Exploring the bulk of the BL Lacertae object population

Keane et al. (2016) have recently reported the discovery of a new fast radio burst, FRB150418, with a promising radio counterpart at 5.5 and 7.5 GHz -a rapidly decaying source, falling from 200-300 µJy to 100 µJy on timescales of ∼6 d. This transient source may be associated with an elliptical galaxy at redshift z = 0.492, providing the first firm spectroscopic redshift for a FRB and the ability to estimate the density of baryons in the intergalactic medium via the combination of known redshift and radio dispersion of the FRB. An alternative explanation, first suggested by Williams & Berger (2016b), is that the identified counterpart may instead be a compact AGN. The putative counterpart's variation may then instead be extrinsic, caused by refractive scintillation in the ionized interstellar medium of the Milky Way, which would invalidate the association with FRB150418. We examine this latter explanation in detail and show that the reported observations are consistent with scintillating radio emission from the core of a radio-loud active galactic nucleus (AGN) having a brightness temperature T b 10 9 K. Using numerical simulations of the expected scattering for the line of sight to FRB150418, we provide example images and light curves of such an AGN at 5.5 and 7.5 GHz. These results can be compared with continued radio monitoring to conclusively determine the importance of scintillation for the observed radio variability, and they show that scintillation is a critical consideration for continued searches for FRB counterparts at radio wavelengths.

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“…Falcke et al 2004). This radio power is also consistent with the low-power end of the known blazars (Liuzzo et al 2013;Massaro et al 2015).…”

Section: The Origin Of J0716-1900supporting

confidence: 80%

Interstellar Scintillation and the Radio Counterpart of the Fast Radio Burst FRB 150418

Akiyama

Johnson

2016

ApJL

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“…In addition, the accurate match of the source positions measured with VLBI and Gaia, and the high compactness (the ratio of the VLBI and VLA flux densities close to 1) imply that the jet is oriented at a small viewing angle (e.g. Liuzzo et al 2013). However, the VLA experiment (Schinzel et al 2017) was carried out more than 2 years before our EVN observations.…”

Section: Discussionmentioning

confidence: 99%

The loud and the quiet: searching for radio counterparts of two radio-weak BL Lac candidates with VLBI

Cao

Frey

Gabányi

et al. 2018

Monthly Notices of the Royal Astronomical Society: Letters

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BL Lac objects are known to have compact jets inclined to our line of sight at a small angle, showing prominent radio emission. Two radio-weak BL Lac candidates with no counterparts in current radio surveys were recently reported by Massaro et al. (2017). Both sources were selected as candidate low-energy counterparts of unassociated Fermi γ-ray sources. We carried out very long baseline interferometry (VLBI) observations with the European VLBI Network (EVN) at 5 GHz to explore their radio properties at milli-arcsecond (mas) scale. One target, J1410+7405, is clearly detected with the EVN. Its measured 5-GHz flux density, 2.4 mJy, is consistent with recent interferometric measurements with the Karl G. Jansky Very Large Array, suggesting that the radio emission is confined to the inner < ∼ 10 mas region. J1410+7405 is therefore identified as a radio-loud jetted active galactic nucleus, and its brightness temperature exceeds ∼ 10 9 K. Its properties are similar to those of other γ-ray detected BL Lac objects. On the other hand, the second target, J0644+6031, remains undetected with the EVN with a 6-σ brightness upper limit of 0.12 mJy beam −1 . This source is thus radio-quiet, confirming its peculiarity, or possibly questioning its BL Lac nature.

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“…The first one was identified as a BL Lac because of γ-ray emission detected by the EGRET instrument on-board the Compton Gamma-Ray Observatory, but it is not present in the third Fermi Large Area Telescope source catalog. Furthermore, Liuzzo et al (2013) did not detect it with the VLA at 8.4 GHz, so we consider it a dubious association. The above analysis leads to the conclusion that in the 0.1 < z ≤ 0.15 redshift range, our selection method recovered all BZB but two sources characterized by a high Dn(4000) value.…”

Section: Discussionmentioning

confidence: 67%

Looking for the least luminous BL Lacertae objects

Capetti

Raiteri

2015

A&A

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Among active galactic nuclei, BL Lac objects show extreme properties that have been interpreted as the effect of relativistic beaming on the emission from a plasma jet oriented close to the line of sight. The Doppler amplification of the jet emission makes them ideal targets for studying jet physics. In particular, low-power BL Lacs (LPBL) are very interesting because they probe the jet formation and emission processes at the lowest levels of accretion. However, they are difficult to identify since their emission is swamped by the radiation from the host galaxy in most observing bands. In this paper we propose a new LPBL selection method based on the mid-infrared emission, in addition to the traditional optical indices. We considered the radio-selected sample of Best & Heckman (2012, MNRAS, 421, 1569 and cross-matched it with the WISE all-sky survey. In a new diagnostic plane including the W2−W3 color and the Dn(4000) index, LPBL are located in a region scarcely populated by other sources. By filtering objects with small emission line equivalent width, we isolated 36 LPBL candidates up to redshift 0.15. Their radio luminosity at 1.4 GHz spans the range log L r = 39.2−41.5 [erg s−1 ]. Considering the completeness of our sample, we analyzed the BL Lac luminosity function (RLF), finding a dramatic paucity of LPBL with respect to the extrapolation of the RLF toward low power. This requires a break in the RLF located at log L r ∼ 40.6 [erg s−1 ]. The consequent peak in the BL Lacs number density is possibly the manifestation of a minimum power required to launch a relativistic jet.

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Exploring the bulk of the BL Lacertae object population

Cited by 14 publications

References 47 publications

Interstellar Scintillation and the Radio Counterpart of the Fast Radio Burst FRB 150418

Interstellar Scintillation and the Radio Counterpart of the Fast Radio Burst FRB 150418

The loud and the quiet: searching for radio counterparts of two radio-weak BL Lac candidates with VLBI

Looking for the least luminous BL Lacertae objects

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