Einstein Probe detects an X-ray flare in the globular cluster M62

ATel #17183; Y. L. Wang (NAO, CAS; ICE, CSIC-IEEC), T. Y. Lian (NAO, CAS), A. Li (BNU), C. C. Jin (NAO, CAS) on behalf of the Einstein Probe (EP) team:
on 15 May 2025; 07:25 UT
Credential Certification: Yilong Wang (wangyilong@nao.cas.cn)

Tweet

We report on the detection of an X-ray flare from the globular cluster M62 by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The source, designated EP J170112.6-300645, was first detected by WXT in a single pointing observation with an exposure time of 1.1 ks at 2025-05-09T14:40:13(UTC). The position of the flare is R.A. = 255.296 deg, Dec. = -30.144 deg, with an uncertainty of 2.8 arcmin. No source was hitherto detected in previous WXT observations at this position. The unabsorbed 0.5-4 keV flux is about 8.5e-11 erg/s/cm^2, almost two orders of magnitude higher than the quienscent upper limit of ~1.2e-12 erg/s/cm^2, which is estimated from a long-exposure WXT observation on May 8, 2024, assuming a power-law spectrum with a photon index of 2.0 and Galactic absorption with NH = 3.2e21 cm^-2. The source was detected by WXT for the second time at 2025-05-10T12:40:38(UTC) with a similar unabsorbed flux level of ~5.9e-11 ergs/s/cm^2.

A follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP was performed at 2025-05-12 18:52:43(UTC), about 42 hours after the second EP-WXT detection, with an exposure of 2362s. Within the WXT error circle, an X-ray source was detected at R.A. = 255.3026 deg, DEC = -30.1126 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic), coincident with the position of M62. Within the FXT error circle, there are several known X-ray sources, including CVs, qLMXB, BH, and MSP. Therefore, the possibility for this event of being a flare from a known X-ray source cannot be ruled out. We list below all the known X-ray sources in M62 which fall into the FXT error circle, as detected by Chandra (Oh et al., 2020).

-------------------------------------------------------------------------------------------------------

Source | R.A. (J2000) | Dec. (J2000) | Sep. (arcsec) | Identification

-------------------------------------------------------------------------------------------------------

s19 | 17:01:12.870 | -30:06:53.26 | 8.52 | CV

s21 | 17:01:13.220 | -30:06:50.38 | 9.22 | BH

s22 | 17:01:12.701 | -30:06:48.87 | 3.65 | MSP

s24 | 17:01:13.376 | -30:06:45.83 | 9.77 | CV

s25 | 17:01:12.848 | -30:06:45.44 | 2.91 | CV

s26 | 17:01:13.365 | -30:06:42.98 | 9.91 | -

s27 | 17:01:13.106 | -30:06:42.22 | 7.00 | qLMXB

s29 | 17:01:13.111 | -30:06:38.54 | 9.30 | CV

-------------------------------------------------------------------------------------------------------

The FXT spectrum in the 0.5-10 keV band can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 3.2e21 cm^-2 and a photon index of 2.77(-/+0.44). The unabsorbed 0.5-10 keV flux is 1.04 (-0.14, +0.16) e-12 erg/s/cm^2 (90% C. L.), which is over one order of magnitude lower than the WXT observations, demonstrating a rapid decay in the source' s X-ray emission.

We performed a target-of-opportunity observation with the Swift-XRT which started at 2025-05-12T15:20:24 (UTC), with an exposure time of 1.7 ks. No significant source was detected within the WXT position error. Assuming a power-law spectrum with a photon index of 2.0 and Galactic absorption with NH = 3.2e21 cm^-2, we estimate an unabsorbed flux upper limit of ~4e-13 ergs/s/cm^2 in the 0.3-10 keV band.

We have scheduled a second FXT observation to monitor this source.

Multi-wavelength follow-up observations are encouraged.

We thank the Neil Gehrels Swift Observatory team for making the ToO observation possible.

Reference: Oh K., Hui C. Y., Li K. L., Kong A. K. H., 2020, MNRAS, 498, 292