Very High Energy Γ-ray Emission Research Articles

The Parsec-scale Jet of the Neutrino-emitting Blazar TXS 0506+056

Abstract Recently the IceCube Collaboration detected very high energy (VHE) neutrinos and associated them with the blazar TXS 0506+056, raising a possible association of VHE neutrinos with this and other individual blazars. Very long baseline interferometry (VLBI) is so far the only technique enabling the imaging of the innermost jet at milliarcsec resolution (parsec scale), where the high-energy emission possibly originates from. Here, we report on the radio properties of the parsec-scale jet in TXS 0506+056 derived from the analysis of multiepoch, multifrequency archive VLBI data. The half opening angle of the jet beam is about 3.°8, and the jet inclination angle is about 20°. The overall jet structure shows a helical trajectory with a precessing period of 5–6 yr, likely originating from instabilities operating at parsec scales. The calculated beaming parameters (Doppler boosting factor, bulk Lorentz factor) suggest a moderately relativistic jet. The parsec-scale magnetic field strength is estimated in the contexts of core shift and variability, and is in general agreement in the range of 0.2–0.7 G. And it is found to decrease from a relatively larger value during the quiescent period before the ongoing flare. This suggests a conversion of magnetic field energy density to particle energy density that helps accelerate injected particles at the jet base and results in variable shocked emission. The neutrino event could be associated with the onset of energetic particle injection into the jet. This scenario then supports the lepto-hadronic origin of the VHE neutrinos and γ-ray emission owing to a cospatial origin.

A Very High Energy γ-Ray Survey toward the Cygnus Region of the Galaxy

Abstract We present results from deep observations toward the Cygnus region using 300 hr of very high energy (VHE) γ-ray data taken with the VERITAS Cerenkov telescope array and over 7 yr of high-energy γ-ray data taken with the Fermi satellite at an energy above 1 GeV. As the brightest region of diffuse γ-ray emission in the northern sky, the Cygnus region provides a promising area to probe the origins of cosmic rays. We report the identification of a potential Fermi-LAT counterpart to VER J2031+415 (TeV J2032+4130) and resolve the extended VHE source VER J2019+368 into two source candidates (VER J2018+367* and VER J2020+368*) and characterize their energy spectra. The Fermi-LAT morphology of 3FGL J2021.0+4031e (the Gamma Cygni supernova remnant) was examined, and a region of enhanced emission coincident with VER J2019+407 was identified and jointly fit with the VERITAS data. By modeling 3FGL J2015.6+3709 as two sources, one located at the location of the pulsar wind nebula CTB 87 and one at the quasar QSO J2015+371, a continuous spectrum from 1 GeV to 10 TeV was extracted for VER J2016+371 (CTB 87). An additional 71 locations coincident with Fermi-LAT sources and other potential objects of interest were tested for VHE γ-ray emission, with no emission detected and upper limits on the differential flux placed at an average of 2.3% of the Crab Nebula flux. We interpret these observations in a multiwavelength context and present the most detailed γ-ray view of the region to date.

A search for very high-energy flares from the microquasars GRS 1915+105, Circinus X-1, and V4641 Sgr using contemporaneous H.E.S.S. and RXTE observations

Context. Microquasars are potential γ-ray emitters. Indications of transient episodes of γ-ray emission were recently reported in at least two systems: Cyg X-1 and Cyg X-3. The identification of additional γ-ray-emitting microquasars is required to better understand how γ-ray emission can be produced in these systems.Aim. Theoretical models have predicted very high-energy (VHE) γ-ray emission from microquasars during periods of transient outburst. Observations reported herein were undertaken with the objective of observing a broadband flaring event in the γ-ray and X-ray bands.Methods. Contemporaneous observations of three microquasars, GRS 1915+105, Circinus X-1, and V4641 Sgr, were obtained using the High Energy Spectroscopic System (H.E.S.S.) telescope array and the Rossi X-ray Timing Explorer (RXTE) satellite. X-ray analyses for each microquasar were performed and VHE γ-ray upper limits from contemporaneous H.E.S.S. observations were derived.Results. No significant γ-ray signal has been detected in any of the three systems. The integral γ-ray photon flux at the observational epochs is constrained to be I(>560 GeV) < 7.3 × 10−13 cm−2 s−1, I(>560 GeV ) < 1.2 × 10−12 cm−2 s−1, and I(>240 GeV) < 4.5 × 10−12 cm−2 s−1 for GRS 1915+105, Circinus X-1, and V4641 Sgr, respectively.Conclusions. The γ-ray upper limits obtained using H.E.S.S. are examined in the context of previous Cherenkov telescope observations of microquasars. The effect of intrinsic absorption is modelled for each target and found to have negligible impact on the flux of escaping γ-rays. When combined with the X-ray behaviour observed using RXTE, the derived results indicate that if detectable VHE γ-ray emission from microquasars is commonplace, then it is likely to be highly transient.

MAGIC highlights

The present generation of Imaging Air Cherenkov Telescopes (IACTs) has greatly improved our knowledge on the Very High Energy (VHE) side of our Universe. The MAGIC IACTs operate since 2004 with one telescope and since 2009 as a two telescope stereoscopic system. I will outline a few of our latest and most relevant results: the discovery of pulsed emission from the Crab pulsar at VHE, recently found to extend up to 400 GeV and along the “bridge” of the light curve, the measurement of the Crab nebula spectrum over three decades of energy, the discovery of VHE γ-ray emission from the PWN 3C 58, the very rapid emission of IC 310, in addition to dark matter studies. The results that will be described here and the planned deep observations in the next years will pave the path for the future generation of IACTs.

IMPLICATIONS OF THE VERY HIGH ENERGY GAMMA-RAY DETECTION OF THE QUASAR 3C279

The Major Atmospheric Gamma-Ray Imaging Cherenkov Telescope very high energy (VHE) γ-ray astronomy collaboration recently reported the detection of the quasar 3C279 at >100 GeV γ-ray energies. Here, we present simultaneous optical (BVRI) and X-ray (RXTE PCA) data from the day of the VHE detection and discuss the implications of the snapshot spectral energy distribution for jet models of blazars. A one-zone synchrotron-self-Compton (SSC) origin of the entire spectral energy distribution (SED), including the VHE γ-ray emission is highly problematic as it would require an unrealistically low magnetic field. The measured level of VHE emission could, in principle, be interpreted as Compton upscattering of external radiation (e.g., from the broad-line regions, BLRs). However, such an interpretation would require either an unusually low magnetic field of B ∼ 0.03 G, or (in order to achieve approximate equipartition between magnetic field at B ∼ 0.25 G and relativistic electrons) an unrealistically high Doppler factor of Γ ∼ 140. In addition, such a model fails to reproduce the observed X-ray flux. Furthermore, both versions of leptonic one-zone models produce intrinsic VHE γ-ray spectra steeper than measured, even in the case of the lowest plausible extragalactic γγ absorption. We therefore conclude that a simple one-zone, homogeneous leptonic jet model is not able to plausibly reproduce the SED of 3C279 including the recently detected VHE γ-ray emission. This as well as the lack of correlated variability in the optical with the VHE γ-ray emission and the substantial γγ opacity of the BLR radiation field to VHE γ-rays suggests a multi-zone model in which the optical emission is produced in a different region than the VHE γ-ray emission. In particular, an SSC model with an emission region far outside the BLR reproduces the simultaneous X-ray–VHE γ-ray spectrum of 3C279. Alternatively, a hadronic model is capable of reproducing the observed SED of 3C279 reasonably well, both in scenarios in which only the internal synchrotron field serves as targets for pγ pion production, and with a substantial contribution from external photons, e.g., from the BLR. However, either version of the hadronic model requires a rather extreme jet power of up to Lj ∼ 1049 erg s−1, compared to a requirement of Lj ∼ 2 × 1047 erg s−1 for a multi-zone leptonic model.

Discovery of Very High Energy γ-Ray Emission from the Low-Frequency-peaked BL Lacertae Object BL Lacertae

The MAGIC collaboration observed BL Lacertae for 22.2 hr during 2005 August to December and for 26 hr during 2006 July to September. The source is the historical prototype and eponym of a class of low-frequency-peaked BL Lacertae (LBL) objects. A very high energy (VHE) γ-ray signal was discovered with a 5.1 σ excess in the 2005 data. Above 200 GeV, an integral flux of (0.6 ± 0.2) × 10-11 cm-2 s-1 was measured, corresponding to approximately 3% of the Crab flux. The differential spectrum between 150 and 900 GeV is rather steep with a photon index of -3.6 ± 0.5. The light curve shows no significant variability during the observations in 2005. For the first time a clear detection of VHE γ-ray emission from an LBL object was obtained with a signal below previous upper limits. The 2006 data show no significant excess. This drop in flux follows the observed trend in optical activity.

MODELING THE MULTIWAVELENGTH SPECTRA AND VARIABILITY OF 3C 66A IN 2003–2004

The BL Lac object 3C 66A was the target of an intensive multiwavelength monitoring campaign organized in 2003–2004. During the campaign, its spectral energy distribution (SED) was measured and flux measurements from radio to X-ray frequencies as well as upper limits in the very high energy (VHE) γ-ray regime were obtained. Here, we reproduce the SED and optical spectral variability pattern observed during our multiwavelength campaign using a time-dependent leptonic jet model. Our model could successfully simulate the observed SED and optical light curves and predict an intrinsic cutoff value for the VHE γ-ray emission at ~4 GeV implying the effect of the optical depth due to the intergalactic infrared background radiation (IIBR) to be negligible. Also, the contribution of external Comptonization (EIC), due to the presence of a broad-line region (BLR), in the emission of γ-ray photons could be significant early-on when the emission region is very close to the central engine but as it travels farther out, the production mechanism of hard X-ray and γ-ray photons becomes dominated by synchrotron self-Compton mechanism (SSC).

Modeling the Spectral Energy Distribution and Variability of 3C 66A during the WEBT Campaign of 2003–2004

The BL Lac object 3C 66A was observed in an extensive multiwavelength monitoring campaign (the Whole Earth Blazar Telescope [WEBT] campaign) from 2003 July until 2004 April. The spectral energy distribution (SED) was measured over the entire electromagnetic spectrum, with flux measurements from radio to X-ray frequencies and upper limits in the very high energy (VHE) γ-ray regime. Here we use a time-dependent leptonic jet model to reproduce the SED and optical spectral variability observed during our multiwavelength campaign. Our model simulations could successfully reproduce the observed SED and optical light curves and predict an intrinsic cutoff value for the VHE γ-ray emission at ~4 GeV. The effect of the optical depth due to the intergalactic infrared background radiation (IIBR) on the peak of the high-energy component of 3C 66A was found to be negligible. Also, the presence of a broad-line region (BLR) in the case of 3C 66A may play an important role in the emission of γ-ray photons when the emission region is very close to the central engine, but farther out, the production mechanism of hard X-ray and γ-ray photons becomes rapidly dominated by synchrotron self-Compton emission. We further discuss the possibility of an observable X-ray spectral variability pattern. The simulated results do not predict observable hysteresis patterns in the optical or soft X-ray regimes for major flares on multiday timescales.

Constraints on the geometry of the VHE emission in LS 5039 from photon–photon deabsorption

Evidence for orbital modulation of the very high energy (VHE) γ-ray emission from the high-mass X-ray binary and microquasar LS 5039 has recently been reported by the HESS collaboration. The observed flux modulation was found to go in tandem with a change in the GeV–TeV spectral shape, which may partially be a result of γγ absorption in the intense radiation field of the massive companion star. However, it was suggested that γγ absorption effects alone cannot be the only cause of the observed spectral variability since the flux at ∼200 GeV, which is near the minimum of the expected γγ absorption trough, remained essentially unchanged between superior and inferior conjunction of the binary system. In this paper, a detailed parameter study of the γγ absorption effects in this system is presented. For a range of plausible locations of the VHE γ-ray emission region and the allowable range of viewing angles, the deabsorbed, intrinsic VHE γ-ray spectra and total VHE photon fluxes and luminosities are calculated and compared to luminosity constraints based on Bondi-Hoyle limited wind accretion onto the compact object in LS 5039. Based on these arguments, it is found that (1) it is impossible to choose the viewing angle and location of the VHE emission region in a way that the intrinsic (deabsorbed) fluxes and spectra in superior and inferior conjunction are identical; consequently, the intrinsic VHE luminosities and spectral shapes must be fundamentally different in different orbital phases, (2) if the VHE luminosity is limited by wind accretion from the companion star and the system is viewed at an inclination angle of i ≳ 40°, the emission is most likely beamed by a larger Doppler factor than inferred from the dynamics of the large-scale radio outflows, (3) the still poorly constrained viewing angle between the line of sight and the jet axis is most likely substantially smaller than the maximum of ∼64° inferred from the lack of eclipses. (4) Consequently, the compact object is more likely to be a black hole rather than a neutron star. (5) There is a limited range of allowed configurations for which the expected VHE neutrino flux would actually anti-correlate with the observed VHE γ-ray emission. If hadronic models for the γ-ray production in LS 5039 apply, a solid detection of the expected VHE neutrino flux and its orbital modulation with km 3 scale water-Cherenkov neutrino detectors might require the accumulation of data over more than 3 years.

Discovery of the binary pulsar PSR B1259-63 in very-high-energy gamma rays around periastron with HESS

We report the discovery of very-high-energy (VHE) γ-ray emission of the binary system PSR B1259−63/SS 2883 of a radio pulsar orbiting a massive, luminous Be star in a highly eccentric orbit. The observations around the 2004 periastron passage of the pulsar were performed with the four 13 m Cherenkov telescopes of the HESS experiment, recently installed in Namibia and in full operation since December 2003. Between February and June 2004, a γ-ray signal from the binary system was detected with a total significance above 13σ. The flux was found to vary significantly on timescales of days which makes PSR B1259−63 the first variable galactic source of VHE γ-rays observed so far. Strong emission signals were observed in pre- and post-periastron phases with a flux minimum around periastron, followed by a gradual flux decrease in the months after. The measured time-averaged energy spectrum above a mean threshold energy of 380 GeV can be fitted by a simple power law F0(E/ 1T eV) −Γ with a photon index Γ= 2.7 ± 0.2stat ± 0.2sys and flux normalisation F0 = (1.3 ± 0.1stat ± 0.3sys) × 10 −12 TeV −1 cm −2 s −1 .T his detection of VHE γ-rays provides unambiguous evidence for particle acceleration to multi-TeV energies in the binary system. In combination with coeval observations of the X-ray synchrotron emission by the RXTE and INTEGRAL instruments, and assuming the VHE γ-ray emission to be produced by the inverse Compton mechanism, the magnetic field strength can be directly estimated to be of the order of 1 G.

Propeller spin-down and the non-thermal emission from AE Aquarii

To explain the observed multiwavelength emission from the cataclysmic variable star AE Aquarii, we propose a model based upon the assumption that the compact component of this binary consists of a white dwarf with a surface field strength of 1 MG, rotating through a clumpy ring near the circularization radius. We argue that MHD instabilities like the Kelvin–Helmholtz instabilities will result in the white dwarf field mixing with the gas in this region and becoming highly sheared. The Poynting flux pumped into the region, and hence angular momentum transferred in the process, is enough to accelerate material centrifugally from the system. We also show that magnetic reconnection in this region will result in runaway electron acceleration to energies of 300 MeV or more, which will radiate through the synchrotron process up to frequencies of the order of ν∼1015 Hz. We show that, because the radiation is emitted in a gaseous medium, the spectrum will be suppressed below the so-called Razin–Tsytovich frequency, which is νR∼2000 GHz, which agrees with the observations. Magnetic shear in the ejection region will also induce field-aligned currents in the magnetosphere. Huge potential differences (double layers) can be generated in the circuit where the gas density becomes very low. This will occur close to the white dwarf. In this region potentials of the order of Φ∥=300 TV (TV = teravolt) can be generated. Synchrotron losses on electrons will restrict them to energies of the order of ℰ50 keV. As protons and ions are not affected by synchrotron losses, energies in excess of 1 TeV are possible. Very high-energy (VHE) γ-ray emission can be produced if these proton and ion beams collide with the clumpy gas in, or outside, the ejection zone. We show that pulsed VHE γ-ray emission is possible, if the gas particle density of the target matter Ng>1013 cm−3, which is compatible with the typical gas particle densities Ng∼1016 cm−3 of the blobs inside the clumpy accretion stream from the secondary star. We show that the VHE γ-ray bursts that were reported by two groups can be explained in terms of an exploding double layer, which will result in a cataclysmic burst of VHE protons and ions over short time-scales. The total power released in the process can be Pγ,b∼1034 erg s−1, which is equal to the spin-down power of the white dwarf, and is enough to explain the reported VHE γ-ray bursts. This implies that the whole reservoir of spin-down power has gone into the production of VHE particles and γ-ray emission.