The environment of theγ-ray emitting SNR G338.3−0.0: a hadronic interpretation for HESS J1640−465

Abstract

The supernova remnant (SNR) G338.3−0.0 spatially correlates with HESS J1640−465, which is considered the most luminous γ-ray source associated with a SNR in our Galaxy. The X-ray pulsar PSR J1640−4631 has been recently discovered within the SNR shell, which could favor a leptonic origin for the detected very-high-energy (VHE) emission. In spite of this, the origin of the VHE radiation from HESS J1640−465 has not been unambiguously clarified so far. Indeed, a hadronic explanation cannot be ruled out by current observations. On the basis of atomic (HI) and molecular ( 12 CO) archival data, we determine, for the first time, the total ambient density of protons in the region of the G338.3−0.0/HESS J1640−465 system, a critical parameter for understanding the emission mechanisms at very high energies. The value obtained is in the 100−130 cm −3 range. Besides this, we developed a new hadronic model to describe the spectral energy distribution (SED) of the HESS J1640−465 source, which includes the latest total γ-ray cross-section for protonproton collisions available in the literature. By using the assessed ambient proton density, we found that the total energy in accelerated protons required to fit the data is 5.4 +4.7 −2.3 × 10 49 erg and 1.6−0.7 × 10 50 erg for a source distance of 8.5 and 13 kpc, respectively. The case where the source distance is 8.5 kpc agrees with the typical scenario in which the energy released is on the order of 10 51 erg and ∼10% of that energy is transferred to the accelerated protons, whereas the case corresponding to a source distance of 13 kpc requires either a higher value of the energy released in the explosion or a larger energy fraction to accelerate protons.

Interstellar gas towards the TeV γ-ray sources HESS J1640−465 and HESS J1641−463

“…From the corrected spectra and, following §3, mass and density estimates of the atomic gas contained in each component were calculated. Similar to the work by Supan et al (2016), we find that, when comparing the molecular gas traced by CO and the atomic gas traced by HI, the physical parameters of the atomic gas are a small fraction of molecular gas. The calculated masses of atomic gas in each component can be found in Table 4 in §5 and in Table A2 in the appendix.…”

“…We note here that the hadronic modelling of HESS J1640−465 done in Supan et al (2016) used the parametrisation of the γ-ray differential cross-section in the proton-proton interactions from Kafexhiu et al (2014). Using ambient proton densities of ∼ 10 2 cm −3 , they found the total energy in accelerated protons to be ∼ 10 49 − 10 50 erg, consistent to first-order with a SNR scenario.…”

“…Images of the HI integrated velocity maps can be found in the appendix Figure A2. We note that a study of atomic gas using SGPS data towards HESS J1640−465 was carried out by Supan et al (2016), which focused on the velocity ranges from −121 to −111 km/s and from −40 to −25 km/s. We find that our results are similar to those presented towards HESS J1640−465.…”

“…However, a contribution to the detected flux by a pulsar or PWN could not be ruled out. Recent work by Supan et al (2016) looked at the SED of HESS J1640−465 using latest data from H.E.S.S. and Fermi-LAT observations, together with an updated hadronic γ-ray model (Kafexhiu et al 2014) and archival atomic (HI) and molecular data.…”

Constraining the origin of the puzzling source HESS J1640-465 and the PeVatron candidate HESS J1641-463 using Fermi-LAT observations

“…Now, we consider the hypothesis that the γ-ray emission detected from HESS J1640−465 is produced by particles accelerated at the shocks of the SNR G338.3−0.0. Since a SNR origin for such emission was proposed by a number of authors (Abramowski et al 2014b,a;Tang et al 2015;Lau et al 2017;Supan et al 2016), we limit the discussion to few essential issues that emerged from the new observational results presented in this paper.…”