Brightest Non-Thermal X-ray Emission Observed from Eta Carinae
ATel #13738; Kenji Hamaguchi (UMBC/GSFC), David A. Espinoza-Galeas (CUA/GSFC), Michael Francis Corcoran (CUA/GSFC), Christopher Russell (CUA/GSFC), Julian Pittard (University of Leeds), Thomas Madura (San Jose State University), Hiromitsu Takahashi (Hiroshima University), Theodore Gull (NASA/GSFC \& STScI), Noel Richardson (ERAU), Augusto Damineli (IAG/USP), Desmond Hillier (U. Pittsburgh), Anthony Moffat (U. Montreal), Gerd Weigelt (MPIfR), Andy Pollock (U. Sheffield)
on 15 May 2020; 19:10 UT
Credential Certification: Kenji Hamaguchi (kenji.hamaguchi@umbc.edu)
Subjects: X-ray, Gamma Ray, Star
The NuSTAR X-ray observatory pointed at the massive colliding wind binary system, eta Carinae, for 90 ksec on 2020 April 13-14 (phi_orb = 4.0297) as its thermal X-ray emission was gradually recovering from its minimum flux in mid February (ATel #13636) near its most recent periastron passage. The observation marked the highest non-thermal X-ray flux above ~25 keV among the 15 NuSTAR observations of eta Carinae obtained since 2014. The X-ray emission showed no significant time variation during the observation. The observed non-thermal X-ray flux assuming a gamma =1.65 power-law spectrum was 3.0e-12 ergs cm-2 s-1 between 30-50 keV, which is a factor of 2 larger than the non-thermal flux at this phase in the previous orbital cycle (phi_orb = 3.0275, 2014 Sept. 26). The bright non-thermal emission in this cycle may be related to an early recovery of the thermal X-ray emission, which showed a factor of 3 larger thermal flux in the 10-15 keV band than that at nearly the same phase in the previous cycle. The non-thermal energy flux ratio between 30-50 keV compared to the thermal energy flux between 10-30 keV was ~0.22. For comparison, this ratio at phi_orb = 3.17 (a later orbital phase in the previous cycle), was ~0.42, about a factor of 2 larger. Since eta Carinae's thermal emission varied similarly after the full recovery in the four previous binary cycles, we expect that the non-thermal to thermal ratio will also increase to or above ~0.4 before phi_orb ~ 4.17, near 2021 February. Considering that the thermal emission is also expected to increase until it recovers totally, the non-thermal X-ray emission may further increase by a factor or ~2 in the next few months. Monitoring eta Carinae in particular in the highest energy domain is strongly encouraged.
