Evidence for a Neutral Iron Line Generated by MeV Protons from Supernova Remnants Interacting with Molecular Clouds

Abstract

Supernova remnants (SNRs) have been prime candidates for Galactic cosmic-ray accelerators. When lowenergy cosmic-ray protons (LECRp) collide with interstellar gas, they ionize neutral iron atoms and emit the neutral iron line (Fe I Kα) at 6.40 keV. We search for the iron K-shell line in seven SNRs from the Suzaku archive data of the Galactic plane in the 6 • l 40 • , |b| < 1 • region. All these SNRs interact with molecular clouds. We discover Fe I Kα line emissions from five SNRs (W28, Kes 67, Kes 69, Kes 78, and W44). The spectra and morphologies suggest that the Fe I Kα line is produced by interactions between LECRp and the adjacent cold gas. The proton energy density is estimated to be 10-100 eV cm −3 , which is more than 10 times higher than that in the ambient interstellar medium.

Impact of Low-Energy Cosmic Rays on Star Formation

“…Emission arising from the n = 2 → 1 transition is referred to as Kα and typically occurs in the X-ray regime. The Fe Kα line at 6.4 keV has been observed in the Galactic centre and from molecular clouds near supernova remnants, but different studies of these regions have interpreted the emission as the product of ionisation by electrons (Yusef-Zadeh et al, 2007), low-energy protons (Tatischeff et al, 2012;Nobukawa et al, 2018) and X-ray photons (Ponti et al, 2010). While Fe Kα line emission indicates that energetic particles or photons are present, it can be difficult to distinguish which is the dominant ionising mechanism.…”

Can a cosmic ray carrot explain the ionization level in diffuse molecular clouds?

“…Thus, in order to reconcile predicted and measured ionization rates, one should either invoke a new source of ionization inside MCs, or question the validity of assuming the Voyager 1 spectrum to be representative of the whole CR spectrum in Galaxy. Several possibilities have been put forward: i) the possible presence of MeV CR accelerators inside MCs (see e.g Padovani et al 2015Padovani et al , 2016; ii) inhomogeneities in the distribution of low energy CRs in the Galaxy (see e. g Cesarsky 1975;Gabici & Montmerle 2015;Nobukawa et al 2015Nobukawa et al , 2018; iii) the existence of a still unknown CR component emerging at energies below few MeV (the smallest energy detected by Voyager 1). Such component, called carrot, was first proposed by Meneguzzi et al (1971) to explain the abundances of light elements, and has recently been reconsidered by Cummings et al (2016) (who called it suprathermal tail).…”

Low-energy Cosmic Rays in Supernova Remnants Interacting with Molecular Clouds

“…The observed value in the five SNRs is 1.0 +0.7 −0.4 keV , which favors the proton origin. After our paper in 2018, the number of samples is increasing now; more than 10 SNRs (including previously found ones, 3C391 and Kes 79) shows the 6.4 keV line probably induced by CR particles [6,7,8,11,12,13,14]. For example, we recently found the 6.4 keV line from a famous SNR, IC 443, with 3.8σ as reported in Nobukawa et al (2019b) [14].…”

“…We reported in 2018 that the 6.4 keV line is found from five SNRs, W28, W44, Kes 67, Kes 69, and Kes 78 [8]. Figure 1 is a stacked spectrum of these SNRs from which a background spectrum is already subtracted.…”

“…Comparison of the 6.4 keV line intensity calculated with the CR escaping model[18], and the observed ones in W28, W44, and IC 443. The values measured byNobukawa et al (2018) [8]. The values measured by Nobukawa et al (2019b)[14].…”