Kavli Affiliate: Kejia Lee
| First 5 Authors: Rui Wang, Zhen Yan, Zhiqiang Shen, KeJia Lee, Yajun Wu
| Summary:
The Gum Nebula is a faint supernova remnant extending about 40 degrees across
the southern sky, potentially affecting tens of background pulsars. Though the
view that the Gum Nebula acts as a potential scattering screen for background
pulsars has been recurrently mentioned over the past five decades, it has not
been directly confirmed. We chose the strong background pulsar PSR~B0740$-$28
as a probe and monitored its diffractive interstellar scintillation (DISS) at
2.25~$&$~8.60~GHz simultaneously for about two years using the Shanghai Tian
Ma Radio Telescope (TMRT). DISS was detected at both frequencies and quantified
by two-dimensional autocorrelation analysis. We calculated their scattering
spectral index $alpha$ and found that 9/21 of the observations followed the
theoretical predictions, while 4/21 of them clearly showed $alpha < 4$. This
finding provides strong support for anomalous scattering along the pulsar line
of sight, due to the large frequency lever arm and the simultaneous features of
our dual-frequency observations. In comparison to the 2.25~GHz observations,
scintillation arcs were observed in 10/21 of the secondary spectrum plots for
8.60~GHz observations. Consequently, the highest frequency record for pulsar
scintillation arc detection was updated to 8.60~GHz. Our fitting results were
the most direct evidence for the view that the Gum Nebula acts as the
scattering screen for background pulsars, because both the distance
($245^{+69}_{-72}$~pc) and transverse speed ($22.4^{+4.1}_{-4.2}$~km/s) of the
scintillation screen are comparable with related parameters of the Gum Nebula.
Our findings indicated that anisotropic scattering provides a superior
explanation for the annual modulation of scintillation arcs than isotropic
scattering. Additionally, the orientation of its long axis was also fitted.
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