TeV Emission Research Articles

Optical spectroscopy of BL Lac objects: TeV candidates

ABSTRACT We investigate the spectroscopic optical properties of gamma-ray sources detected with high significance above 50 GeV in the Third Catalog of Hard Fermi-LAT Sources and that are good candidates as TeV emitters. We focus on the 91 sources that are labelled by the Fermi team as BL Lac (BLL) objects or blazar candidates of uncertain type (BCUs), are in the Northern hemisphere, and are with unknown or uncertain redshift. We report here on GTC (Gran Telescopio Canarias) spectra (in the spectral range 4100–7750 Å) of 13 BCUs and 42 BLL objects. We are able to classify the observed targets as BLL objects and each source is briefly discussed. The spectra allowed us to determine the redshift of 25 objects on the basis of emission and/or absorption lines, finding 0.05 < z < 0.91. Most of the emission lines detected are due to forbidden transition of [O iii] and [N ii]. The observed line luminosity is found to be lower than that of quasi-stellar objects (QSOs) at similar continuum and could be reconciled with the line–continuum luminosity relationship of QSOs if a significant beaming factor is assumed. Moreover, for five sources we found intervening absorption lines that allow to set a spectroscopic lower limit of the redshift. For the remaining 25 sources, for which the spectra are lineless, a lower limit to z is given, assuming that the host galaxies are giant ellipticals.

On the Origin of the Gamma-Ray Emission toward SNR CTB 37A with Fermi-LAT

Abstract The middle-aged supernova remnant (SNR) CTB 37A is known to interact with several dense molecular clouds through the detection of shocked H2 and OH 1720 MHz maser emission. In the present work, we use eight years of Fermi-LAT Pass 8 data, with an improved point-spread function and an increased acceptance, to perform detailed morphological and spectral studies of the γ-ray emission toward CTB 37A from 200 MeV to 200 GeV. The best fit of the source extension is obtained for a very compact Gaussian model with a significance of 5.75σ and a 68% containment radius of 0.°116 ± 0.°014stat ± 0.°017sys above 1 GeV, which is larger than the TeV emission size. The energy spectrum is modeled as a LogParabola, resulting in a spectral index α = 1.92 ± 0.19 at 1 GeV and a curvature β = 0.18 ± 0.05, which becomes softer than the TeV spectrum above 10 GeV. The SNR properties, including a dynamical age of 6000 yr, are derived assuming the Sedov phase. From the multiwavelength modeling of emission toward the remnant, we conclude that the nonthermal radio and GeV emission is mostly due to the reacceleration of preexisting cosmic rays (CRs) by radiative shocks in the adjacent clouds. Furthermore, the observational data allow us to constrain the total kinetic energy transferred to the trapped CRs in the clouds. Based on these facts, we infer a composite nature for CTB 37A to explain the broadband spectrum and to elucidate the nature of the observed γ-ray emission.

Striped Blandford/Znajek jets from advection of small-scale magnetic field

ABSTRACT Black hole – accretion disc systems are the central engines of relativistic jets from stellar to galactic scales. We numerically quantify the unsteady outgoing Poynting flux through the horizon of a rapidly spinning black hole endowed with a rotating accretion disc. The disc supports small-scale, concentric, flux tubes with zero net magnetic flux. Our general relativistic force-free electrodynamics simulations follow the accretion on to the black hole over several hundred dynamical time-scales in 3D. For the case of counter-rotating accretion discs, the average process efficiency reaches up to 〈ϵ〉 ≈ 0.43, compared to a stationary energy extraction by the Blandford/Znajek process. The process efficiency depends on the cross-sectional area of the loops, i.e. on the product l × h, where l is the radial loop thickness and h its vertical scale height. We identify a strong correlation between efficient electromagnetic energy extraction and the quasi-stationary setting of ideal conditions for the operation of the Blandford/Znajek process (e.g. optimal field line angular velocity and fulfillment of the so-called Znajek condition). Remarkably, the energy extraction operates intermittently (alternating episodes of high and low efficiency) without imposing any large-scale magnetic field embedding the central object. Scaling our results to supermassive black holes, we estimate that the typical variability time-scale of the system is of the order of days to months. Such time-scales may account for the longest variability scales of TeV emission observed, e.g. in M87.

Halo fraction in TeV-bright pulsar wind nebulae

The discovery of extended TeV emission around the Geminga and PSR B0656+14 pulsars, with properties consistent with free particle propagation in the interstellar medium (ISM), has led to the suggestion of “TeV halos” as a separate source class, which is distinct from pulsar wind nebulae. This has sparked considerable discussion on the possible presence of such halos in other systems. In defining halos as regions where the pulsar no longer dominates the dynamics of the interstellar medium, yet where an over-density of relativistic electrons is present, we make an assessment of the current TeV source population associated with energetic pulsars in terms of size and estimated energy density. Based on two alternative estimators, we conclude that a large majority of the known TeV sources have emission originating in the zone that is energetically and dynamically dominated by the pulsar (i.e. the pulsar wind nebula), rather than from a surrounding halo of escaped particles diffusing into the ISM. Furthermore, whilst the number of established halos will surely increase in the future since there is a known large population of older, less energetic pulsars, we find that it is unlikely that such halos contribute significantly to the total TeV γ-ray luminosity from electrons accelerated in pulsar wind nebulae due to their lower intrinsic surface brightness.

Constraints on the Intergalactic Magnetic Field from Bow Ties in the Gamma-Ray Sky

Abstract Pair creation on the cosmic infrared background and subsequent inverse-Compton scattering on the cosmic microwave background potentially reprocesses the TeV emission of blazars into faint GeV halos with structures sensitive to intergalactic magnetic fields (IGMF). Previous work has shown that these halos are then highly anisotropic and extended. If the coherence length of the IGMF is greater than the inverse-Compton cooling length of the pairs, then the orientation of the gamma-ray halo will be correlated with the direction of the magnetic field which is unknown and expected to change for each source. In order to constructively add each source we then use angular power spectra which are insensitive to the jet orientation. By looking at known GeV blazars detected by Fermi, we exclude the existence of an IGMF with coherence lengths >100 Mpc at greater than 3.9σ with current-day strengths in the range 10−16 to 10−15 G, and at 2σ from 10−17 to 10−14 G. This provides a direct measurement of the nonexistence of gamma-ray halos, providing an important check on previous results.

Discovery of a Spatially Extended GeV Source in the Vicinity of the TeV Halo Candidate 2HWC J1912+099: a TeV Halo or Supernova Remnant?

Abstract Observations by the High-Altitude Water Cherenkov Observatory (HAWC) and Milagro have detected spatially extended TeV sources surrounding middle-aged (t ∼ 100–400 kyr) pulsars like Geminga and PSR B0656+14, which have been named “TeV halos,” representing very extended TeV pulsar wind nebulae (PWNe) powered by relatively old pulsars. A few more HAWC-detected sources have been suggested to be TeV halo candidates. In this paper, we search for possible GeV counterparts of three TeV halo candidates with Fermi Large Area Telescopes. We detect a new spatially extended GeV source in the vicinity of the TeV halo candidate 2HWC J1912+099, which is also detected by the High Energy Stereoscopic System (H.E.S.S.) (HESS J1912+101). We find that the size of the GeV source is significantly larger than that of the TeV emission measured by H.E.S.S., and a spatial template characteristic of a PWN can fit the GeV data. We suggest that the GeV source is an extended PWN powered by the central middle-aged pulsar PSR J1913+1011. This discovery favors the TeV halo scenario for the TeV source 2HWC J1912+099 (HESS J1912+101), although the possible shell-like morphology measured by H.E.S.S. challenges this interpretation. Alternatively, the TeV emission could be dominated by a supernova remnant via the hadronic process. Future multi-wavelength observations of the source and more precise measurements of the spatial profile of the TeV emission will be useful to distinguish between the two scenarios.

Arcminute-scale studies of the interstellar gas towards HESS J1804−216: Still an unidentified TeVγ-ray source

AbstractThe Galactic TeV${\gamma}$-ray source${\mathrm{HESS\,J}1804{-}216}$is currently an unidentified source. In an attempt to unveil its origin, we present here the most detailed study of interstellar gas using data from the Mopra Southern Galactic Plane CO Survey, 7- and 12-mm wavelength Mopra surveys and Southern Galactic Plane Survey of HI. Several components of atomic and molecular gas are found to overlap${\mathrm{HESS\,J}1804{-}216}$at various velocities along the line of sight. The CS(1-0) emission clumps confirm the presence of dense gas. Both correlation and anti-correlation between the gas and TeV${\gamma}$-ray emission have been identified in various gas tracers, enabling several origin scenarios for the TeV${\gamma}$-ray emission from${\mathrm{HESS\,J}1804{-}216}$. For a hadronic scenario,${\mathrm{SNR\,G}8.7{-}0.1}$and the progenitor supernova remnant (SNR) of${\mathrm{PSR\,J}1803{-}2137}$require cosmic ray (CR) enhancement factors of${\mathord{\sim} 50}$times the solar neighbour CR flux value to produce the TeV${\gamma}$-ray emission. Assuming an isotropic diffusion model, CRs from both these SNRs require a slow diffusion coefficient, as found for other TeV SNRs associated with adjacent ISM gas. The morphology of gas located at 3.8 kpc (the dispersion measure distance to${\mathrm{PSR\,J}1803{-}2137}$) tends to anti-correlate with features of the TeV emission from${\mathrm{HESS\,J}1804{-}216}$, making the leptonic scenario possible. Both pure hadronic and pure leptonic scenarios thus remain plausible.

Radio study of the extended TeV source VER J1907+062

ABSTRACT This paper aims to provide new insights on the origin of the TeV source VER J1907+062 through new high-quality radio observations. We used the Karl G. Jansky Very Large Array (VLA) to observe the whole extension of VER J1907+062 at 1.5 GHz with a mosaicking technique and the PSR J1907+0602 in a single pointing at 6 GHz. These data were used together with 12CO and atomic hydrogen observations obtained from public surveys to investigate the interstellar medium in the direction of VER J1907+062. The new radio observations do not show any evidence of a pulsar wind nebula (PWN) driven by the pulsars present in the field and no radio counterpart to the proposed X-ray PWN powered by PSR J1907+0602 is seen in the new VLA image at 6 GHz down to a noise level of 10 $\mu$Jy beam−1. Molecular clouds were discovered over the eastern, southern, and western borders of the radio shell of G40.5−0.5, suggesting an association with this supernova remnant. We explored several scenarios for the origin of VER J1907+062. We propose as the most probable scenario one in which the TeV emission is produced by two separated γ-ray sources located at different distances: one of leptonic origin and associated with a PWN powered by PSR J1907+0602 at ∼3.2 kpc and another of hadronic origin and produced by the interaction between G40.5−0.5 and the surrounding molecular gas at ∼8.7 kpc.

A two-fluid model for black-hole accretion flows: Particle acceleration, outflows, and TeV emission

ABSTRACT The multi-wavelength spectrum observed from M87 extends from radio wavelengths up to TeV γ-ray energies. The radio through GeV components have been interpreted successfully using SSC models based on misaligned blazar jets, but the origin of the intense TeV emission detected during flares in 2004, 2005, and 2010 remains puzzling. It has been previously suggested that the TeV flares are produced when a relativistic proton jet originating in the core of M87 collides with a molecular cloud (or stellar atmosphere) located less than one parsec from the central black hole. We explore this scenario in detail here using a self-consistent model for the acceleration of relativistic protons in a shocked, two-fluid ADAF accretion disc. The relativistic protons accelerated in the disc escape to power the observed jet outflows. The distribution function for the jet protons is used to compute the TeV emission produced when the jet collides with a cloud or stellar atmosphere. The simulated broadband radiation spectrum includes radio, X-ray, and GeV components generated via synchrotron, as well as TeV emission generated via the production and decay of muons, positrons, and electrons. The self-consistency of the model is verified by computing the relativistic particle pressure using the distribution function, and comparing it with the relativistic particle pressure obtained from the hydrodynamical model. We demonstrate that the model is able to reproduce the multi-wavelength spectrum from M87 observed by VERITAS and HESS during the high-energy flares in 2004, 2005, and 2010.

VER J2227+608: A Hadronic PeVatron Pulsar Wind Nebula?

Abstract We report the detection of GeV γ-ray emission from the very-high-energy γ-ray source VER J2227+608 associated with the “tail” region of supernova remnant (SNR) G106.3+2.7. The GeV γ-ray emission is extended and spatially coincident with molecular clouds traced by CO emission. The broadband GeV to TeV emission of VER J2227+608 can be well fitted by a single power-law function with an index of 1.90 ± 0.04, without obvious indication of spectral cutoff toward high energies. The pure leptonic model for the γ-ray emission can be marginally ruled out by the X-ray and TeV data. In the hadronic model, the low energy content of CRs and the hard γ-ray spectrum, in combination with the center bright source structure, suggest that VER J2227+608 may be powered by the Pulsar wind nebula instead of shocks of the SNR. And the cutoff energy of the proton distribution needs to be higher than ∼400 TeV, which makes it an attractive PeVatron candidate. Future observations by the upcoming Large High Altitude Air Shower Observatory and the Cherenkov Telescope Array in the north could distinguish these models and constrain the maximum energy of cosmic rays in SNRs.

Gravitational-wave follow-up with CTA after the detection of GRBs in the TeV energy domain

ABSTRACT The recent discovery of TeV emission from gamma-ray bursts (GRBs) by the MAGIC and H.E.S.S. Cherenkov telescopes confirmed that emission from these transients can extend to very high energies. The TeV energy domain reaches the most sensitive band of the Cherenkov Telescope Array (CTA). This newly anticipated, improved sensitivity will enhance the prospects of gravitational-wave follow-up observations by CTA to probe particle acceleration and high-energy emission from binary black hole and neutron star mergers, and stellar core-collapse events. Here we discuss the implications of TeV emission on the most promising strategies of choice for the gravitational-wave follow-up effort for CTA and Cherenkov telescopes more broadly. We find that TeV emission (i) may allow more than an hour of delay between the gravitational-wave event and the start of CTA observations; (ii) enables the use of CTA’s small size telescopes that have the largest field of view. We characterize the number of pointings needed to find a counterpart. (iii) We compute the annual follow-up time requirements and find that prioritization will be needed. (iv) Even a few telescopes could detect sufficiently nearby counterparts, raising the possibility of adding a handful of small-sized or medium-sized telescopes to the network at diverse geographic locations. (v) The continued operation of VERITAS/H.E.S.S./MAGIC would be a useful compliment to CTA’s follow-up capabilities by increasing the sky area that can be rapidly covered, especially in the United States and Australia, in which the present network of gravitational-wave detectors is more sensitive.

X-Ray Monitoring of the Magnetar CXOU J171405.7–381031 in Supernova Remnant CTB 37B

Abstract We present the results of our 8 yr X-ray monitoring campaign on CXOU J171405.7−381031, the magnetar associated with the faint supernova remnant (SNR) CTB 37B. It is among the youngest by inferred spin-down age, and most energetic in spin-down power of magnetars, and may contribute, at least partially, to the GeV and TeV emission coincident with the SNR. We use a series of Chandra, XMM-Newton, and NuSTAR observations to characterize the timing and spectral properties of the magnetar. The spin-down rate of the pulsar almost doubled in <1 yr and then decreased slowly to a more stable value. Its X-ray flux varied by ≈50%, possibly correlated with the spin-down rate. The 1–79 keV spectrum is well-characterized by an absorbed blackbody plus power-law model with an average temperature of kT = 0.62 ± 0.04 keV and photon index Γ = 0.92 ± 0.16, or by a Comptonized blackbody with kT = 0.55 ± 0.04 keV and an additional hard power law with Γ = 0.70 ± 0.20. In contrast with most magnetars, the pulsed signal is found to decrease with energy up to 6 keV, which is apparently caused by mixing with the hard spectral component that is pulse-phase shifted by ≈0.43 cycles from the soft X-rays. We also analyze the spectrum of the nearby, diffuse nonthermal source XMMU J171410.8−381442, whose relation to the SNR is uncertain.

The GeV Emission in the Field of the Star-forming Region W30 Revisited

Abstract We present a detailed study of the γ-ray emission from the direction of the star-forming region W30 based on a decade of the Fermi Large Area Telescope data in the 0.3–300 GeV photon energy range. The morphological and spectral analyses allow us to resolve the γ-ray emission into two extended structures from different origins. One of them mostly overlaps with the supernova remnant (SNR) G8.7−0.1 and has a soft spectrum that resembles the spectra of other middle-aged SNRs interacting with molecular clouds. The other shows remarkable spatial and spectral consistency with the TeV emission from HESS J1804−216, and its spectrum could be naturally explained by inverse Compton scattering of electrons like a number of TeV γ-ray emitting pulsar wind nebulae. Thus we attribute this source to the nebula around the pulsar PSR J1803−2137.

3C 84: Observational Evidence for Precession and a Possible Relation to TeV Emission

3C 84 (NGC 1275, Perseus A) is a bright radio source at the center of an ongoing merger, where HST observations show two colliding spiral galaxies. 3C 84 holds promise to improve our understanding about how of the activity of active galactic nuclei, the formation of supermassive binary black holes, feedback processes, and galaxy collisions are interrelated. 3C,84 is one of only six radio galaxies, which reveal TeV emission. The origin of this TeV emission is still a matter of debate. Our present study is based on high resolution radio interferometric observations (15 GHz) of the pc-scale jet in this complex radio galaxy. We have re-modeled and re-analyzed 42 VLBA observations of 3C 84, performed between 1999.99 and 2017.65. In order to enable a proper alignment of the VLBA observations, we developed a method of a “differential” alignment whereby we select one reference point and minimize the deviations from this reference point in the remaining epochs. As a result, we find strong indication for a precession of the 3C 84 jet—not only for its central regions, but also for the outer lobe at 10 mas distance. These findings are further supported by our kinematic precession modeling of the radio flux-density monitoring data provided by the University of Michigan Radio Observatory and the Owens Valley Radio Observatory, which yields a precession time scale of about 40 yr. This time scale is further supported by literature maps obtained about 40 yr ago (1973 and 1974.1) which reveal a similar central radio structure. We suggest that the TeV flare detected by MAGIC may correlate with the precession of 3C 84, as we disentangle a projected reversal point of the precessing motion that correlates with the flaring time. This may physically be explained by a precessing jet sweeping over a new region of so far undisturbed X-ray gas which would then lead to shock-produced TeV-emission. In addition, we perform a correlation analysis between the radio data and GeV data obtained by the Fermi Gamma-ray Space Telescope and find that the γ -ray data are lagging the radio data by 300–400 days. A possible explanation could be that the radio and the GeV data stem from different emission regions. We discuss our findings and propose that the detected jet precession can also account for the observed cavities in the X-ray emission on kpc-scales.

On the TeV Halo Fraction in gamma-ray bright Pulsar Wind Nebulae

The discovery of extended TeV emission around the Geminga and PSR B0656+14 pulsars, with properties consistent with free particlepropagation in the interstellar medium (ISM), has sparked considerable discussion on the possible presence of such halos in othersystems. Here we make an assessment of the current TeV source population associated with energetic pulsars, in terms of size andestimated energy density. Based on two alternative estimators we conclude that a large majority of the known TeV sources haveemission originating in the zone energetically and dynamically dominated by the pulsar (i.e. thepulsar wind nebula), rather than froma halo of particles diffusing in to the ISM. Furthermore, whilst the number of established halos will surely increase in the future, wefind that it is unlikely that such halos contribute significantly to the total TeVγ-ray luminosity from electrons accelerated in PWN

H.E.S.S. andSuzakuobservations of the Vela X pulsar wind nebula

Context.Pulsar wind nebulae (PWNe) represent the most prominent population of Galactic very-high-energy gamma-ray sources and are thought to be an efficient source of leptonic cosmic rays. Vela X is a nearby middle-aged PWN, which shows bright X-ray and TeV gamma-ray emission towards an elongated structure called the cocoon.Aims.Since TeV emission is likely inverse-Compton emission of electrons, predominantly from interactions with the cosmic microwave background, while X-ray emission is synchrotron radiation of the same electrons, we aim to derive the properties of the relativistic particles and of magnetic fields with minimal modelling.Methods.We used data from theSuzakuXIS to derive the spectra from three compact regions in Vela X covering distances from 0.3 to 4 pc from the pulsar along the cocoon. We obtained gamma-ray spectra of the same regions from H.E.S.S. observations and fitted a radiative model to the multi-wavelength spectra.Results.The TeV electron spectra and magnetic field strengths are consistent within the uncertainties for the three regions, with energy densities of the order 10−12erg cm−3. The data indicate the presence of a cutoff in the electron spectrum at energies of ~ 100 TeV and a magnetic field strength of ~6μG. Constraints on the presence of turbulent magnetic fields are weak.Conclusions.The pressure of TeV electrons and magnetic fields in the cocoon is dynamically negligible, requiring the presence of another dominant pressure component to balance the pulsar wind at the termination shock. Sub-TeV electrons cannot completely account for the missing pressure, which may be provided either by relativistic ions or from mixing of the ejecta with the pulsar wind. The electron spectra are consistent with expectations from transport scenarios dominated either by advection via the reverse shock or by diffusion, but for the latter the role of radiative losses near the termination shock needs to be further investigated in the light of the measured cutoff energies. Constraints on turbulent magnetic fields and the shape of the electron cutoff can be improved by spectral measurements in the energy range ≳ 10 keV.

Decade‐long X‐ray observations of HESS J0632+057

AbstractWe present the results of a decade of X‐ray observations of the gamma ray loud binary HESS J0632+057 and interpret the available broadband data in view of the system geometry and emission mechanisms. We have performed an analysis of all X‐ray data available to date from Swift, XMM‐Newton, Chandra, NuSTAR, and Suzaku. We refine the orbital period of the system to be days (95% c.l.), consistent with previous studies but measured with significantly better accuracy. We report on a hydrogen column density and spectral slope variation along the orbit. We argue that the observed variability can be explained within an “inclined disk” model in which the orbit of the compact object is inclined to the disk of the Be star. We show that the observed X‐ray to TeV emissions can originate from a broken cut‐off power‐law population of electrons and describe a way in which future X‐ray/TeV observations can distinguish between the proposed model and the alternative flip‐flop emission scenario of this system.

Multiwavelength Investigation of Pulsar Wind Nebula DA 495 with HAWC, VERITAS, and NuSTAR

Abstract Pulsar wind nebula DA 495 (G65.7+1.2) was detected in TeV gamma-rays by the High Altitude Water Cerenkov Observatory in 2017 (2HWC J1953+294). Follow-up observations by the Very Energetic Radiation Imaging Telescope Array System confirmed the association between 2HWC J1953+294 and DA 495 and found the TeV emission to be spatially coincident with the radio emission first reported in 1968. The detection of TeV gamma-rays from DA 495, along with past X-ray detection up to 10 keV, prompted high-energy X-ray observations as part of the NuSTAR Galactic Legacy Survey. We present the results of these NuSTAR observations, combined with archival Chandra and XMM-Newton observations, and confirm the previous X-ray photon index of Γ 2 – 20 keV = 2.0 ± 0.1 . We find no spectral cutoff up to 20 keV. With the spectral information for DA 495 extended to TeV gamma-rays, we were able to perform analytical modeling to test leptonic and hadronic emission scenarios. The leptonic models can explain the broadband emission, but also imply a diffuse X-ray nebula of similar extent to the radio and TeV nebulae, which cannot be confirmed by our observations. The hadronic models can simultaneously explain the spectrum and the spatial extent in all wavelengths; however, we need a very high magnetic field strength pervading the radio and TeV nebulae and a surprisingly high particle kinetic energy. These requirements deepen the mystery of the physical nature of DA 495. Future observations in radio to infrared bands and spatially resolved γ-rays can further constrain the physical conditions and radiation mechanisms in DA 495.

Inverse Compton Scattering of Starlight in the Kiloparsec-scale Jet in Centaurus A: The Origin of Excess TeV γ-Ray Emission

Abstract Centaurus A (Cen A) is the nearest active radio galaxy, which has kiloparsec-scale jets and giant lobes detected by various instruments in radio and X-ray frequency ranges. The Fermi-Large Area Telescope and High Energy Stereoscopic System (HESS) confirmed that Cen A is a very high-energy (VHE; >0.1 TeV) γ-ray emitter with a known spectral softening in the energy range from a few GeV to TeV. In this work, we consider a synchrotron self-Compton model in the nucleus for the broadband spectrum below the break energy and an external Compton model in kiloparsec-scale jets for the γ-ray excess. Our results show that the observed γ-ray excess can be suitably described by the inverse Compton scattering of the starlight photons in the kiloparsec-scale jets, which is consistent with the recent tentative report by HESS on the spatial extension of the TeV emission along the jets. Considering the spectral fitting results, the excess can only be seen in Cen A, which is probably due to two factors: (1) the host galaxy is approximately 50 times more luminous than other typical radio galaxies and (2) the core γ-ray spectrum quickly decays above a few MeV due to the low maximum electron Lorentz factor of γ c = 2.8 × 103 resulting from the large magnetic field of 3.8 G in the core. By the comparison with other γ-ray detected radio galaxies, we found that the magnetic field strength of relativistic jets scales with the distance from the central black holes d with B(d) ∝ d −0.88 ± 0.14.

Tentative evidence of spatially extended GeV emission from SS433/W50

We have analyzed ten years of theFermiLarge Area Telescope data toward the SS433/W50 region. With the latest source catalog and diffuse background models, theγ-ray excess from SS433/W50 is detected with a significance of ~6σin the photon energy range of 500 MeV–10 GeV. Our analysis indicates that an extended flat disk morphology is preferred over a point-source description, suggesting that the GeV emission region is much larger than that of the TeV emission detected by the High Altitude Water Cherenkov (HAWC) telescope. The size of the GeV emission is instead consistent with the extent of the radio nebula W50, a supernova remnant being distorted by the jets, so we suggest that the GeV emission may originate from this supernova remnant. The spectral result of the GeV emission is also consistent with a supernova remnant origin. Furthermore, we derive the GeV flux upper limits on the TeV emission region, which put moderate constraints on the leptonic models to explain the multiwavelength data.