Physical properties of the molecular cloud, N4, in SS 433: Evidence for an interaction of molecular cloud with the jet from SS 433

Abstract

We conducted observations and analyses of the molecular cloud, N4, which is located at ∼40 pc from SS 433 and the same line of sight as that of the radio shell, in 12CO(J = 1–0), 12CO(J = 3–2), 13CO(J = 3–2), and grand-state OH emissions. N4 has a strong gradient of the integrated intensity of 12CO(J = 1–0, 3–2) emission at the northern, eastern, and western edges. The main body of N4 also has a velocity gradient of ∼0.16 km s−1 (20″)−1. A velocity shift by up to 3 km s−1 from the systemic velocity at ∼49 km s−1 is detected at only the northwestern part of N4. The volume density of the molecular hydrogen gas and the kinematic temperature are estimated at eight local peaks of 12CO(J = 1–0) and 13CO(J = 3–2) emissions by the RADEX code. The calculated $n_{\rm (H_2)}$ is an order of 103 cm−3, and Tk ranges from ∼20 to ∼56 K. The mass of N4 is estimated to be ∼7300 M⊙. The thermal and turbulent pressures in N4 are estimated to be ∼105 K cm−3 and ∼107 K cm−3, respectively. The relation of the thermal and turbulent pressures in N4 tends to be similar to that of the molecular clouds in the Galactic plane. However, these values are higher than those in the typical molecular clouds in the Galactic plane. Several pieces of circumstantial evidence representing the physical properties of N4 and comparison with the data of infrared and X-ray radiation suggest that N4 is interacting with a jet from SS 433. However, no gamma-ray radiation is detected toward N4. Compared to the previous study, it is hard to detect the gamma-ray radiation by cosmic-ray proton origin due to the low sensitivity of the current gamma-ray observatories. No OH emission was detected toward N4 due to the low sensitivity of the observation and antenna beam dilution.

Spatially resolved study of the SS 433/W 50 west region with Chandra: X-ray structure and spectral variation of non-thermal emission

“…We have detected a new knot, w1.5, located between the previously known knots, w1 and w2 (figure 1). As shown in figure 6d, the location of the knot w1.5 coincides with the peak of the 12 CO emission, which is from the molecular cloud, N4 (Yamamoto et al 2008;Su et al 2018;Yamamoto et al 2022). The figure also shows that 12 CO emission has a clear positive correlation with X-ray absorption column densities.…”

“…The figure also shows that 12 CO emission has a clear positive correlation with X-ray absorption column densities. Yamamoto et al (2022) reported that the temperature of this cloud is higher than that of typical clouds in the Galactic plane, and a velocity shift of up to 3 km s −1 from the velocity of the entire cloud (∼ 49 km s −1 ) was detected only in the northwestern part. These results suggest that most of the cloud is located in front of the jet, and a part of the cloud is interacting with the jet.…”

“…Alternatively, we propose scenarios in which the magnetic field is amplified by some mechanism (e.g., plasma compression; instabilities) near the central shell of W50. Another new finding is the detection of the X-ray knot w1.5, which is at the location of the molecular cloud (Yamamoto et al 2022). A possible origin of the knot is an enhanced magnetic field by a jet-cloud interaction.…”

“…The cloud must survive without fully evaporating under such an environment (e.g., Cowie & McKee 1977;Liu et al 2020). In fact, Yamamoto et al (2022) argued from the intensity gradient of the 12 CO emission that the envelope of the cloud has been stripped away and that only the dense part of the cloud survives. It is beyond the scope of this paper to discuss further and will be studied in future works.…”

Molecular clouds at the eastern edge of radio nebula W50

“…The western-side clouds (N1-N4) are closer to SS 433, and Liu et al (2020) observed N1, N2, and N3 with higher spatial resolution and concluded that a part of them is located within W50. Recently, Yamamoto et al (2022) reported the observation of N4. The molecular cloud has an asymmetric spectrum for CO emission and an extreme temperature gradient.…”