Chandra detection of new outburst by XMMU J174445.5-295044

ATel #17903; Craig Heinke (U. Alberta), Farhad Yusef-Zadeh (Northwestern U)
on 17 Jul 2026; 16:58 UT
Credential Certification: Craig Heinke (cheinke@virginia.edu)

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The X-ray source XMMU J174445.5-295044 shows rapid variability, high and variable obscuration, and infrared spectra matching an M2 III giant nature (Bahramian et al. 2014, MNRAS, 441, 640). The accreting object is thought to be a neutron star, based on their inferred distance of 3.1+1.8-1.1 kpc, and a peak 10-25 keV flux seen by INTEGRAL of 1.5e-10 erg/cm^2/s (Atel #4000; see also Atels #12843,#13095,#15236), indicating an intrinsic Lx (2-10) >4e34 erg/s.

A Chandra observation on July 22, 2025 showed XMMU J174445.5-295044 (5.5' off-axis) with Fx(2-10 keV)=2+1.5-0.6 e-12 erg/cm^2/s, when fit by a power-law, with photon index 2.4+-1.2 and N_H=11+-5e22 cm^-2, consistent with its most common flux and spectrum.

New Chandra observations, also 5.5' off-axis, between June 29 and July 11, 2026 show it with a countrate 30 times higher (0.02 vs. averaging 0.6 ct/s). Accurate spectral modeling is complicated by the high count rate and large off-axis angle. Modeling the pileup in XSPEC with the Davis (2001) model, with psffrac=0.85 and nregions=3, we estimate the 2-10 keV flux at 3e-10 ergs/cm^2/s, though different pileup modeling choices permit a range of a factor of up to 3 in inferred flux. Assuming the 3.1 kpc distance, we infer Lx(2-10) between 5e34 and 4e35 erg/s. The spectrum is remarkably hard (photon index 0-0.4), with N_H again consistent with 9-11e22 cm^-2. Rapid variability occurs, by a factor of 2 in countrate within 5000 seconds.