High-energy particle acceleration at the radio-lobe shock of Centaurus A
J. H. Croston, R. P. Kraft, M. J. Hardcastle, M. Birkinshaw, D. M. Worrall, P. E. J. Nulsen, R. F. Penna, G. R. Sivakoff, A. Jordan, N. J. Brassington, D. A. Ev… 展开作者
J. H. Croston, R. P. Kraft, M. J. Hardcastle, M. Birkinshaw, D. M. Worrall, P. E. J. Nulsen, R. F. Penna, G. R. Sivakoff, A. Jordan, N. J. Brassington, D. A. Evans, W. R. Forman, M. Gilfanov, J. L. Goodger, W. E. Harris, C. Jones, A. M. Juett, S. S. Murray, S. Raychaudhury, C. L. Sarazin, R. Voss, K. A. Woodley
We present new results on the shock around the southwest radio lobe of Centaurus A using data from the Chandra Very Large Programme observations. The X-ray spectrum of the emission around the outer southwestern edge of the lobe is well described by a single power-law model with Galactic absorption -- thermal models are strongly disfavoured, except in the region closest to the nucleus. We conclude that a significant fraction of the X-ray emission around the southwest part of the lobe is synchrotron, not thermal. We infer that in the region where the shock is strongest and the ambient gas density lowest, the inflation of the lobe is accelerating particles to X-ray synchrotron emitting energies, similar to supernova remnants such as SN1006. This interpretation resolves a problem of our earlier, purely thermal, interpretation for this emission, namely that the density compression across the shock was required to be much larger than the theoretically expected factor of 4. We estimate that the lobe is expanding to the southwest with a velocity of ~2600 km/s, roughly Mach 8 relative to the ambient medium. We discuss the spatial variation of spectral index across the shock region, concluding that our observations constrain gamma_max for the accelerated particles to be 10^8 at the strongest part of the shock, consistent with expectations from diffusive shock acceleration theory. Finally, we consider the implications of these results for the production of ultra-high energy cosmic rays (UHECRs) and TeV emission from Centaurus A, concluding that the shock front region is unlikely to be a significant source of UHECRs, but that TeV emission from this region is expected at levels comparable to current limits at TeV energies, for plausible assumed magnetic field strengths.
Radio and $γ$-ray measurements of large lobes of several radio galaxies provide adequate basis for determining whether emission in these widely separated spectral regions is largely by energetic electrons. This is very much of interest as there is of yet no unequivocal evidence for a significant energetic proton component to account for $γ$-ray emission by neutral pion decay. A quantitative assessment of the proton spectral distribution necessitates full accounting of the local and background radiation fields in the lobes; indeed, doing so in our recent analysis of the spectral energy distribu…
The origin of recently discovered PeV neutrinos is an unsolved problem. In this work we consider a hadronic scenario to produce PeV neutrinos from a region around giant lobes of Centaurus A. Although ultrahigh-energy cosmic rays (UHECRs) are accelerated and confined by giant lobes, they can escape to be later injected in the inter-group medium where galaxies near the giant lobes provides the condition to confine them. UHECRs interact with low-energy photons and protons producing high-energy photons and neutrinos. We found that the IC35 event cannot be generated neither inside the giant lobes n…
Pierre Auger observatory reported the distribution of arrival directions of the highest energy cosmic rays. These events were collected in 10 years of operations with declinations between -90$^\circ$ and +45$^\circ$. The IceCube neutrino telescope reported the detection of 82 extraterrestrial neutrinos in the High-Energy Starting Events catalog. The highest-energy neutrino event (IC35) reported in this catalog had an energy of 2004$^{+236}_{-262}$ TeV and its reconstruction was centered at RA=$208.4^\circ$ and DEC=$-55.8^\circ$ (J2000). Being Centaurus A the nearest radio-loud active Galactic …
Mass entrainment and turbulence-driven acceleration of ultra-high energy cosmic rays in Centaurus A
对FR I射电星系半人马座A在射电、X射线和伽马射线波段的观测提供了轻子加速到几TeV的证据,以及强子加速到几十EeV的线索。综合现有的观测约束的物理条件和粒子含量的喷流,内叶和巨大的半人马座A的瓣,我们的目标是评估其作为一个超高能宇宙射线源的可行性。我们应用了几种确定喷流功率的方法,并证实了各种功率估计值~ 1 x 10^43 erg s^-1的一致性。采用标度关系的基础上,以前的结果为3C 31,我们估计粒子数密度的射流,包括通过X射线观测可用的无线电。我们的模型与喷流相容,喷流通过热气体的外部卷吸作用吸收约3 x 10^21 g/s的物质,通过含喷流恒星的内部卷吸作用吸收约7 x 10^22 g/s的物质。这导致辐射粒子和磁场产生的内部波瓣压力与我们推导出的外部压力之间的不平衡。基于对其他FR I源外部环境的了解,我们估计巨瓣中的热压力为1.5 x 10^-12达因·厘米^-2,由此我们推导出温度的下限约为1.6 x 10^8 K。利用动力学和浮力参数,我们推断~ 440-645百万年和~ 560百万年分别为巨型裂片的声音交叉和浮力年龄,与它们的光谱年龄不一致。我们重新调查粒子加速的可行性,通过随机过程中的叶片,把新的约束的能量和湍流输入的叶片。相同的“非常热”的温度,允许夹带计算和丢失的压力之间的自洽性,也允许随机UHECR加速模型的工作。
The origin of the 3.55 keV excess reported in various galaxies and galaxy clusters remains unknown. Non-observations of the line in dwarf spheroidal galaxies (dSphs) create tension with galaxy cluster observations when considering a direct Dark Matter (DM) $\rightarrow γ$ decay. This discrepancy could be explained by a DM $\rightarrow a \rightarrow γ$ decay, where the DM particles decay to axion-like particles (ALPs), which then convert to photons in a magnetic field. The 3.55 keV line from a decay to ALPs therefore has very different morphology from a direct decay to photons, as it would only…
Here we re-analyze the archival {\it Chandra} data for the central parts of the Centaurus\,A radio galaxy, aiming for a systematic investigation of the X-ray emission associated with the inner radio lobes in the system, and their immediate surroundings. In particular, we focus on four distinct features characterized by the soft excess with respect to the adjacent fields. Those include the two regions located at kpc distances from the nucleus to the West and East, the extended bow-shock structure to the South, and a fragment of a thin arc North from the center. The selected North, West, and Sou…
The origin of ultra-high energy cosmic rays (UHECRs) is still unknown. It has recently been proposed that UHECR anisotropies can be attributed to starburst galaxies or active galactic nuclei. We suggest that the latter is more likely and that giant-lobed radio galaxies such as Centaurus A and Fornax A can explain the data.
Far-infrared synchrotron properties of the inner lobes of the radio galaxy Centaurus A revealed with the Herschel observatory
Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA)(宇宙科学研究所);Department of Physics and Astronomy, Kwansei Gakuin University(关西学院大学物理与天文学系);Division of Liberal Arts, Kogakuin University of Technology & Engineering(工机大学人文教育部);National Astronomical Observatory of Japan(日本国立天文台);Advanced Research Laboratories, Tokyo City University(东京都市大学先进研究实验室);Department of Physics, Saitama University(埼玉大学物理学系)
We use 2.0 Msec of Chandra observations to investigate the cocoon shocks of Cygnus A and some implications for its lobes and jet. Measured shock Mach numbers vary in the range 1.18-1.66 around the cocoon. We estimate a total outburst energy of $\simeq 4.7\times10^{60}\rm\ erg$, with an age of $\simeq 2 \times 10^{7}\rm\ yr$. The average postshock pressure is found to be $8.6 \pm 0.3 \times 10^{-10}\rm\ erg\ cm^{-3}$, which agrees with the average pressure of the thin rim of compressed gas between the radio lobes and shocks, as determined from X-ray spectra. However, average rim pressures are f…
Centaurus B (Cen B) is one of the closest and brightest radio-loud galaxy in the southern sky. This radio galaxy, proposed as a plausible candidate for accelerating ultra-high-energy cosmic rays (UHECRs), is near the highest-energy neutrino event reported (IC35) in the High-Energy Starting Events catalog. Pierre Auger observatory reported the highest energy comic rays during 10 years of collecting data with some of them around this source. In this paper, the analysis of the gamma-ray spectrum and the light curve above 200 MeV is presented with nine years of cumulative Fermi-LAT data around Cen…
While there is overwhelming observational evidence of AGN-driven jets in galaxy clusters and groups, if and how the jet energy is delivered to the ambient medium remains unanswered. Here we perform very high resolution AGN jet simulations within a live, cosmologically evolved cluster with the moving mesh code AREPO. We find that mock X-ray and radio lobe properties are in good agreement with observations with different power jets transitioning from FR-I to FR-II-like morphologies. During the lobe inflation phase, heating by both internal and bow shocks contributes to lobe energetics, and $\sim…
The origin of ultra-high energy cosmic rays (UHECRs) has been an open question for decades. Here, we use a combination of hydrodynamic simulations and general physical arguments to demonstrate that UHECRs can in principle be produced by diffusive shock acceleration (DSA) in shocks in the backflowing material of radio galaxy lobes. These shocks occur after the jet material has passed through the relativistic termination shock. Recently, several authors have demonstrated that highly relativistic shocks are not effective in accelerating UHECRs. The shocks in our proposed model have a range of non…
Context: Radio-loud AGNs are thought to possess various sites of particle acceleration, which gives rise to the observed non-thermal spectra. Stochastic turbulent acceleration (STA) and diffusive shock acceleration (DSA) are commonly cited as potential sources of high-energy particles in weakly magnetized environments. Together, these acceleration processes and various radiative losses determine the emission characteristics of these extra-galactic radio sources. Aims: The purpose of this research is to investigate the dynamical interplay between the STA and DSA in the radio lobes of FR-II radi…
We report a 3D MHD simulation study of the interactions between radio galaxies and galaxy-cluster-media shocks in which the incident shock normals are orthogonal to the bipolar AGN jets. Before shock impact, light, supersonic jets inflate lobes (cavities) in a static, uniform ICM. We examine three AGN activity scenarios: 1) continued, steady jet activity; 2) jet source cycled off coincident with shock/radio lobe impact; 3) jet activity ceased well before shock arrival (a "radio phoenix" scenario). The simulations follow relativistic electrons (CRe) introduced by the jets, enabling synthetic ra…
The powerful FR II radio galaxy Cygnus A exhibits primary and secondary hotspots in each lobe. A 2 Msec Chandra X-ray image of Cygnus A has revealed an approximately circular hole, with a radius of 3.9 kpc, centered on the primary hotspot in the eastern radio lobe, hotspot E. We infer the distribution of X-ray emission on our line-of-sight from an X-ray surface brightness profile of the radio lobe adjacent to the hole and use it to argue that the hole is excavated from the radio lobe. The surface brightness profile of the hole implies a depth at least 1.7 $\pm$ 0.3 times greater than its proje…
The origin of ultrahigh energy cosmic rays (UHECRs) is an open question. In this proceeding, we first review the general physical requirements that a source must meet for acceleration to 10-100 EeV, including the consideration that the shock is not highly relativistic. We show that shocks in the backflows of radio galaxies can meet these requirements. We discuss a model in which giant-lobed radio galaxies such as Centaurus A and Fornax A act as slowly-leaking UHECR reservoirs, with the UHECRs being accelerated during a more powerful past episode. We also show that Centaurus A, Fornax A and oth…
We examine the remnant phase of radio galaxies using three-dimensional hydrodynamical simulations of relativistic jets propagating through cluster environments. By switching the jets off once the lobes have reached a certain length we can study how the energy distribution between the lobes and shocked intra-cluster medium compares to that of an active source, as well as calculate synchrotron emission properties of the remnant sources. We see that as a result of disturbed cluster gas beginning to settle back into the initial cluster potential, streams of dense gas are pushed along the jet axis …
Centaurus A (Cen~A) is the nearest active radio galaxy, which has kiloparsec (kpc) 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 broad band spectrum {below the break energy} and an external Compton model in kpc-scale jets for the $γ$…
Jet feedback from active galactic nuclei (AGN) harboured by brightest cluster galaxies is expected to play a fundamental role in regulating cooling in the intracluster medium (ICM). While observations and theory suggest energy within jet lobes balances ICM radiative losses, the modus operandi of energy communication with the ICM remains unclear. We present simulations of very high resolution AGN-driven jets launching in a live, cosmological galaxy cluster, within the moving mesh code AREPO. As the jet propagates through the ICM the majority of its energy, which is initially in the kinetic form…