Unlocking the Secrets of M87: What the Energy Densities Reveal About This Galactic Jet
"Scientists delve into the magnetic fields and relativistic electrons within M87's innermost jet to understand its formation and behavior."
The formation of relativistic jets in active galactic nuclei (AGNs) has been a long-standing puzzle in astrophysics. Estimating magnetic field energy density (UB) and electron energy density (Ue) is vital to understanding how these jets form, yet determining whether UB or Ue dominates at the base of a jet remains a challenge. Any deviation from equipartition (Ue/UB ≈ 1) is crucial for understanding relativistic jet formation, but obtaining a reliable estimate of Ue/UB at a jet base has proven difficult.
M87, a nearby giant radio galaxy, is the ideal subject for studying jet bases due to its proximity and supermassive black hole. M87 has been extensively studied across the electromagnetic spectrum, from radio waves to very high-energy (VHE) gamma rays. Causality arguments based on a VHE gamma-ray outburst in February 2008 suggest that the VHE emission region is less than 56 Rs, where Rs is the Schwarzschild radius. With VLBA resolution at 43 GHz reaching approximately 0.21×0.43 mas, corresponding to 5.3 × 10^16 × 1.1 × 10^17 cm, or 30 × 60 Rs, M87 offers an unparalleled opportunity for detailed investigation.
Recent advancements in VLBI observations have revealed the innermost structure of the M87 jet, including the frequency and core-size relation, and the distance and core-size relation, down to approximately 16 Schwarzschild radii (Rs). This makes the jet base of M87 the perfect environment for studying Ue/UB in close proximity to a central engine. Recent M87 observations provide two significant advancements, motivating this work. H11 successfully measured core-shift phenomena at the jet base at multiple frequencies. Also, core sizes were recently measured providing data for analysis.
What Can We Learn from the Radio Core at 43GHz?
This analysis focuses on the radio core at 43GHz. Select VLBA data observed after 2009 (H13) with good qualities to measure the core width and fit a single, full-width-half-maximum (FWHM) Gaussian to the observed core in the perpendicular direction to the jet axis to derive the core width (OFWHM). Using OFWHM at 43GHz, a model-independent value of Ue/UB in the 43GHz core of M87 can be estimated for the first time.
- Assume a uniform and isotropic distribution of relativistic electrons in the radio core at 43 GHz.
- Assume an isotropic tangled magnetic field. Hereafter, B is denoted as the magnetic field strength perpendicular to the direction of electron motion. Then, the total field strength is given by Btot = √3B.
- Assume the emission region is spherical with its radius R measured at the comoving frame. The radius is defined as 2R = θobs D where D is the distance to a source. A factor A defined as θobs = AθFWHM and 1 ≤ A ≤ 1.8 is assumed.
Implications for Relativistic Jet Models
Based on VLBA observation data at 43 GHz, Ue/UB at the base of the M87 jet is explored. Using the basic theory of synchrotron radiation with the simplest geometry of one-zone sphere model for the radio core at 43 GHz. The upper limit of total jet power Ljet is imposed based on various previous works. The findings:
The allowed range of B is 2 G < B < 13 G in the observed radio core at 43GHz with its diameter 0.11 mas ((16 R$)). This estimate of B is basically close to previous estimates in the literature, although fewer assumptions have been made in this work. This result excludes a strong magnetic field such as B ~ 103-4 G which is frequently assumed in previous works in order to activate Blandford-Znajek process. Although M87 has been a prime target for testing relativistic MHD jet simulation studies powered by black-hole spin energy, our result provides a very stringent limit on the maximum B,one of the critical parameters in relativistic MHD jets model.
The allowed region of Ue/UB is obtained in the allowed θobs and ye,min plane. The resultant Ue/UB contains both the region of Ue/UB > 1 and Ue/UB < 1. The deviation from Ue/UB ≈ 1 is not very large. It is found that the allowed range is 0.18 ≤ Ue/UB ≤ 66. This result gives a tight constraint against relativistic MHD models since they seem to postulate much larger UB/Ue at a jet-base than the maximum value obtained in this work. Therefore, the obtained Ue/Uß in this work gives a tight constraint on the initial conditions in relativistic MHD models.