Identifying a Point-Symmetrical Morphology in the Core-Collapse Supernova Remnant W44

Abstract

I identify a point-symmetrical morphology in the core-collapse supernova remnant (CCSNR) W44 compatible with shaping by three or more pairs of jets in the jittering jets explosion mechanism (JJEM). Motivated by recent identifications of point-symmetrical morphologies in CCSNRs and their match to the JJEM, I revisit the morphological classification of CCSNR W44. I examine a radio map of W44 and find the outer bright rim of the radio map to possess a point-symmetric structure compatible with shaping by two energetic pairs of opposite jets rather than an S-shaped morphology shaped by a precessing pair of jets. An inner pair of filaments might hint at a third powerful pair of jets. More pairs of jets were involved in the explosion process. This study adds to the growing evidence that the JJEM is the primary explosion mechanism of core-collapse supernovae.

1. Introduction

The jittering jets explosion mechanism (JJEM) is a theoretical mechanism for exploding core-collapse supernovae (CCSNe), in which several to a few tens of pairs of jets explode the star. Intermittent accretion disks around the newly born neutron star (NS) launch the exploding jets in stochastically varying directions and powers (e.g., [1,2,3,4,5]; for recent quantitative values of the different parameters of the JJEM, see [6]). Most jets are choked inside the core, deposit their energy, and explode the star. Some pairs of jets might leave imprints on the CCSN remnant (CCSNR), e.g., by forming dense clumps, inflating bubbles or ears, breaking out through the main ejecta, and leaving behind a nozzle.

Since the intermittent accretion disks launch pairs of opposite jets, these structural features come in pairs. Pairs of structural features not along the same line form a point-symmetric morphology. As such, the JJEM predicts that many CCSNRs possess point-symmetric morphologies, but not all CCSNRs. The recent identification of point-symmetric morphologies in more than ten CCSNRs confirms one of the fundamental predictions of the JJEM (for a review of 2024 results and list of the point-symmetric CCSNe, see [7]).

The effect of the NS natal kick, instabilities during the explosion process, and the interaction of the ejecta with a circumstellar material and the interstellar medium cannot account for all properties of the identified point-symmetric morphologies of these CCSNRs [8]. These processes smear any point-symmetrical morphology. These make it challenging to identify point-symmetrical morphologies in some cases. For these challenging identifications, any additional CCSNR with an identified point-symmetric morphology is a treasure to JJEM researchers.

The magnetorotational explosion mechanism (e.g., [9,10,11,12,13,14]), which is an active topic of research (e.g., [15,16,17]), requires the pre-collapse core to have a fast rotation; after the explosion, it has one fixed axis of symmetry. Because it has one fixed axis of symmetry, the magnetorotational explosion mechanism cannot explain point-symmetric CCSNRs with two or more symmetry axes. Also, because it requires rapid pre-collapse core rotation (e.g., [18]), this mechanism operates only in rare cases, i.e., ≃1% of CCSNe, (e.g., [19]); this mechanism assumes that most CCSNe are jetless and explode by the neutrino-driven explosion mechanism.

Spin-kick alignment is observed in many pulsars (e.g., [20,21,22]). Both the magnetorotational explosion mechanism (e.g., [15]) and the JJEM [23] can explain this alignment. However, while there are many simulations of the magnetorotational explosion mechanism (e.g., [15,24]), only two studies have simulated the interaction of jittering jets with the core during the explosion mechanism [1], and they are limited in their extent. Namely, the JJEM assumes the launching of jets, but there are no simulations showing the launching of jets.

As I show in this study, the CCSNR W44 (G34.7-0.4, 3C 392) is also challenging. The earlier W44 morphological classification was of an elongated structure with an S-shape morphology (Section 2). Motivated by the recent identification of point-symmetrical CCSNRs, I revisit its morphological classification and find it to be point-symmetric (Section 3). In Section 4, I discuss alternatives that cannot account for the point-symmetric morphology, and in Section 5, I summarize this short study and its significant contribution in establishing the JJEM as the primary explosion mechanism of CCSNe.

2. Previous S-Shape Classification

In this section, I discuss the previous morphological classification of W44 concerning a jet-driven explosion.

SNR W44 was mapped in several wavebands, e.g., gamma ray (e.g., [25]), X-ray (e.g., [26,27,28,29,30]), visible (e.g., [31,32,33]), IR (e.g., [34,35]), and radio (e.g., [36,37,38,39,40,41]). SNR W44 is close to the Galactic plane and interacts with a molecular cloud (e.g., [31,42,43,44,45,46]). It has a pulsar, PSR B1853+01 and its distance is ≃3 kpc (e.g., [47]). In Figure 1, I present a composite image of W44 from the NASA site. The image shows the complicated structure of W44, particularly its filamentary texture and unequal west–east sides. I mark the two axes of the two energetic jets that I propose shaped W44 during the explosion process (Section 3).

W44 has an elongated morphology (e.g., [38]). Ref. [48] identified two ears in IR images of W44. Figure 2 is from their paper. They mark the base of each ear that they identified (thin double-headed yellow arrows), the middle of each ear (thin double-headed red arrows), and the center of W44 in the symmetrical morphology they identified (thick orange double-headed arrow). They aimed to calculate the energy of the jets that inflated the ears.

Ref. [48] identified an S-shaped morphology, as the three-segment line on Figure 2 depicts. They also noted that the ears they defined with the S-shaped morphology appear in the IR image but not in the X-ray or radio. Motivated by identifying eleven point-symmetric CCSNRs in the past two years, I revisit the morphological classification of W44.

3. Identifying a Point-Symmetric Morphology in W44

Considering W44’s complicated structure, I prefer to examine the radio map, which is the most detailed. I determine whether W44, or part of it, possesses a point-symmetric morphology.

I found the radio image from [49] to fulfill my goals; I present this image in Figure 3 (the same image in the four panels). They mark the location of six regions with strong OH (1720 MHz) emission on that image, which I do not refer to in this study. On Figure 3a, I have added two red arrows pointing at the structural features I term dents, where the bright rim sharply bends inward.

In Figure 3b, I mark several western structural features. I draw a line (orange) on the bright western rim of W44. I mark three structural features outside and attached to the rim on the west (dashed pale blue lines). I draw a dotted yellow line on an inner filament on the southwest. The yellow double-headed arrow points at the two dents. One double-line double-headed red arrow is along the long dimension of W44, through the center of the point-symmetrical structure that I mark with an asterisk; the other goes through the center and points at two opposite protrusions of the bright rim. The centers of the three double-headed arrows are at the asterisk. Figure 3c presents all the structural marks of Figure 3b, with their 180° rotational symmetric marks. Namely, I take all the structural marks from Figure 3b and rotate them by 180° around the asterisk. Figure 3c shows the point-symmetrical structure that I identify in W44, primarily the two bright rims and the two inner filaments.

To allow a better visual inspection of the rim on the west and east and the two inner filaments, on Figure 3d I displace outward the marks of the two rims and the two inner filaments. I also mark the location of the pulsar-wind nebula (PWN; e.g., [50,51]). This PWN likely influenced the filament on the southwest; hence, I do not expect a perfect point symmetry between the two opposite filaments marked by the dotted yellow lines in Figure 3b–d.

I suggest that each of the two dents are the zones where two jet-inflated structures (bubble/lobes/ears) touch each other on the bright rim. Similar dents are observed between jet-inflated bubbles in some planetary nebulae and clusters of galaxies. In Figure 4, I present an image of the cooling flow cluster RBS 797 from [52]. They mark (white dashed lines) the rims of the four jet-inflated bubbles, i.e., inflated by two pairs of jets. Two adjacent rims have different directions where they meet, forming the dent. The similarity of the structures of the dents in W44 with those in RBS 797, which are known to be inflated by jets, supports the jet-shaped modeling. In the case of the CCSNR W44, more than two pairs of jets were involved. For example, I suggest that a third pair of jets shaped the two opposite filaments.

4. Answering Possible Counterarguments

Let me answer some possible arguments against my interpretation of a jet-shaped point-symmetrical structure.

(1) Counterargument: There are many filaments and clumps in the radio image of W44, and one can find a point-symmetrical structure just by chance.

• Answer: As I indicated above, the point symmetry is not expected to be perfect because of several smearing processes. Considering these smearing processes, I crudely estimate the probability of chance alignment in a point symmetry.

When two opposite jets shape ejecta, they form an axial-symmetric structure. The structure of W44 has a long axis, the long double-headed arrow in Figure 3. To examine the possibility of axial symmetry, in panel b of Figure 5 I mirror the structure I marked on the western side, with the long axis being the mirror. I marked the mirrored structures with dashed lines. It is evident that the bright rim on the east is not a mirror image of the one on the west. Also, only a short segment of the inner filament fits the northeast filament. The dashed blue line matches the bump in the northeast; however, the bump in the northeast is much brighter and somewhat smaller than that in the northwest. There are some axial-symmetry signatures, but they are not as strong as the point-symmetric ones. The mirror symmetry fails to account for the most prominent morphological features, the dent, nose, and the general rim line. This strengthens the claim for point symmetry.

However, there is still some probability that the point symmetry results from chance alignment. For example, the dent in the southeast is a chance crossing of two filaments. The opening angle of the northwest dent is 130°, while that in the southeast is 120°. The chance of two filaments crossing each other in the southeast within ±10° from the opposite side is 20/180 = 0.11. However, not only are the angles point-symmetric, but so are the locations of the two dents. Overall, I crudely estimate the chance alignment to a point-symmetric morphology of the bright rims on the two sides, the dents, the noses, and the inner filaments to be < 3%, and more likely < 1%.

(2) Counterargument: The IR and X-ray images do not show a point-symmetrical structure, as marked on the radio images.

Answer: The IR image (Figure 2) shows many of the structures of the radio image (Figure 3), but not all structures. For example, the IR shows part of the rims but is missing other parts, like the east dent. This situation resembles other point-symmetrical morphologies of CCSNe, e.g., Cassiopeia A’s IR and X-ray images reveal point-symmetrical morphologies but do not fully overlap [54]. The X-ray map of W44 shows the inner part, so it does not show the point symmetry of the outer rims. It follows an S-shape morphology [48], a point-symmetrical morphology type, but it seems that the X-ray-emitting gas fills the space inside the rims and filaments.

(3) Counterargument: Interaction with the cloud surrounding W44 shaped its morphology.

Answer: In general, interaction with ambient gas cannot explain the properties of the point-symmetrical structure of CCSNRs [8]. In the case of W44, the protrusions from the dents (pale blue dashed lines) imply that no dense ambient clumps form the dents. Other protrusions outside the rims on both sides show that it is not the ambient medium that shapes the point-symmetrical morphology. The ambient medium can shape these protrusions; hence, the protrusions are not equal on both sides, i.e., the point symmetry is not perfect. Furthermore, the ambient gas cannot shape the two inner filaments of the point-symmetrical morphology.

(4) Counterargument: Post-explosion jets shaped the point-symmetrical structure.

Answer: The point-symmetrical structural features involve the entire CCSNR with an elongated structure. Hence, the energy of the shaping jets must be a large fraction of the explosion energy. It is unlikely that post-explosion jets carry such energy, and it is not clear why post-explosion jets should jitter.

5. Summary

I examined the complicated structure of CCSNR W44 (Figure 1 and Figure 2) by marking some structural features on the west side of a radio image of W44 (Figure 3b), and then rotated it around a central point (the asterisk). The rotated markings overlap with similar structures on the east side (Figure 3c,d), signifying the point-symmetrical morphology of W44. I attribute the shaping of the rims (orange lines) to two pairs of jets (red double-headed arrows). A third pair might have shaped the inner filaments. The dents are the zones where two jet-inflated structures meet. Such jet-shaped dents are observed in planetary nebulae and clusters of galaxies, one of which I present in Figure 4.

In Section 4, I presented answers to some possible counterarguments to my claims. Mainly, I explained why the cloud that CCSNR W44 interacts with cannot account for the point-symmetrical morphology. This interaction instead smears point-symmetrical structures.

The result of this short study, which identifies a point-symmetrical morphology in CCSNR W44, is of great importance to the JJEM. According to the JJEM, stochastic pairs of jets explode most (possibly all) CCSNe, implying an explosion with a point-symmetric morphology. However, most pairs of jets are choked in the core, leaving no observable imprints on the CCSNR morphology. Instabilities during the explosion process, the NS natal kick and a PWN when existing, and interaction with the circumstellar material and the interstellar medium smear point-symmetrical structures that the shaping pairs of jets form. These smearing processes lead to imperfect point-symmetrical morphologies and can even completely erase the point-symmetric structures.

For that, any CCSNR that reveals a point-symmetrical structure is a treasure to the development of the JJEM. SNR W44 is one of only 12 CCSNRs with an identified point-symmetrical morphology [7]. This study shows how a new analysis might reveal a different morphology, motivating more profound studies of CCSNRs for point-symmetrical morphologies.

This study’s new identification of a point-symmetrical CCSNR, W44, adds to the growing evidence that the JJEM is the primary explosion process of CCSNe, even when an energetic magnetar supplies extra post-explosion energy (e.g., [55]).