MAXI J2101-802: likely a giant X-ray flare as observed with EP-FXT follow-up
ATel #17188; W. Chen (NAO, CAS), Y. F. Liang (PMO, CAS), C. Y. Dai (NJU), Y. Liu (NAO, CAS) on behalf of the Einstein Probe (EP) team:
on 16 May 2025; 15:39 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
Subjects: X-ray, Transient
Following the detection of the MAXI J2101-802 (Negoro et al., ATel #17186), we performed an observation with the Follow-up X-ray Telescope (FXT) on board Einstein Probe.
The observations began at 2025-05-16T06:02:06 (UTC), about 5 hours after the MAXI/GSC detection. The exposure time is 2368 seconds. Preliminary analysis shows there are three sources detected within the nominal MAXI error circle:
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| Source Name | RA | DEC | Flux (erg/s/cm2) | Flux_err | eROSITA flux
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|EPF_J205754.1-800706| 314.4754 | -80.1185 | 1.15e-13 | 4.68e-14 | 2.30e-14
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|EPF_J210600.6-800347| 316.5023 | -80.0630 | 3.54e-13 | 8.35e-14 | 3.86e-14
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|EPF_J205602.4-795828| 314.0100 | -79.9746 | 2.26e-13 | 8.19e-14 | < 4.03e-13
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For these 3 sources, no significant variations in fluxes were found compared to previous measurements. The FXT flux is measured in the 0.5-10 keV band. The positional uncertainty of FXT is 10 arcsec (radius, 90% confidence level including both statistical and systematic errors).
Interestingly, we notice that a RS CVn Variable identified as CF Oct (RA=312.4076, Dec=-80.1332; distance=224 pc)Â was detected by FXT approximately 30 arcminutes away from the MAXI position, with a flux of 1.82(+/-0.05) x 10^(-10) erg/s/cm2, corresponding to a luminosity of 1.1(+/-0.03) x 10^(33) erg/s, which is about an order of magnitude higher than its historical fluxes. We consider that the MAXI detected source is likely an X-ray flare from this object, though the separation is slightly larger than the nominal error circle given by MAXI.
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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.
