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A diagnostic on the orbital spectrum of LS 5039 with Fermi-LAT | | Zhi Chang
; Shu Zhang
; Yu-Peng Chen
; Long Ji
; Ling-Da Kong
; | | Date: |
17 Jul 2019 | | Abstract: | LS 5039 is a well-known $gamma$-ray binary system which consists of an
unknown compact object and a massive companion O star. It shows rather stable
emissions at high energies over years and hence serves as an ideal laboratory
for investigating the emission mechanism for such peculiar systems which emit
prominent $gamma$-rays. To this end, we take the orbital phase resolved energy
spectrum as observed by $Fermi$-LAT over 10 years. We divide the orbit into
four orbital phases, each with an orbital phase range of 0.25, centered at
0.00, 0.25, 0.50 and 0.75 respectively, where the phase 0.0 is the periastron
and phase 0.5 is the apastron. The phases around 0.25 and 0.75 are symmetric
and hence are supposed to have identical local acceleration environment. The
spectral analysis shows that, the $Fermi$-LAT spectra are largely different
from these two symmetric orbital phases: the emission from orbital phase 0.25
turns out to be significantly stronger than that from 0.75. This result does
not fit a scenario that $gamma$-rays are Doppler boosted emission from bow
shock tails if LS 5039 has a shock configuration similar to PSR B1259-63, and
indicates that the inverse Compton scatterings between the shock accelerated
plasma and the stellar particle environment is the underline procedure. Also we
find that the previous report for a disappearance of the orbital modulation at
3--20 GeV is due to the similar spectral turn-over energies of the different
orbital phases. The spectral properties of periastron and apastron regions are
addressed in the context of the measurements in phase regions around 0.25 and
0.75. | | Source: | arXiv, 1907.7342 | | Services: | Forum | Review | PDF | Favorites | |
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