X-ray observation of the cometary interloper C/2025 N1 (3I/ATLAS) by XRISM/Xtend
ATel #17523; D. Ishi, Y. Kanemaru, K. Fukushima, S. Ogawa, Y. Uchida, T. Yoneyama, R. Iizuka (JAXA/ISAS), Chris Baluta (NASA/GSFC), Y. Terada (Saitama U.), E. Costantini (SRON), M. Guainazzi (ESA/ESTEC), R. Kelley (NASA/GSFC), K. Matsushita (TUS), R. Petre, B. Williams (NASA/GSFC), H. Yamaguchi, S. Watanabe (JAXA/ISAS), M. Tashiro (Saitama U.)
on 3 Dec 2025; 08:13 UT
Credential Certification: Yoshiaki Kanemaru (kanemaru.yoshiaki@jaxa.jp)
The follow-up XRISM observation has been performed for the interstellar comet C/2025 N1, also known as 3I/ATLAS, discovered by the ATLAS survey telescope on July 01, 2025. This comet was followed by many instruments in various wavebands, such as NOT in optical, IRTF in infrared, and MeerKAT in radio (ATels #17263, #17283, and #17473) following the discovery. An unanticipated ToO observation of this interloper was requested, and it began at 23:20 UT on November 26, 2025, and ended at 20:38 UT on November 28, 2025, with a 60-ks exposure using the Director's Discretionary Time allocation for the XRISM observing cycle 2. In order to track the apparent motion along the sky plane of the comet, we conducted 14 slew maneuvers during the observation, keeping the comet near the on-axis position on the detector plane.
After applying noisy-pixel screening, instrument background subtraction, vignetting correction, point-source removal, and apparent-motion correction to the Xtend data, we constructed a comet-centered image in the 0.3–1.0 keV band, which reveals plausible extended X-ray emission over approximately 5 arcmin of radius. Because the X-ray Mirror Assembly (XMA) for Xtend has a half-power diameter of 1.4 arcmin, the emission appears broader than its point-spread function. It is important to note that the vignetting effect caused by the XMA intrinsically produces such a brightness distribution for a flat emission, whose profile has a broader radial distribution (FWHM ∼17–18 arcmin) than the extent of the diffuse emission. Nonetheless, it may be premature to conclude that this diffuse emission is attributable to the atmospheric coma associated with the comet at this time. Further analysis using fully processed data is required.
A preliminary spectral analysis on the Xtend data shows that the emission from the 5 arcmin region centered on the comet is globally reproduced by the empirical background model, comprising Milky Way Halo (MWH, kT = 0.2 keV), Local Hot Bubble (LHB, kT = 0.1 keV), Cosmic X-ray Background (CXB, Γ = 1.4), and neutral Oxygen line emission (OI, E = 0.525 keV). The MWH and CXB components are subject to the Galactic absorption with a column density of NH = 1.77 × 1020 cm-2. The emission measures and brightness for these components are consistent with previous studies, as EMMWH = 0.002 cm-6 pc, EMLHB = 0.014 cm-6 pc, and SCXB = 8.4 cm-2 s-1 keV-1 sr-1. However, this model fails to explain the soft band with Gaussian-like residual structures at around 0.4 and 0.6 keV. The energies suggest that these deficits likely correspond to the CVI, NVI, and OVII lines. If these excess components come from the comet, the most plausible origin is charge-exchange emission between comet volatiles and the solar wind.
Follow-up monitors are encouraged.
