AstroNote 2026-275

SN 2026sqf was discovered on 2026-07-08 in NGC 3310 (Wiggins 2026, TNS Astronomical Transient Report No. 313386) and classified as a Type Ia-CSM supernova (Midavaine AstroNote 2026-230, and Rehemtulla AstroNote 2026-240). At a distance of ~19 Mpc it is the closest SN Ia-CSM to date, and the first of its class with a candidate progenitor system detected in pre-explosion imaging (Szalai et al. 2026, arXiv:2608.13321). Here we report on our Chandra X-ray observations of SN2026sqf acquired starting on 2026-09-19, at an epoch of ~2 months post explosion under a DDT proposal (PI D. Milisavljevic). In the first ~10ks of ACIS-S data we find clear evidence of an X-ray source at the location of the optical transient with high statistical significance (~10 sigma, Gaussian equivalent). The 0.5-10 keV net count-rate is ~2e-3 c/s. The observed spectrum can be fitted with a rising spectral slope Fnu~nu indicative of large absorption in significant excess to the Milky Way contribution along the line of sight (which is ~1.5e+20 cm-2; HI4PI Collaboration, et al. 2016, A&A, 594, A116). The observed flux is ~1e-13 ergs/cm^2/s (0.3-10 keV). Adopting an intrinsic bremsstrahlung thermal spectrum with T>10 keV, as appropriate for SNe with shock interaction with a dense medium (e.g., Margutti et al., 2017, ApJ, 835, 140), we infer an intrinsic NH~2e23 cm-2. The observed luminosity is Lx~5e39 erg/s  (0.3-10 keV), and the unabsorbed luminosity for the absorbed bremsstrahlung spectrum above is Lx>=1e40 erg/s (0.3-10 keV). 

Among SNe Ia, only the Ia-CSM sub-class has been detected in the X-rays (i.e., SN2012ca at ~500-800 days since explosion, Bochenek et al., 2018, MNRAS, 473, 336).  This represents the earliest X-ray detection of a Type Ia SN. 

Further soft and hard X-ray observations are planned. 

We are grateful to the Chandra Director’s Office and the entire Chandra team for making these observations possible.