EP J182730.0-095633(EP240904a): recent follow-up observations by EP-FXT and NuSTAR

ATel #16825; H. Q. Cheng (NAOC, CAS), A. Marino, F. Coti Zelati, N. Rea (ICE-CSIC, IEEC), L. Tao, H. Feng (IHEP, CAS), H. W. Pan (NAOC, CAS), X. F. Zhao, Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, J. Zhang, S. N. Zhang (IHEP, CAS), Y. Liu, C. Zhang, Z. X. Ling, C. C. Jin, W. Yuan (NAOC, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the Einstein Probe team
on 21 Sep 2024; 01:49 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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After its discovery (Atel #16805), EP J182730.0-095633 has been monitored daily by the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) from September 6th to 17th, 2024. Preliminary analysis shows that the EP-FXT spectra can be well fitted with an absorbed power-law model, with the best-fit photon index in the range of ~2-2.5. The observed flux within 0.5-10 keV evolved from ~1e-10 erg/s/cm^2 on September 6th to ~8e-12 erg/s/cm^2 on September 17th, showing a decreasing trend. No type-I X-ray bursts were detected.

We perform a target-of-opportunity (ToO) observation with the Nuclear Spectroscopic Telescope Array Mission (NuSTAR) for a total of 27 ks on September 11th. No type-I X-ray bursts were detected by visual inspection of the light curve. The NuSTAR spectrum is background-dominated above 30 keV. The 4-30 keV average spectrum can also be fitted with an absorbed power-law model, with the best-fit column density of 2.7(+/-1.3)e22 cm^-2 and photon index of 1.88(+/-0.03). The observed flux within the 0.5-10 keV band is 6.8(-0.4/+0.5)e-11 ergs/s/cm^2. All uncertainties are quoted at 90 per cent confidence level. We conducted preliminary searches for periodic signals over the frequency range 1-1000 Hz using the PRESTO (Ransom et al. 2002, AJ, 124, 1788) and Stingray (Huppenkothen et al. 2019, ApJ, 881, 39) software packages, without finding any plausible candidate. The power-density spectrum is dominated by band-limited noise below a few Hz, and displays a QPO-like feature at a frequency of around 0.04 Hz.

The X-ray spectral and timing properties are compatible with EP J182730.0-095633 being a new transient low-mass X-ray binary in outburst. This is supported by the radio flat spectrum detected in ATCA observations (Atel #16817), likely indicative of the presence of a compact jet.

EP-FXT will continue to monitor the source in the next few days.

We thank the NuSTAR PI, Fiona Harrison, for approving the DDT request, and the NuSTAR SOC for carrying out the observation. 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).