SRG/eROSITA X-ray shadowing study of giant molecular clouds

Abstract

Context. SRG/eROSITA is situated in a halo orbit around L2 where the highly variable solar wind charge exchange (SWCX) emission from Earth’s magnetosheath is expected to be negligible. The soft X-ray foreground emissions from the local hot bubble (LHB) and the remaining heliospheric SWCX emissions could be studied in unprecedented detail with eROSITA All-Sky Survey (eRASS) data in a 6-month cadence and better spectral resolution than ROSAT. Aims. We aim to use eRASS data of the sight lines towards three giant molecular clouds away from the Galactic plane to isolate and study the soft X-ray diffuse foreground emission. These X-ray shadows will serve as calibration baselines for the future three-dimensional structural study of the LHB. Methods. We conducted spectral analysis on the diffuse X-ray spectra of these clouds from the first four eRASSs to estimate and separate the heliospheric SWCX contribution from the LHB emission. Results. We find the density of the LHB to be independent of the sight line with ne ~ 4 × 10−3 cm−3, but not the temperature. We report a lower temperature of kTLHB = 0.084 ± 0.004 keV towards Chamaeleon II & III (Cha II & III) than Ophiuchus (Oph) and Corona Australis (CrA), in which we measured 0.102 ± 0.006 and 0.112 ± 0.009 keV, respectively. We measured the emission measure of the LHB to be ~2 × 10−3 cm−6 pc at medium Galactic latitudes (|b| ~ 20°). A monotonic increase in the SWCX contribution has been observed since the start of 2020, coincidental with the beginning of solar cycle 25. For Oph, SWCX has dominated the LHB in the 0.3−0.7 keV band intensity since eRASS2. We observed lower SWCX contributions in Cha II & III and CrA, consistent with the expected decreasing solar wind ion density at high heliographic latitudes.