EP240408a: SALT optical spectroscopy and NICER X-ray follow-up observations
ATel #16589; N. Rea, L. Galbany, F. Coti Zelati, A. Marino (ICE-CSIC, IEEC), D. Buckley, I. Monageng (SAAO), E. Kuulkers (ESA), H. Q. Cheng, J. Hu, C. Jin, H. Sun, W. Yuan, X. Wu, H. Zhou (NAOC, CAS), P. G. Jonker (RU, SRON), P. O'Brien (Univ. of Leicester), A. Rau (MPE), H. Feng, L. Tao (IHEP, CAS), on behalf of the Einstein Probe team and a larger collaboration
on 16 Apr 2024; 09:53 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Nanda Rea (rea@ice.csic.es)
Subjects: Optical, X-ray, Transient
The X-ray transient EP240408a (Hu et al. GCN #36053), discovered by the Einstein Probe (EP)-WXT on 2024 April 8 at 17:56:30 (UTC), was observed by Swift XRT (Hu et al. GCN #36053) and EP-FXT (Rea et al. Atel #16584) on April 10, revealing a fading X-ray counterpart. GROND observations (Rau et al. GCN #36059) pinpointed a single near-infrared source within the Swift XRT error circle with magnitudes J= 19.9(0.3) and H= 20.5(0.5), also detected in archival observations in the z and K bands.
To identify the nature of the near-infrared counterpart, we performed optical spectroscopy using the Robert Stobie Spectrograph (RSS; Burgh et al. 2003, SPIE, 4841, 1463) mounted on the 11m Southern African Large Telescope, as part of the SALT Large Science Program on Transients. Two spectra each with an exposure of 1100 s were acquired starting on April 11 at 22:29 UTC (3.19 days after the EP-WXT trigger), using the PG0700 grating and covering the wavelength range from 2850 to 6765AA. The spectrum was noisy and showed a single significant narrow emission line at 4752AA, and possibly a few other lines at a lower significance.
Furthermore, NICER has been monitoring the X-ray transient since April 10 at 13:35 (UTC), for a duration of over 4 days and a total on-source exposure time of ~25ks. The X-ray lightcurve shows a decay in the count rate from ~8 to ~0.8 cts/s, with at least 6 flares superposed and a possible plateau phase starting ~2.5 days after the discovery flare and lasting ~10^5 s. The overall decay is broadly compatible with a linear function with a slope of around -2e-05 counts/s^2. The NICER power spectrum does not show significant coherent signals up to a Fourier frequency of 2 kHz. However, red noise is present.
The nature of this transient still remains uncertain. However, the absence of prominent broad emission lines in the optical spectrum suggests that this transient is unlikely to be a Galactic accreting compact object in outburst.
Further optical and X-ray observations are strongly encouraged.
