EP-FXT observation of the Galactic transient EP260812c/EP J175003.0-324244/MAXI J1750-327 and detection of a low-frequency quasi-periodic oscillation signal

ATel #17969; G. L. Huang (IHEP), A. Li (BNU), C. Y. Wang (THU), W. D. Zhang (NAOC) on behalf of the Einstein Probe (EP) team
on 15 Aug 2026; 07:41 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

Tweet

The Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission performed a follow-up observation of the X-ray transient EP260812c/MAXI J1750-327 (ATel #17963, ATel #17964) on 2026-08-14 15:24:02 (UTC), with a net exposure of 6 ks. A bright uncatalogued X-ray source was detected within the WXT error circle at R.A. = 267.5123 and Dec. = -32.7123 (J2000). The source position has an uncertainty radius of 10 arcseconds (90% confidence level, including both statistical and systematic uncertainties). We therefore designate this source EP J175003.0-324244. Its position is approximately 149 arcseconds away from the previously detected X-ray source 1eRASS J175013.2-324130, indicating that EP260812c/EP J175003.0-324244/MAXI J1750-327 is a new X-ray transient.

The power spectrum density (PSD) of the EP-FXT lightcurve reveals a significant, narrow quasi-periodic oscillation (QPO) signal on top of the band-limited-noise continuum, with a frequency of approximately 0.45 Hz, a full width at half maximum (FWHM) of 0.08 Hz, and a fractional rms amplitude of about 7%. It's first harmonic is also significantly detected at a frequency of around 0.94 Hz, an FWHM of 0.23 Hz, and a fractional rms amplitude of around 4%. Based on the properties of the QPOs and the continuum PSD, we tentatively identify this feature as a type-C low-frequency QPO, implying that the source is likely a candidate black hole X-ray binary.

We fit the photon spectrum obtained with FXT-A in the partial-window mode. Due to the high flux of the source, the spectrum is affected by the pile-up effect. To mitigate this effect, while extracting the source spectrum we exclude the central part (within 120 arcseconds) of the PSF. We caution, however, that residual pile-up may still be present even after this exclusion. The energy spectrum can be fitted with an absorbed power-law model. The best-fitting parameters are a hydrogen column density of 1.04 (+/-0.02) e22 cm-2 and a photon index of 1.91 (+/-0.02). The observed 0.5-10.0 keV flux obtained from the spectral fit is 2.84 (+/-0.03) e-8 erg/s/cm^2.

Further follow-up observations are encouraged to identify the nature of this source.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

Einstein Probe detection of a likely new X-ray transient