016) catalogs. This watchlist, implemented using the ANTARES broker (Matheson et al 2021), led to the discovery of outbursts from XTE J1859+226 (Bellm 2021a;Bellm et al 2023) and Swift J1357.2-0933 (Bellm 2021b).…”