The radio/gamma-ray connection from 120 MHz to 230 GHz

Radio loud active galactic nuclei are composed of different spatial features, each one characterized by different spectral properties in the radio band. Among them, blazars are the most common class of sources detected at gamma-rays by Fermi, and their radio emission is dominated by the flat spectrum compact core. In this contribution, we explore the connection between emission at high energy revealed by Fermi and at radio frequencies. Taking as a reference the strong and very highly significant correlation found between gamma rays and cm-λ radio emission, we explore the different behaviours found as we change the energy range in gamma rays and in radio, therefore changing the physical parameters of the zones involved in the emitted radiation. We find that the correlation weakens when we consider (1) gamma rays of energy above 10 GeV (except for high synchrotron peaked blazars) or (2) low frequency radio data taken by the Murchison Widefield Array; on the other hand, the correlation strengthens when we consider mm-λ data taken by Atacama Large Millimeter Array (ALMA)

Energetic Particles of Cosmic Accelerators I: Galactic Accelerators

The high-energy universe has revealed that energetic particles are ubiquitous in the cosmos and play a vital role in the cultivation of cosmic environments on all scales. Our pursuit of more than a century to uncover the origins and fate of these cosmic energetic particles has given rise to some of the most interesting and challenging questions in astrophysics. Energetic particles in our own galaxy, galactic cosmic rays (GCRs), engage in a complex interplay with the interstellar medium and magnetic fields in the galaxy, giving rise to many of its key characteristics. For instance, GCRs act in concert with galactic magnetic fields to support its disk against its own weight. GCR ionization and heating are essential ingredients in promoting and regulating the formation of stars and protostellar disks. GCR ionization also drives astrochemistry, leading to the build up of complex molecules in the interstellar medium. GCR transport throughout the galaxy generates and maintains turbulence in the interstellar medium, alters its multi-phase structure, and amplifies magnetic fields. GCRs could even launch galactic winds that enrich the circumgalactic medium and alter the structure and evolution of galactic disks. As crucial as they are for many of the varied phenomena in our galaxy, there is still much we do not understand about GCRs. While they have been linked to supernova remnants (SNRs), it remains unclear whether these objects can fully account for their entire population, particularly at the lower (approximately less than 1 GeV per nucleon) and higher (~PeV) ends of the spectrum. In fact, it is entirely possible that the SNRs that have been found to accelerate CRs merely re-accelerate them, leaving the origins of the original GCRs a mystery. The conditions for particle acceleration that make SNRs compelling source candidates are also likely to be present in sources such as protostellar jets, superbubbles, and colliding wind binaries (CWBs), but we have yet to ascertain their roles in producing GCRs. For that matter, key details of diffusive shock acceleration (DSA) have yet to be revealed, and it remains to be seen whether DSA can adequately explain particle acceleration in the cosmos. This White Paper is the first of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV gamma-ray astronomy will bring to understanding their energetic particle phenomena. For the case of GCRs, MeV astronomy will: 1) Search for fresh acceleration of GCRs in SNRs; 2) Test the DSA process, particularly in SNRs and CWBs; 3) Search for signs of CR acceleration in protostellar jets and superbubbles

A Panchromatic View of Relativistic Jets in Narrow-Line Seyfert 1 Galaxies

The discovery by the Large Area Telescope on board Fermi of variable γ-ray emission from radio-loud narrow-line Seyfert 1 (NLSy1) galaxies revealed the presence of a possible third class of Active Galactic Nuclei (AGN) with relativistic jets in addition to blazars and radio galaxies. Considering that NLSy1 are usually hosted in spiral galaxies, this finding poses intriguing questions about the nature of these objects and the formation of relativistic jets. We report on a systematic investigation of the γ-ray properties of a sample of radio-loud NLSy1, including the detection of new objects, using 7 years of Fermi-LAT data with the new Pass 8 event-level analysis. In addition we discuss the radio-to-very-high-energy properties of the γ-ray emitting NLSy1, their host galaxy, and black hole mass in the context of the blazar scenario and the unification of relativistic jets at different scales

Flaring γ-Ray Emission from High Redshift Blazars

High redshift blazars are among the most powerful objects in the Universe. Although they represent a significant fraction of the extragalactic hard X-ray sky, they are not commonly detected in γ-rays. High redshift ( z > 2 ) objects represent < 10 per cent of the active galactic nuclei (AGN) population observed by Fermi so far, and γ-ray flaring activity from these sources is even more uncommon. The characterization of the radio-to-γ-ray properties of high redshift blazars represents a powerful tool for the study of the energetics of such extreme objects and the Extragalactic Background Light. This contribution will present results of multi-band campaigns, from radio to γ-rays, on PKS 0836+710, PKS 2149−306, and TXS 0536+145. The latter is the highest redshift detection of a flaring γ-ray blazar so far. At the peaks of their respective flares these sources reached an apparent isotropic gamma-ray luminosity of about 10 50 erg·s − 1 , which is comparable with the luminosity observed from the most powerful blazars. The physical properties derived from the multi-wavelength observations of these sources are then compared with those shown by the high redshift population

Probing the Radio Counterpart of Gamma-ray Flaring Region in 3C 84

The radio source 3C 84 associated with the radio/giant elliptical galaxy NGC 1275 is one of the best targets to probe the radio counterpart of the γ-ray emitting region. Although this source shows clear time variability in γ-ray bands, no clear correlation in radio light curve was found on the timescale of –ray variability. The location of the γ-ray flaring region has been an open question. In this proceeding, we firstly review our previous findings from radio observations. Next we present our new results based on the Very Long Baseline Array (VLBA) data at 43 GHz. We discover the limb-brightened structure in the “restarted” jet associated with the 2005 radio outburst. In 1990s, the jet structure was rather ridge-brightening than limbbrightening, despite the observations were done with similar angular resolution. This indicates that the radio jet morphology in terms of the transverse structure has been indeed changed recently. This change in the morphology shows an interesting agreement with the time variation of the γ-ray flux density, i.e., the γ-ray flux density in 1990s was more than 7 times lower than the current one. We argue the possibility that the transition from ridge-brightening to limb-brightening is related to the γ-ray time variability

The optical-gamma correlation in BL Lacertae

We present multifrequency light curves of BL Lacertae from February 2008 to October 2012. Lowenergy data (optical and millimetre) were acquired in the framework of a GASP-WEBT project. High-energy data (ultraviolet, X-ray, and γ-ray) come from observations of the Swift, RXTE, and Fermi satellites. After a period of moderate activity, in May 2011 the source suddenly started to flare at γ and optical-UV frequencies. Activity at millimetre wavelengths and X rays began 3-4 months later. This behaviour offered a good opportunity to study the correlation among flux variability in different bands, in particular between the best-sampled optical and γ-ray light curves. However, even in this fortuitous case, we can only define a general correlation with likely no time lag, but with a lag uncertainty of ±1 day. Indeed, the data reveal a complex relationship between the γ and optical fluxes, which cannot be unveiled because of the small gaps in the sampling of this extremely variable source

Probing the Heart of Active Narrow-line Seyfert 1 Galaxies with VERA Wideband Polarimetry

We explored the parsec-scale nuclear regions of a sample of radio-loud narrow-line Seyfert 1 galaxies (NLSy1s) using the VLBI Exploration of Radio Astrometry wideband (at a recording rate of 16 Gbps) polarimetry at 22 and 43 GHz. Our targets include 1H 0323+342, SBS 0846+513, PMN J0948+0022, 1219+044, PKS 1502+036, and TXS 2116-077, which are all known to exhibit γ -ray emission indicative of possessing highly beamed jets similar to blazars. For the first time, we unambiguously detected Faraday rotation toward the parsec-scale radio core of NLSy1s, with a median observed core rotation measure (RM) of 2.7 × 10 ^3 rad m ^−2 (or 6.3 × 10 ^3 rad m ^−2 for redshift-corrected). This level of RM magnitude is significantly larger than those seen in the core of BL Lacertae objects (BLOs; a dominant subclass of blazars), suggesting that the nuclear environment of NLSy1s is more gas-rich than that in BLOs. Interestingly, the observed parsec-scale polarimetric properties of NLSy1s (low core fractional polarization, large core RM and jet–EVPA misalignment) are rather similar to those of flat-spectrum radio quasars (FSRQs). Our results are in accordance with the scenario that NLSy1s are in an early stage of active galactic nucleus evolution with their central black hole masses being smaller than those of more evolved FSRQs

Discovery of TXS 1515-273 at VHE gamma-rays and modelling of its Spectral Energy Distribution

In February 2019, a flaring state of the extreme blazar candidate TXS 1515-273 was registered by the Fermi-LAT, which triggered observations with the MAGIC telescopes and the X-ray satellites Swift, XMM-Newton and NuStar. The observations led to the discovery of the source at very-high-energy (VHE, 100 GeV < E < $ 100 TeV) gamma-ray energies and the detection of short time scales of variability (1 h) in several X-ray bands. The analysis of the observed variability helped us to constrain the physical parameters of the emission region. Thanks to the high-quality X-ray data, the synchrotron peak location was determined. The source was classified as a high synchrotron peaked source during the flaring activity. We constructed the broadband spectral energy distribution from radio to TeV. We interpreted it assuming leptonic emission and taking into account the constraints from the X-ray variability. We tested two scenarios: a simple one-zone model and a two-component model. Both models were found to describe the data well from X-ray to VHE gamma ray, but the two-zone model allows for a more accurate modelling of the emission at radio and optical energies.ISSN:1824-803

Multiwavelength observations in 2019-2020 of a new very-high-energy gamma-ray emitter: the flat spectrum radio quasar QSO B1420+326

The flat-spectrum radio quasar QSO B1420+326 underwent an enhanced γ-ray flux state seen by Fermi-LAT at the turn of 2019/2020. Compared to the low state both the position and luminosity of the two spectral energy distribution peaks changed by at least two orders of magnitude. The high state resulted in the discovery of the very-high-energy (>100 GeV) γ-ray emission from the source by the MAGIC telescopes. The organized multiwavelength campaign allow us to trace the broadband emission of the source through different phases of the flaring activity. The source was observed by 20 instruments in radio, near-infrared, optical, ultra-violet, X-ray and γ-ray bands. We use dedicated optical spectroscopy results to estimate the accretion disc and the dust torus luminosity. The optical spectroscopy shows a prominent FeII bump with flux evolving together with the continuum emission and a MgII line with varying equivalent width. The γ-ray flare was accompanied by a rotation of the optical polarization vector and emission of a new superluminal radio knot. We model spectral energy distributions in different flare phases in the framework of combined synchrotron-self-Compton and external Compton scenario in which the shape of the electron energy distribution is determined from cooling processes.ISSN:1824-803