RadioAstron Observations of the Quasar 3C273: a Challenge to the Brightness Temperature Limit (1601.05806v2)

Abstract: Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of $10^{11.5}$ K. Relativistic boosting can increase its observed value, but apparent brightness temperatures much in excess of $10^{13}$ K are inaccessible using ground-based very long baseline interferometry (VLBI) at any wavelength. We present observations of the quasar 3C273, made with the space VLBI mission RadioAstron on baselines up to 171,000 km, which directly reveal the presence of angular structure as small as 26 $\mu$as (2.7 light months) and brightness temperature in excess of $10^{13}$ K. These measurements challenge our understanding of the non-thermal continuum emission in the vicinity of supermassive black holes and require a much higher Doppler factor than what is determined from jet apparent kinematics.

Observational Constraints on the Brightness Temperature of Quasar 3C 273

The paper "RadioAstron Observations of the Quasar 3C\,273: a Challenge to the Brightness Temperature Limit" by Kovalev et al. offers a detailed re-evaluation of fundamental assumptions regarding the brightness temperature of quasar 3C 273. This paper exploits the unprecedented capabilities of the space VLBI mission RadioAstron, which enables high-resolution observations of celestial phenomena by using space-based interferometry.

Key Observations and Results

Using baselines extending as far as 171,000 km, the authors explored the angular resolutions of 26 µas, detecting brightness temperatures exceeding $10^{13}$ K. These observations are significant as they challenge the traditionally accepted inverse Compton cooling limit of about $10^{11.5}$ K for incoherent synchrotron emission in quasars, suggesting that relativistic boosting and other standard models may underestimate the actual brightness temperatures, which could verify at least $10^{13}$ K.

Previous studies of 3C 273 offered estimates of core brightness temperatures using ground-based VLBI techniques, but they lacked the necessary angular resolution to provide conclusive evidence beyond a few $10^{12}$ K. Kovalev et al.’s method significantly enhances observational sensitivity, thanks to the RadioAstron's extended baseline and space-based platform, debunking the notion that such high apparent brightness temperatures are mere observational artifacts or overestimations caused by interstellar scintillation.

Implications and Challenges

The findings pose several implications for theoretical models of quasar emissions:

• Doppler Factor Estimations: The traditional Doppler boosting factors derived from jet kinematics appear insufficient to explain the extreme brightness temperatures observed. This discrepancy indicates that either the jet speed measurements are inaccurate, or a different, possibly novel emission mechanism may be at work.
• Alternative Emission Mechanisms: The work suggests the potential need for models beyond incoherent synchrotron radiation, as existing models do not accommodate such high brightness temperatures. The authors consider mechanisms like synchrotron radiation from relativistic protons or coherent processes such as plasma waves, which could better align with observations.

Prospects for Future Research

While the paper provides robust data challenging existing thermal limits, it also opens questions requiring further investigation. This includes the need for multi-wavelength observations to discern the frequency-independent characteristics of brightness temperature limits. Future work should aim to extend these observations across numerous active galactic nuclei to validate the generality of these results in broader astrophysical contexts.

Additionally, complementary studies could explore theoretical expositions to resolve the apparent contradictions, reconciling observed jet velocities with the required high Doppler factors without necessitating extraneous or unconventional emission models. This could also involve more sophisticated simulations integrating both radiative and kinetic modeling of jets.

In conclusion, the research presented in this paper reshapes current understanding of quasar 3C 273’s core characteristics and presents a formidable challenge to astrophysics, urging a reevaluation of theoretical models in light of empirically derived constraints on brightness temperature in quasars. This paper acts as a catalytic point for expanding the observational prowess and theoretical insights into the behavior of active galactic nuclei and their fundamental physical processes.