Near-ultraviolet radiation toward molecular cloud N4 in W 50/SS 433: Evidence for direct interaction of the jet with molecular cloud

Abstract

We compared the molecular clouds in the western part of SS 433 with near-ultraviolet radiation data obtained from GALEX. Near-ultraviolet radiation is prominently confirmed toward only N4, while no near-ultraviolet radiation is detected toward N1, N2, or N3. The radiative region of near-ultraviolet radiation is nearly the same as the CO-emitting region in N4, and does not extend beyond the jet seen in X-ray radiation. Near-ultraviolet radiation cannot be explained solely by broad-band continuous radiation and may originate from line emissions. The intensity of near-ultraviolet radiation exhibits an anti-correlation with that of ¹³CO(J = 3–2) emission. This anti-correlation, along with strong far-infrared radiation in the region with weaker near-ultraviolet radiation intensity compared to its surroundings, suggests that near-ultraviolet radiation originates from behind the molecular cloud, heating up the interstellar dust in N4. Subsequently, the dust in N4 reradiates in the far-infrared band. In the same region, a high peak T_MB ratio of ¹²CO(J = 3–2)|$/$|¹²CO(J = 1–0) of ∼0.9, and a high kinetic temperature of T_k ∼ 56 K in the molecular cloud, indicate that CO molecules are highly excited, and the molecular cloud is heated through photoelectric heating. This heating results from electrons released due to the photoelectric effect caused by the phenomenon where interstellar dust absorbs near-ultraviolet radiation. In terms of the timescale of near-ultraviolet radiation originating from line emissions, near-ultraviolet radiation towards N4 cannot be explained by the shock of the blast wave from a supernova that created W 50. These findings also suggest that N4 directly interacts with the jet from SS 433. As a result of this direct interaction, near-ultraviolet radiation is emitted from an interacting layer between the jet and N4.

1 Introduction

The microquasar SS 433 is well known as an X-ray binary at nearly the center of the supernova remnant W 50. SS 433 emanates a powerful jet with a velocity of ∼0.26c (Margon 1984). Radio continuum radiation at 1.4 GHz has been identified by Dubner et al. (1998), and the shell of this radiation is elongated in the east–west direction due to bursts from the powerful jet.

The distance to SS 433 has been estimated in several ways (e.g., Vermeulen et al. 1993; Dehnen & Binney 1998; Dubner et al. 1998; Blundell & Bowler 2004; Lockman et al. 2007; Gaia Collaboration 2018, 2021). The most reliable estimation of the distance is based on a deep-integration radio image of SS 433 on the arcsecond scale, considering the speed and precession period of the jet (Blundell & Bowler 2004; Lockman et al. 2007). The estimation results in a distance of ∼5.5 kpc.

X-ray observations have revealed the spatial distribution and properties of the jet emanating from SS 433 (e.g., Safi-Harb & Ögelman 1997; Kotani 1998; Moldowan et al. 2005; Brinkmann et al. 2007). The X-ray jet extends by ∼0.^○7 to the west and ∼1^○ to east, corresponding to ∼70 pc and ∼100 pc at a distance of 5.5 kpc, respectively. High-energy gamma-ray radiation in the range of a few hundred GeV to 100 TeV has been detected by Fermi-LAT, HAWC (High Altitude Water Cherenkov Observatory), and more recently LHAASO (Large High Altitude AirShower Observatory) toward the W 50/SS 433 system (e.g., Abeysekara et al. 2018; Fang et al. 2020; Cao et al. 2024). The origin of such GeV–TeV gamma-ray radiation is considered to be inverse Compton scattering.

Yamamoto et al. (2022) revealed an association of mid- to far-infrared radiation with the molecular cloud N4 (see next paragraph for details of N4), indicating that possibly the interstellar dust in N4 heats up as a result of the interaction of the jet from SS 433.

CO observations toward the W 50/SS 433 system have been conducted by several authors (e.g. Huang et al. 1983; Yamamoto et al. 2008, 2022; Su et al. 2018; Liu et al. 2020; Sakemi et al. 2023). Yamamoto et al. (2008) identified the molecular clouds associated with the W 50/SS 433 system. Four molecular clouds exist in the western part of SS 433, and are labeled N1–N4. Yamamoto et al. (2022) revealed the detailed physical properties of N4, which is located nearest to SS 433 and along the same line of sight as the X-ray jet. The kinetic temperature, T_k, of the molecular cloud is estimated to be ∼56 K at maximum in the western part of N4 from ¹²CO(J = 1–0, 3–2) and ¹³CO(J = 3–2). Several observational pieces of evidence indicate that N4 is possibly interacting with the jet from SS 433.

Toward N4, GeV–TeV gamma-ray, X-ray, mid–far-infrared, and radio radiation are detected. In this paper, we compare the N4 with ultraviolet radiation, which has never been confirmed in the W 50/SS 433 system, and reveal the detailed nature of N4.

2 Observations

2.1 Nobeyama 45 m telescope

Observations were conducted on the molecular clouds detected by NANTEN observations conducted by Yamamoto et al. (2008) using the Nobeyama 45 m (NRO 45 m) telescope. The front-end system utilized for the observations of the ¹²CO(J = 1–0) emission, with a rest frequency of 115.27120 GHz, was BEARS (25-BEam Array Receiver System; Sunada et al. 2000). The beam size of the telescope at 115 GHz was ∼15″. The backend used was Auto Correlators, with 32 MHz bandwidth mode employed for 25 beams (Sorai et al. 2000). The velocity resolution was ∼0.1 km s⁻¹. The observations were conducted in the On-The-Fly mapping mode, involving scans along the directions of the Galactic longitude and latitude (Mangum et al. 2000; Sawada et al. 2008a). Two orthogonal scans were merged using the basket-weaving method (Emerson & Graeve 1988). The observing grid for the OTF mapping mode was 5″, and the grid size of output data matched that of the James Clerk Maxwell Telescope (JCMT) archival data at |${7_{.}^{\prime \prime}2}$|⁠. The data was smoothed to ∼20″ to reduce the rms noise levels. The final rms noise level of the data was ∼1.4 K ch⁻¹. In this paper, we exclusively utilize the dataset pertaining to the N4 cloud in SS 433 as presented in Yamamoto et al. (2022).

2.2 JCMT archival data

We utilized archival datasets obtained with the JCMT 15 m telescope. The data used in this study corresponds to Proposal ID M18BP027. In our analysis, we exclusively employed datasets of ¹²CO and ¹³CO(J = 3–2) emissions. The frequency resolution of the data was 61 kHz, corresponding to a velocity resolution of ∼0.05 km s⁻¹. For this paper, we further smoothed the velocity resolution to 0.1 km s⁻¹. The spatial grid of the data was |${7_{.}^{\prime \prime}2}$| for a beam size of ∼15″. The data was smoothed to 20″ to facilitate comparison with ¹²CO(J = 1–0). A main beam efficiency, η_MB, of 0.72 was adopted to convert the intensity scale T_a in the archival data to the scale of the main beam temperature, T_MB (Buckle et al. 2009). The final rms noise level for the data of ¹²CO and ¹³CO(J = 3–2) was ∼1.6 and ∼1.9 K ch⁻¹, respectively.

2.3 GALEX archival data

Image data for the ultraviolet (UV) bands are provided by GALEX (Galaxy Evolution Explorer) and are available to the public. GALEX is a UV astronomical satellite launched in 2003 by NASA (Morrissey et al. 2007). GALEX has far-UV (FUV) and near-UV (NUV) bands and is performing an all-sky survey as well as deep surveys in some sky areas. In this analysis, archival datasets from the All-Sky Imaging Survey (AIS) GR6/GR7 (e.g., Bianchi et al. 2014) are utilized. However, in the W 50/SS 433 region, only the NUV detector was operational, as the FUV detector did not function initially. The system angular resolution in the NUV band is |${5^{\prime \prime}_{.}6}$| at full width at half maximum. When comparing the NUV datasets with CO datasets, the data was smoothed to 20″. The sensitivity of the datasets is m_AB ∼ 21 mag before smoothing.

3 Availability of GALEX Data

Figure 1 shows datasets of GALEX in the field where the present analyzed region is included. Figure 1a represents an artifact flag map. The details are described in the caption of the figure and table 1. In figure 1a, four distinct linear structures are seen, traversing the center of the field: two extending from the upper right to lower left, and two from the upper left to lower right in yellow. These are identified as number 17, indicating “edge + brteddge” artifacts. Additionally, a prominent linear structure in green, stretching from the upper right to the lower left across the center of the field in figure 1a is marked as number 136, indicating “varmask + varpix.” These artifact structures are expected to be removed in figure 1b. However, the linear structure around (α, δ) (J2000.0) ∼ (⁠|${19{^{\rm h}}9{^{\rm m}}56{^{\rm s}}}$|⁠, |${5{^\circ }24^{\prime }0^{\prime \prime }}$|⁠) in figure 1b can not be conclusively identified as either real radiation or a residual error of the artifact. This structure lies outside the radio shell of the W 50/SS 433 system (see figure 2) and beyond the scope of this paper. There is an overlap of artifact and radiation near (α, δ) (J2000.0) ∼ (⁠|${19{^{\rm h}}10{^{\rm m}}20{^{\rm s}}}$|⁠, |${5{^\circ }7^{\prime }16^{\prime \prime }}$|⁠) in the north-eastern part of the NUV radiation area. The extent of accurate artifact removal is uncertain. However, for the purposes of this study, the NUV radiation around (α, δ) (J2000.0) ∼ (⁠|${19{^{\rm h}}10{^{\rm m}}20{^{\rm s}}}$|⁠, |${5{^\circ }7^{\prime }16^{\prime \prime }}$|⁠) in figure 1b is considered authentic.

4 Results

Figure 2 shows a composite image of three colors. They are red, green, and blue: red is NUV radiation; green is the 1.4 GHz radio continuum; blue is the 0.1–2.4 keV X-ray. They are superposed on an integrated intensity map of ¹²CO(J = 1–0) emission. NUV radiation is prominently confirmed toward N4, while no NUV radiation is observed toward N1 or N2, which are situated at the leading edge of the radio continuum shell. Liu et al. (2020) mentioned that N3 is situated in the radio continuum shell, and at the leading edge of the X-ray jet. However, no NUV radiation is seen toward N3 either. As mentioned in Yamamoto et al. (2022), N4 is situated within the radio continuum shell and along the same line of sight as the X-ray jet emanating from SS 433. The distribution of NUV radiation in N4 is also confirmed on the same line of sight as the X-ray jet, and does not extend beyond that of the X-ray radiation.

Notably, strong and extended NUV radiation is concentrated around (α, δ) (J2000.0) ∼ (⁠|${19{^{\rm h}}11{^{\rm m}}5{^{\rm s}}}$|⁠, |${5{^\circ }16^{\prime }30^{\prime \prime }}$|⁠). This extended source of NUV radiation could originate from HD 179124, characterized as a B9V spectral type star, positioned at an estimated distance of ∼380 pc from the Sun as inferred from its parallax. Therefore, this radiation source is unrelated to the W 50/SS 433 system. While many small, point-like sources are also confirmed toward the W 50/SS 433 system, they are not associated with the W 50/SS 433 system.

The upper left-hand image in figure 3 provides an intensity distribution of NUV radiation in N4 superposed on the distribution of peak T_MB of ¹²CO(J = 1–0) emission, obtained through Gaussian fitting and depicted in contours. The bright NUV radiating region is split into two parts, west and east. The area between these two bright NUV regions corresponds to a local peak in ¹²CO(J = 1–0) emission, which we will discuss in more detail in the next section. The intensity distribution of the region with bright NUV radiation exhibits a patchy nature, with intensity fluctuations of less than a factor of 2. The spectra shown in figure 3 represent the ¹²CO(J = 1–0) spectra for each region, labeled with characters in the upper left-hand image. Additional details regarding these spectra can be found in the caption of figure 3. The detection of weak CO emission around the molecular cloud, as defined in Yamamoto et al. (2022), suggests that the extent of the molecular cloud is greater than previously reported in Yamamoto et al. (2022), and that it closely matches the extent of NUV radiation. Consequently, this implies a connection between NUV radiation and the molecular cloud. Further details regarding the relationship between NUV radiation and the molecular cloud will be discussed in the next section.

5 Discussion and summary

First, we discuss the properties of NUV radiation. The radio continuum radiation at 1.4 GHz toward N4 is weak and does not exhibit any morphology related to the shape of the N4 cloud. Yamamoto et al. (2022) have identified mid-/far-infrared radiation toward N4, tracing thermal dust radiation with a temperature below 100 K, and PAH emission. This thermal dust radiation has been detected only in the infrared to radio wavelengths, indicating that infrared radiation presented in Yamamoto et al. (2022) is not related to current NUV radiation. If radiation from much hotter sources, such as stars with temperatures of thousands of Kelvin or more, were present, extended radiation would also be detectable in the near-infrared and visible wavelengths. However, such extended radiation in the near-infrared and visible wavelengths has not been detected in 2MASS or optical datasets. Cheung (2002) reported that the broad-band spectrum at radio, optical, and UV wavelengths at knot D in 3C279 originates from synchrotron radiation. However, in the present case, no counterpart of NUV radiation at other wavelengths is detected, indicating that the origin of NUV radiation is not synchrotron radiation. These results suggest that NUV radiation in N4 is not a part of broadband continuous radiation. Another possibility for the NUV radiation is emission lines. Numerical simulation by Lehmann et al. (2020, 2022) suggests the presence of UV radiation in the shocked layer. In the GALEX NUV band, ranging from 1750 Å to 2800 Å, strong line emissions such as C iii] 1909 Å and Mg ii 2800 Å are observed. While these specific line emissions in NUV bands are not included in Lehmann et al. (2020) or Lehmann et al. (2022), emissions such as Lyα, Lyβ, and Hα emissions are reported. Simulations show Hα emission in the shocked layer, but towards N4, Hα emission has not been detected (details of Hα emission are described in the latter half of this section). C iii] 1909 Å and Mg ii 2800 Å are often detected in distant galaxies and/or QSOs (e.g., Steidel & Sargent 1991; Bunker et al. 2023). If the origin of NUV radiation toward N4 is emission lines, C iii] and Mg ii become candidates for the NUV radiation. Sonneborn et al. (1997) monitored the intensity of several line emissions including C iii] emission in the UV/NUV bands after an outburst in SN1987A. They found that the intensity of all of observed line emissions peaked at |$\sim 400\:$|d after the outburst, and disappeared at ∼2000 d. This indicates that the radiation timescale of line emissions in the UN/NUV bands is several years after a strong shock. Present NUV radiation seen in N4 suggests that N4 is now under a strong shock condition. If the age of W 50 is ∼10⁵ yr (Panferov 2017), the blast wave of the supernova that formed W 50 has already reached the boundary of the cavity created by the massive star before it becomes a compact star, and line emissions in the UV/NUV bands due to the effect of the supernova have already disappeared. This suggests that the supernova is not the origin of NUV radiation toward N4.

Figure 4 shows a detailed comparison of the spatial distribution of NUV radiation with (figure 4a) integrated intensity of ¹³CO(J = 3–2) emission, (figure 4b) peak T_MB ratio of ¹²CO(J = 3–2)|$/$|¹²CO(J = 1–0) emission, and (figure 4c) AKARI N160 (Doi et al. 2015). In figure 4a, the intensity of NUV decreases at the intense ¹³CO(J = 3–2) emitting area around (α, δ) (J2000.0) ∼ (⁠|${19{^{\rm h}}10{^{\rm m}}10.5{^{\rm s}}}$|⁠, |${5{^\circ }4^{\prime }42^{\prime \prime }}$|⁠). The intensity of the extended ¹³CO(J = 3–2) emitting area also shows an anti-correlation with that of NUV radiation. In figure 5, the peak T_MB ratio higher than 0.6 extends over N4, indicating that the CO molecules in N4 are relatively excited compared to the typical molecular clouds in the Galactic plane (e.g., Sawada et al. 2008b). Particularly, the peak T_MB ratio in figure 4b and figure 5 is as high as ∼0.9 where the intensity of NUV radiation decreases. The region with a T_k of ∼56 K estimated in Yamamoto et al. (2022) corresponds to the region where the intensity of NUV decreases. In figure 4c, far-infrared radiation peaks in the region where NUV radiation decreases at (α, δ) (J2000.0) ∼ (⁠|${19{^{\rm h}}10{^{\rm m}}10{^{\rm s}}}$|⁠, |${5{^\circ }4^{\prime }44^{\prime \prime }}$|⁠). These results suggest that NUV radiation is absorbed by interstellar dust in N4 and the heated dust re-radiates in the far-infrared wavelength. If this is true, NUV radiation is radiated from behind the molecular cloud. The molecular cloud is heated up by photoelectric heating through electrons released due to the photoelectric effect caused by the phenomenon where interstellar dust absorbs NUV radiation. As mentioned in section 4, while N3 is also situated within the radio continuum shell (Liu et al. 2020), no NUV radiation has been detected. T_k of N3 has been estimated to be ∼30 K, which is higher than that of typical molecular clouds in the Galactic plane. They conclude that the heating source for N3 is cosmic rays, not X-rays, as the heating rate from the X-ray radiation is three orders of magnitude lower than the cooling time-scale. In contrast, the heating source for N4 is identified as an X-ray jet. This difference in heating sources may influence the physical phenomena that produce NUV radiation. The details of this will be discussed in a forthcoming paper.

Figure 6 shows a schematic view of the current condition of N4. The results discussed in this section, along with the findings in Yamamoto et al. (2022), suggest that N4 is currently interacting directly with the jet from SS 433. Atoms of carbon and/or magnesium etc. are ionized and excited in the interacting layer between the jet and N4. Then, those emissions are detected as the NUV radiation with GALEX. Figure 7 presents a three-color composite image; red is Hα emission by IPHAS (the INT Photometric Hα Survey of the Northern Galactic Plane; Barentsen et al. 2014); green is an intensity map of ¹²CO(J = 1–0) emission integrating from 45 to 60 km s⁻¹; blue is a map identical to the green one but integrating from 25 to 35 km s⁻¹. The molecular cloud depicted in blue is identified as a giant molecular filament situated in front of the W 50/SS 433 system, with an estimated distance of ∼1.7 kpc (Lin et al. 2020). This foreground component overlaps with two-thirds of N4 at the southeastern part. The N(H₂) of the foreground component is estimated to be a few times 10²¹ cm⁻², corresponding to an A_v of 2 to 5. Considering A_v in both the foreground component and N4, as well as the number of stars identified in the Hα image, both inside and outside N4, Hα emission does not seem to be significantly dimmed by the molecular clouds. Therefore, it is inferred that Hα emission is not being radiated in N4. This may result from the moderate effective interacting velocity between the jet and N4 because Alfven’s critical ionization velocities of H ii and C iii are ∼50.9 km s⁻¹ and ∼19.7 km s⁻¹, respectively. This velocity range of the shock is similar to the velocity simulated in Lehmann et al. (2020). In the case of the Cygnus loop (Danforth et al. 2001), emissions of ionized atoms of oxygen, carbon, nitrogen, etc., provide insights into the detailed shock conditions in the interacting layer between the cavity wall and the blast wave of the supernova. Thus, to gain a better understanding of the details of the interacting layer between the jet and N4, spectroscopic observations in the UV/NUV bands are essential. The interacting layer of the jet and N4 is an important site of shock–cloud interaction. The region of shock-cloud interaction is the place where high-energy gamma-ray radiation is produced as a result of the collision between cosmic-ray protons accelerated by diffusive shock acceleration (Fermi 1949) and the ISM. Strong and long-term shock-cloud interaction continues while the jet emanates from SS 433, and the timescale of shock–cloud interaction in the present case is longer than that in the case of supernova remnants. This implies that the energy of the cosmic-ray protons may increase beyond 1 PeV, and N4 may be a candidate for a PeVatron.

Acknowledgments

This work has been supported by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (19K03912, 21H01128). This work has also been supported in part by the Collaboration Funding of the Institute of Statistical Mathematics “New Perspective of the Cosmology Pioneered by the Fusion of Data Science and Physics”. Open Access funding provided by Nagoya University. This research is based on observations made with the GALEX mission, obtained from the MAST data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 526555. The James Clerk Maxwell Telescope is operated by the East Asian Observatory on behalf of The National Astronomical Observatory of Japan; Academia Sinica Institute of Astronomy and Astrophysics; the Korea Astronomy and Space Science Institute; Center for Astronomical Mega-Science (as well as the National Key R&D Program of China with No. 2017YFA0402700). Additional funding support is provided by the Science and Technology Facilities Council of the United Kingdom and participating universities and organizations in the United Kingdom and Canada. The Nobeyama 45 m radio telescope is operated by Nobeyama Radio Observatory, a branch of National Astronomical Observatory of Japan. This research is based on observations with AKARI, a JAXA project with the participation of ESA. This paper makes use of data obtained as part of the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS, www.iphas.org) carried out at the Isaac Newton Telescope (INT). The INT is operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. All IPHAS data are processed by the Cambridge Astronomical Survey Unit, at the Institute of Astronomy in Cambridge. The bandmerged DR2 catalogue was assembled at the Centre for Astrophysics Research, University of Hertfordshire, supported by STFC grant ST/J001333/1.

References