A VLA Polarimetric Study of the Galactic Center Radio Arc: Characterizing Polarization, Rotation Measure, and Magnetic Field Properties

Abstract: The Radio Arc is one of the brightest systems of non-thermal filaments (NTFs) in the Galactic Center, located near several prominent HII regions (Sickle and Pistol) and the Quintuplet stellar cluster. We present observations of the Arc NTFs using the S-, C-, and X-bands of the Very Large Array interferometer. Our images of total intensity reveal large-scale helical features that surround the Arc NTFs, very narrow sub-filamentation, and compact sources along the NTFs. The distribution of polarized intensity is confined to a relatively small area along the NTFs. There are elongated polarized structures that appear to lack total intensity counterparts. We detect a range of rotation measure values from -1000 to -5800 rad m$\rm^{-2}$, likely caused by external Faraday rotation along the line of sight. After correcting for Faraday rotation, the intrinsic magnetic field orientation is found to generally trace the extent of the NTFs. However, the intrinsic magnetic field in several regions of the Arc NTFs shows an ordered pattern that is rotated with respect to the extent of the NTFs. We suggest this changing pattern may be caused by an additional magnetized source along the line of sight, so that we observe two field systems superposed in our observations. We suggest that the large scale helical segments near the Radio Arc could be components of such a source causing these changes in intrinsic magnetic field, and some variations in the polarization and rotation measure values along the NTFs.

• The paper reveals that the Radio Arc NTFs consist of over 20 sub-filaments with widths under 0.5 pc, showcasing intricate structural organization.
• The paper quantifies polarized intensity as patchy and clumped, with RM values from -1000 to -5800 rad m⁻², indicating strong Faraday rotation.
• The paper demonstrates that, after correcting for Faraday rotation, intrinsic magnetic field orientations generally follow the filaments while suggesting additional superposed magnetized structures.

A VLA Polarimetric Study of the Galactic Center Radio Arc

The paper presented by Paré et al. offers a detailed polarimetric analysis of the non-thermal filaments (NTFs) located near the Galactic Center, particularly focusing on the Radio Arc, using the Very Large Array (VLA) across S-, C-, and X-bands. The study aims to characterize the polarization, rotation measure (RM), and magnetic field properties of these filaments, which are significant features of the unique environment of the Galactic Center.

Key Observations and Results

1. Complex Filamentary Structures: The study reveals that the Arc NTFs are composed of sub-filaments with narrow widths of less than 0.5 pc. These filaments are organized into three bundles aligned approximately perpendicular to the Galactic plane. The high-resolution analysis has uncovered more than 20 individual filaments, presenting complexity beyond what was previously documented.
2. Polarized Intensity and Structure: The polarized intensity of the NTFs is notably patchy and clumped, with coherence observed at the scale of approximately 0.4 to 0.2 pc, depending on the frequency band. Interestingly, elongated polarized structures are found lacking corresponding total intensity features, suggesting the presence of external influences or structures without significant synchrotron emission.
3. Rotation Measure Analysis: The paper identifies a range of RM values from -1000 to -5800 rad m$^{-2}$ across the Arc NTFs. These values are consistent with previous observations of high RM magnitudes in the Galactic Center, indicating strong Faraday rotation largely attributed to external effects, likely caused by intervening magnetized gas along our line of sight.
4. Intrinsic Magnetic Field Orientation: After correcting for Faraday rotation, the study reveals that the intrinsic magnetic field orientation generally traces the extent of the NTFs. However, in specific regions, the field orientation shows an ordered pattern that is rotated with respect to the filaments, potentially suggesting the presence of an additional magnetized structure superposed along the line of sight.

Implications and Speculative Insights

The study provides deep insights into the nature of magnetic fields in the central region of our Galaxy. The presence of such complex magnetic field structures suggests that the Galactic Center serves as a laboratory for studying magnetic fields in extreme environments. The large ranges of RM observed could inform models of electron densities and magnetic field strengths unique to this region.

The observed rotational patterns of the magnetic field and the anomalies in polarization hint at active processes, such as a magnetized plasma interaction, that may not only affect Galactic Center NTFs but could also be applicable in other galactic contexts. Further studies could focus on understanding how the inferred magnetic and plasma dynamics impact star formation and the central black hole's accretion processes.

Future Directions

Given the findings, future research could utilize techniques like RM synthesis to dissect multiple RM components along the line of sight more accurately. Additional multi-band observations could refine the understanding of the magnetic field's structure and its external and internal rotation effects in relation to the NTFs. With the advent of more sensitive radiotelescopes, such as the upcoming Square Kilometer Array, the potential to resolve the enigma of Galactic magnetism and related processes looks promising.