Archival Chandra Upper Limit on the Pre-outburst X-ray Emission of MAXI J1750-327 / EP J175003.0-324244

ATel #17978; Y. J. Yang, D. M. Russell, D. M. Bramich, D. Akl (NYU Abu Dhabi), K. Alabarta, M. C. Baglio (INAF-OAB), P. Saikia (Yale)
on 16 Aug 2026; 19:47 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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Following our identification of the optical counterpart of the black hole candidate X-ray binary MAXI J1750-327 / EP J175003.0-324244 (ATel #17971; see also ATels #17963, #17964, #17969, #17974 and #17977), we searched the archive of NASA's Chandra for previous X-ray coverage at the counterpart coordinates (RA = 17:50:03.038, Dec = -32:42:45.43). We identified a Chandra observation (ObsID 669) obtained on 2000 August 30, approximately 26 years prior to the current outburst. This observation therefore provides an independent constraint on the pre-outburst X-ray emission of the system. No significant Chandra source is detected within the 10" FXT error circle reported in ATel #17969.

The optical counterpart position lies far off-axis on the ACIS-S1 CCD. A Chandra PSF map indicates a 90% encircled-energy radius of approximately 29" at this position. We extracted 0.5-7 keV events using a 30" radius circular source aperture and a concentric background annulus with inner and outer radii of 35" and 80", respectively. The effective exposure was 6.72 ks, and no statistically significant source excess was detected.

Using the CIAO task "srcflux", we derive a 90% upper limit on the 0.5-7 keV source count rate of 7.93e-3 counts/s. An independent aperture-photometry calculation using the CIAO task "aprates", including a correction for the approximately 90% enclosed PSF fraction, gives a consistent upper limit of approximately 7.7e-3 counts/s.

Assuming a power-law spectrum with photon index Gamma = 2.0 and Galactic line-of-sight absorption nH = 4.35e21 cm^-2, WebPIMMS converts the "srcflux" count-rate limit to an unabsorbed 0.5-10 keV flux upper limit of 1.94e-13 erg/cm^2/s, corresponding to an absorbed flux upper limit of 1.43e-13 erg/cm^2/s, which is more than five orders of magnitude lower than the observed 0.5-10 keV outburst flux of 2.84e-8 erg/cm^2/s measured by EP-FXT (ATel #17969). This archival non-detection therefore provides a relatively deep constraint on the pre-outburst X-ray emission of MAXI J1750-327 approximately 26 years before its current activity.