MOJAVE: MONITORING OF JETS IN ACTIVE GALACTIC NUCLEI WITH VLBA EXPERIMENTS. XI. SPECTRAL DISTRIBUTIONS

By T. Hovatta, M. Aller, H. Aller, E. Clausen-Brown, D. Homan, Y. Kovalev, M. Lister, A. Pushkarev, T. Savolainen. 2014

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We have obtained milliarcsecond-scale spectral index distributions for a sample of 190 extragalactic radio jets through the Monitoring of Jets in Active Galactic Nuclei with the VLBA Experiments (MOJAVE) project. The sources were observed in 2006 at 8.1, 8.4, 12.1, and 15.4 GHz, and we have determined spectral index maps between 8.1 and 15.4 GHz to study the four-frequency spectrum in individual jet features. We have performed detailed simulations to study the effects of image alignment and (u, v)-plane coverage on the spectral index maps to verify our results. We use the spectral index maps to study the spectral index evolution along the jet and determine the spectral distributions in different locations of the jets. The core spectral indices are on average flat with a mean value of +0.22 ± 0.03 for the sample, while the jet spectrum is in general steep with a mean index of −1.04 ± 0.03. A simple power-law fit is often inadequate for the core regions, as expected if the cores are partially self-absorbed. The overall jet spectrum steepens at a rate of about −0.001 to −0.004 per deprojected parsec when moving further out from the core with flat spectrum radio quasars having significantly steeper spectra (mean −1.09 ± 0.04) than the BL Lac objects (mean −0.80 ± 0.05). However, the spectrum in both types of objects flattens on average by ∼0.2 at the locations of the jet components indicating particle acceleration or density enhancements along the jet. The mean spectral index at the component locations of −0.81 ± 0.02 corresponds to a power-law index of ∼2.6 for the electron energy distribution. We find a significant trend that jet components with linear polarization parallel to the jet (magnetic field perpendicular to the jet) have flatter spectra, as expected for transverse shocks. Compared to quasars, BL Lacs have more jet components with perpendicular magnetic field alignment, which may explain their generally flatter spectra. The overall steepening of the spectra with distance can be explained with radiative losses if the jets are collimating or with the evolution of the high-energy cutoff in the electron spectrum if the jets are conical. This interpretation is supported by a significant correlation with the age of the component and the spectral index, with older components having steeper spectra.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/a1e02ac31ac8b72fa6f532f21300fe09760f3317

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In Astronomy and Astrophysics

1. Bing Zhang
2. Y. Kovalev
3. A. Pushkarev
4. M. Lister
5. T. Savolainen
6. A. Lobanov
7. J. Zensus
8. S. Burke-Spolaor

In Nuclear and High Energy Physics

1. Y. Kovalev
2. A. Pushkarev
3. M. Lister
4. T. Savolainen
5. A. Lobanov
6. J. Zensus
7. D. Homan
8. M. Aller

Citations in the corpus

• The limb-brightened jet of M87 down to the 7 Schwarzschild radii scale (2018, methodology): “We also adopt a typical optically thin spectral index of the M 87 jet α=−1 (Hovatta et al. 2014).”
• A real-time fast radio burst: polarization detection and multiwavelength follow-up (2014, background): “Some AGN have been observed with more circular polarization than linear (Homan & Lister 2006), however the overall levels of CP were much lower (typically ≥ 0.3”
• The Structure and Dynamics of the Subparsec Jet in M87 Based on 50 VLBA Observations over 17 Years at 43 GHz (2018, methodology): “That result is based partly on the MOJAVE program (Lister & Homan 2005), a recent reanalysis of which gives hints of acceleration and mild superluminal motion (Britzen et al 2017).”
• MOJAVE. XV. VLBA 15 GHz Total Intensity and Polarization Maps of 437 Parsec-scale AGN Jets from 1996 to 2017 (2017, methodology): “In Lister & Homan (2005) we discussed the first epoch linear polarization maps of the original MOJAVE flux-density limited AGN sample, and we described their circular polarization properties in Homan & Lister (2006).”
• Detection of significant cm to sub-mm band radio and γ-ray correlated variability in Fermi bright blazars (2014, methodology): “Apparent jet speed measurements from the VLBI literature (e.g. Lister et al. 2009) are available for 42 sources in our sample.”
• MOJAVE. XIII. PARSEC-SCALE AGN JET KINEMATICS ANALYSIS BASED ON 19 YEARS OF VLBA OBSERVATIONS AT 15 GHz (2016, methodology): “A similar observing method was used in the full polarization MOJAVE observations from 2002 May to 2007 September (VLBA codes BL111, BL123, BL137, and BL149), and is described by Lister & Homan (2005).”
• Kinematics of the jet in M 87 on scales of 100–1000 Schwarzschild radii (2016, methodology): “In a recent publication, α = −1 was estimated for the radio emission on sub-parsec scales in the M 87 jet (Hovatta et al. 2014).”
• MOJAVE. X. PARSEC-SCALE JET ORIENTATION VARIATIONS AND SUPERLUMINAL MOTION IN ACTIVE GALACTIC NUCLEI (2013, background): “The latter data were recently presented by Hovatta et al. (2012) and Pushkarev et al. (2012).”
• THE SWIFT/BAT HARD X-RAY TRANSIENT MONITOR (2006, background): “…monitor (apart from blank sky points and provisional sources) contains 975 sources, covering most known hard X-ray sources, well-localized γ-ray sources, and a strong sampling of flare stars and active galaxies visible in the northern sky as monitored in the MOJAVE program (Lister et al 2009).”
• Jet opening angles and gamma-ray brightness of AGN (2009, methodology): “This research has made use of data from the MOJAVE database tht is maintained by the MOJAVE team (Lister et al. 2009a).”