Detection of hard and soft X-ray emission with NuSTAR and Swift-XRT at the location of SN2024iss

ATel #16624; Raffaella Margutti (UC Berkeley), Brian Grefenstette (Caltech), Ryan Chornock (UC Berkeley)
on 18 May 2024; 04:29 UT
Credential Certification: Raffaella Margutti (rafmargutti@gmail.com)

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We report evidence for rapidly declining (Fx~t^-0.5) soft X-ray emission detected by Swift-XRT at the location of the very young and nearby type-II SN2024iss (TNS Astronomical Transient Report No. 210230 and TNS Classification Report No. 17057) starting ~0.9 days post discovery. A spectrum extracted in the time period 0.9-4.4 days after discovery (total exposure of 8.2 ks) can be fitted with a power-law model with a hard photon index Gamma ~ 0.9 +\- 0.3. We find no evidence for intrinsic absorption. The inferred 0.3-10 keV flux is ~(1.7 +\- 0.4)e-12 erg/s/cm2, which is a luminosity of ~4e+40 erg/s at the distance of ~14.4 Mpc.

Prompted by the presence of a bright source of soft X-ray emission with a hard spectrum detected by Swift-XRT, we started observations with NuSTAR (PI Margutti) on 2024-05-17, 16:11:09 UT, which is around 4.5 days post discovery (O’Neill et al., AstroNote 2024-128). Using the first ~16 ks of NuSTAR observations we find clear evidence for hard-X-ray emission at the location of the supernova with high statistical significance in each of the two NuSTAR modules. A preliminary spectral analysis shows that the data can be modeled with a bremsstrahlung spectrum with temperature T~20 keV in the energy range 3-20 keV with no evidence for intrinsic absorption. The inferred flux is ~2.5e-12 erg/s/cm2 in the 3-80 keV energy range, corresponding to a luminosity of ~6e40 erg/s.

The inferred luminosity is larger than (but comparable with) the unabsorbed luminosity of SNe 2023ixf and 2024ggi at a few days post explosion (Grefenstette 2023, Margutti 2024, ATel #1658). However, in stark contrast with the strong interactors SNe 2023ixf and 2024ggi, the X-ray emission from SN2024iss shows no evidence for intrinsic absorption. This finding is consistent with the lack of narrow features reported in the optical spectra of SN2024iss at early times (Srivastav et al., TNS Classification Report No. 17057 and Wise et al., TNS Classification Report No. 17060), and indicates a less extended high-density circumstellar medium surrounding SN2024iss when compared to SNe 2023ixf and 2024ggi. It also further establishes broad-band X-ray observations of very young type-II SNe as a powerful probe of their circumstellar medium.

Further analysis is ongoing and further observations are planned.

We are extremely grateful to the NuSTAR PI Fiona Harrison and to the entire NuSTAR team (in particular to Murray Brightman) for rapidly approving and executing these observations.