[Paper Review] OVRO 40 m Blazar Monitoring Program: Location of the gamma-ray emission region in blazars by the study of correlated variability at radio and gamma-rays

W. Max-Moerbeck, J. L. Richards+6 more|arXiv (Cornell University)|Mar 8, 2013

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This study investigates the spatial location of gamma-ray emission in blazars by analyzing correlated variability between radio (15 GHz) and gamma-ray (100 MeV–200 GeV) emissions using data from the OVRO 40 m monitoring program and Fermi-LAT. Among 86 bright blazars, only one source (J0238+1636) shows >3σ significant radio-gamma-ray correlation at a lag of −30 ± 8 days, suggesting that gamma-ray emission may originate in the same region as the radio-emitting jet, though more data are needed to confirm this for the broader population.

ABSTRACT

Blazars are powerful, variable emitters from radio to gamma-ray wavelengths. Even though the general picture of synchrotron emission at low energies and inverse Compton at the high energy end is well established, many important aspects of these remarkable objects are still not well understood. For example, even the location of the gamma-ray emission region is still not clearly established, with some theories locating it close to the black hole/accretion disk while others place it at parsec scales in the radio jet. Since mid-2007 we have carried out a large scale monitoring program at 15 GHz using the OVRO 40 m telescope. We are currently observing about 1700 blazars twice per week. The sample includes all the Fermi-LAT detected blazars north of declination -20 degrees. Here, we study the existence of correlated variability between these two bands for 86 sources bright enough to be detected weekly by LAT. The existence of correlated variability can be interpreted as an indication of a related spatial locations for the radio and gamma-ray emission, making the evaluation of its statistical significance a key goal of our program. A study of the statistical significance of these cross-correlations is presented along with a discussion of the Monte Carlo simulations used to evaluate them. More information about the conditions on the radio emission zone can be obtained through polarization monitoring which tells us about the configuration of the magnetic fields in this region. To study radio polarization variability we are building KuPol, a radio polarization receiver for the 12 to 18 GHz band that will replace the current total power receiver at the OVRO 40 meter telescope.

Motivation & Objective

• To determine the spatial location of gamma-ray emission in blazars, which remains uncertain between the inner jet near the black hole and parsec-scale jet regions.
• To test whether correlated variability between radio and gamma-ray bands indicates a common emission region.
• To use statistical significance testing of cross-correlations to infer physical associations between emission zones.
• To improve long-term monitoring with new polarization measurements via the KuPol receiver to study magnetic field evolution.
• To extend the analysis to include major flaring events, such as the 2012 outburst of Mrk 421, to assess variability correlations on long timescales.

Proposed method

• Cross-correlation analysis of weekly binned radio light curves (15 GHz, OVRO 40 m) and gamma-ray light curves (100 MeV–200 GeV, Fermi-LAT) over 2008–2012.
• Use of the discrete correlation function (DCF) to handle uneven sampling in the light curves.
• Monte Carlo simulations (20,000 realizations) to estimate the distribution of random cross-correlation amplitudes and assess significance.
• Power spectral density (PSD) modeling with P(f) ∝ 1/f^β: β_radio = 2.3 (from OVRO sample), β_γ = 1.6 (from bright BL Lacs and FSRQs).
• Statistical significance evaluated via p-values derived from simulated random correlations, with thresholds at 2σ and 3σ.
• Incorporation of extended light curves including the 2012 Mrk 421 flare to assess lag consistency, though with caution due to post-hoc analysis.

Experimental results

Research questions

• RQ1 Is there a significant time lag between radio and gamma-ray variability in Fermi-detected blazars, indicating a physical connection between emission regions?
• RQ2 What is the statistical significance of observed cross-correlations between radio and gamma-ray light curves, after accounting for noise and uneven sampling?
• RQ3 Can the observed correlations constrain the location of the gamma-ray emission region—closer to the black hole or at parsec-scale distances in the jet?
• RQ4 How do major flaring events, such as the 2012 outburst of Mrk 421, affect the observed radio-gamma-ray correlation?
• RQ5 To what extent do polarization variations, as measured by the upcoming KuPol receiver, help trace magnetic field evolution in the emission region?

Key findings

• Among 86 bright Fermi-detected blazars, only 63 showed detectable variability in both radio and gamma-ray bands, limiting the sample for cross-correlation analysis.
• One source, J0238+1636, exhibited a >3σ significant cross-correlation peak at a lag of −30 ± 8 days, with a 99.9% confidence level, indicating potential co-location of radio and gamma-ray emission regions.
• Six additional sources showed >2σ correlations, but five were deemed unreliable due to high noise or slow trends, leaving only J1504+1029 as a credible candidate with a 98.5% significance at −40 ± 13 days.
• The 2012 major flare in Mrk 421 showed two significant cross-correlation peaks: one at 520 days (98.8% significance) and another at −40 days (96.3% significance), though the latter is subject to a posteriori statistical bias.
• The radio band PSD index was measured as β_radio = 2.3, while the gamma-ray band index was β_γ = 1.6, consistent with previous population studies.
• The development of KuPol, a new 12–18 GHz polarization receiver, will enable future studies of magnetic field structure and its role in emission variability.