arXiv subjects

Roger Blandford

Publications and source records attributed to Roger Blandford.

At least 19 recordsLinked to original sources

arXiv2026

A Hierarchical Shock Model of Ultra-High-Energy Cosmic Rays

We propose that a hierarchical shock model$\unicode{x2014}$including supernova remnant shocks, galactic wind termination shocks, and accretion shocks around cosmic filaments and galaxy clusters$\unicode{x2014}$can naturally explain the cosmic ray spectrum from ~1 GeV up to ~200 EeV. While this framework applies to the entire cosmic ray spectrum, in this work, we focus on its implications for ultra-high-energy cosmic rays (UHECRs). We perform a hydrodynamic cosmological simulation to investigate the power processed at shocks around clusters and filaments. The downstream flux from nearby shocks around the local filament accounts for the softer, lower-energy extragalactic component around the ankle, and the upstream escaping flux from nearby clusters accounts for the transition to a hard spectral component at the highest energies. This interpretation is in agreement with UHECR observations. We suggest that a combination of early-Universe galactic outflows, cosmic ray streaming instabilities, and a small-scale turbulent dynamo can increase magnetic fields enough to attain the required rigidities. Our simulation suggests that the available volume-averaged power density of accretion shocks exceeds the required UHECR luminosity density by three orders of magnitude. We show that microgauss magnetic fields at these shocks could explain both the origin of UHECRs and potentially contribute to the diffuse radio synchrotron background below 10 GHz. The shock-accelerated electrons produce a hard radio background without overproducing diffuse inverse Compton emission. These results motivate further observational tests with upcoming facilities to help distinguish accretion shocks from other UHECR sources.

astro-ph.HE↗

arXiv2025

Ultra High Energy Cosmic Rays

Ultra High Energy Cosmic Rays, UHECR, are charged particles with energies between $\sim10^{18}\,{\rm eV}$ and $\sim3\times10^{20}\,{\rm eV}\sim50\,{\rm J}$. They exhibit fundamental physics at energies inaccessible to terrestrial accelerators, challenge experimental physics and connect strongly to astronomical observations through electromagnetic, neutrino and even gravitational wave channels. There has been much theoretical and observational progress in the sixty years that have elapsed since the discovery of UHECR, to divine their nature and identify their sources. The highest energy UHECR appear to be heavy nuclei with rigidity extending up to $\sim10\,{\rm EV}$; A significant ($6.9σ$) dipole anisotropy has been measured but our poor understanding of the Galactic magnetic fields makes this hard to interpret; The UHECR luminosity density is $\sim 10^{44}$ erg Mpc$^{-3}$ yr$^{-1}$ which constrains explanations of their origin; The most promising acceleration mechanisms involve diffusive shock acceleration and unipolar induction; The most promising sources include intergalactic accretion shocks, and relativistic jets from stellar-mass or supermassive black holes. We explore the prospects for using the highest energy events, combined with multimessenger astronomy, to help us solve the riddle of UHECR.

astro-ph.HE↗

arXiv2023

On the Comparison of AGN with GRMHD Simulations: II. M87

Horizon-scale observations of the jetted active galactic nucleus M87 are compared with simulations spanning a broad range of dissipation mechanisms and plasma content in three-dimensional general relativistic flows around spinning black holes. Observations of synchrotron radiation from radio to X-ray frequencies can be compared with simulations by adding prescriptions specifying the relativistic electron-plus-positron distribution function and associated radiative transfer coefficients. A suite of time-varying simulations with various spins, plasma magnetizations and turbulent heating and equipartition-based emission prescriptions (and piecewise combinations thereof) is chosen to represent distinct possibilities for the M87 jet/accretion flow/black hole (JAB) system. Simulation jet morphology, polarization and variation are then "observed" and compared with real observations to infer the rules that govern the polarized emissivity. Our models support several possible spin/emission model/plasma composition combinations supplying the jet in M87, whose black hole shadow has been observed down to the photon ring at 230 GHz by the Event Horizon Telescope (EHT). Net linear polarization and circular polarization constraints favor magnetically arrested disk (MAD) models whereas resolved linear polarization favors standard and normal evolution (SANE) in our parameter space. We also show that some MAD cases dominated by intrinsic circular polarization have near-linear V/I dependence on unpaired electron or positron content while SANE polarization exhibits markedly greater positron-dependent Faraday effects - future probes of the SANE/MAD dichotomy and plasma content with the EHT. This is the second work in a series also applying the "observing" simulations methodology to near-horizon regions of supermassive black holes in Sgr A* and 3C 279.

astro-ph.HE↗

arXiv2023

A Hierarchical Framework for explaining the Cosmic Ray Spectrum using Diffusive Shock Acceleration

The hypothesis that the entire cosmic ray spectrum, from $\lesssim1\,{\rm GeV}$ to $\gtrsim100\,{\rm EeV}$ energy, can be accounted for by diffusive shock acceleration on increasingly large scales is critically examined. Specifically, it is conjectured that Galactic cosmic rays, up to $\sim3\,{\rm PeV}$, are mostly produced by local supernova remnants, from which they escape upstream. These cosmic rays initiate a powerful magnetocentrifugal wind, removing disk mass and angular momentum before passing through the Galactic Wind Termination Shock at a radius $\sim200\,{\rm kpc}$, where they can be re-accelerated to account for observed cosmic rays up to $\sim30\,{\rm PeV}$. The cosmic rays transmitted downstream from more powerful termination shocks associated with other galaxies can be further accelerated at Intergalactic Accretion Shocks to the highest energies observed. In this interpretation, the highest rigidity observed particles are protons; the highest energy particles are heavy nuclei, such as iron. A universal "bootstrap" prescription, coupling the energy density of the magnetic turbulence to that of the resonant cosmic rays, is proposed, initially for the highest energy particles escaping far ahead of the shock front and then scattering, successively, lower energy particles downstream. Observable implications of this general scheme relate to the spectrum, composition and sky distribution of Ultra-High-Energy Cosmic Rays, the extragalactic radio background, the Galactic halo magnetic field and Pevatrons.

astro-ph.HE↗

arXiv2023

Extreme Energy Cosmic Rays "Treasure Maps": a new methodology to unveil the nature of cosmic accelerators

Extreme Energy Cosmic Rays, EECRs -- cosmic rays with energies beyond the GZK cutoff (i.e. greater than 100 EeV) are scarce. Only a few of such events have been detected by air shower experiments and the nature of the primary particles are still unknown. Individual EECRs sources become more prominent, relative to the background, as the horizon diminishes. We show that an event-by-event, composition-dependent observatory would allow us to limit the character of the sources and learn about the intervening magnetic fields, as the deflections in the intervening Galactic and extragalactic magnetic fields depend on the nature of the particle. A major goal here is to provide a methodology to distinguish between steady and transient sources.

astro-ph.HE↗

arXiv2023

On the origin of radio-loudness in active galactic nuclei using far-infrared polarimetric observations

The dichotomy between radio-loud (RL) and radio-quiet (RQ) active galactic nuclei (AGN) is thought to be intrinsically related to radio jet production. This difference may be explained by the presence of a strong magnetic field (B-field) that enhances, or is the cause of, the accretion activity and the jet power. Here, we report the first evidence of an intrinsic difference in the dust polarized emission cores of four RL and five RQ obscured AGN using 89 $μ$m polarization with HAWC+/SOFIA. We find that the thermal polarized emission increases with the nuclear radio-loudness, $R_{20} = L_{\rm 5GHz}/ L_{\rm 20μm}$. The dust emission cores of RL AGN are measured to be polarized, $\sim5-11$%, while RQ AGN are unpolarized, $<1$%. For RQ AGN, our results are consistent with the observed region being filled with an unmagnetized or highly turbulent, disk and/or expanding outflow at scales of $5-130$ pc from the AGN. For RL AGN, the measured $89$ $μ$m polarization arises primarily from magnetically aligned dust grains associated with a $5-130$ pc-scale dusty obscuring structure with a toroidal B-field orientation highly offset, $65\pm22^{\circ}$, with respect to the jet axis. Our results indicate that the size and strength of the B-fields surrounding the AGN are intrinsically related to the strength of the jet power -- the stronger the jet power is, the larger and stronger the toroidal B-field is. The detection of a $\le130$ pc-scale ordered toroidal B-field suggests that a) the infalling gas that fuels RL AGN is magnetized, b) there is a magnetohydrodynamic wind that collimates the jet, and/or c) the jet is able to magnetize its surroundings.

astro-ph.GA↗

arXiv2023

Ultra High Energy Cosmic Ray Source Models: Successes, Challenges and General Predictions

Understanding the acceleration of Ultra High Energy Cosmic Rays is one of the great challenges of contemporary astrophysics. In this short review, we summarize the general observational constraints on their composition, spectrum and isotropy which indicate that nuclei heavier than single protons dominate their spectra above $\sim 5\,{\rm EeV}$, that they are strongly suppressed above energies $\sim50\,{\rm EeV}$, and that the only significant departure from isotropy is a dipole. Constraints based upon photopion and photodisintegration losses allow their ranges and luminosity density to be estimated. Three general classes of source model are discussed - magnetospheric models (including neutron stars and black holes), jet models (including Gamma Ray Bursts, Active Galactic Nuclei and Tidal Disruption Events) and Diffusive Shock Acceleration models (involving large accretion shocks around rich clusters of galaxies). The value of constructing larger and more capable arrays to measure individual masses at the highest energies and probably identifying their sources is emphasized.

astro-ph.HE↗

arXiv2023

Reacceleration of Galactic Cosmic Rays Beyond the Knee at the Termination Shock of a Cosmic-Ray-Driven Galactic Wind

The origin of cosmic rays above the knee in the spectrum is an unsolved problem. We present a wind model in which interstellar gas flows along a non-rotating, expanding flux tube with a changing speed and cross-sectional area. Cosmic rays from Galactic sources, such as supernova remnants, which are coupled to the plasma via Alfvén waves, provide the main pressure source for driving this outflow. These cosmic rays are then subject to diffusive shock reacceleration at the Galactic wind termination shock, which is located at a distance $\sim200\,{\rm kpc}$. Some of the highest-energy reaccelerated particles propagate upstream against the wind and can contribute to the PeV-EeV range of the spectrum. We analyze the conditions under which efficient reacceleration can occur and find that rigidities $\sim$ 10-40 PV can be obtained and that the termination shock may account for half of the proton spectrum measured in IceCube/IceTop experiment. The highest-energy particles that escape downstream from our termination shock, and similar shocks surrounding most galaxies, can be further accelerated by intergalactic shock fronts.

astro-ph.HE↗

arXiv2022

Evidence for PeV Proton Acceleration from Fermi-LAT Observations of SNR G106.3+2.7

The existence of a "knee" at energy ~1 PeV in the cosmic-ray spectrum suggests the presence of Galactic PeV proton accelerators called "PeVatrons". Supernova Remnant (SNR) G106.3+2.7 is a prime candidate for one of these. The recent detection of very high energy (0.1-100 TeV) gamma rays from G106.3+2.7 may be explained either by the decay of neutral pions or inverse Compton scattering by relativistic electrons. We report an analysis of 12 years of Fermi-LAT gamma-ray data which shows that the GeV-TeV gamma-ray spectrum is much harder and requires a different total electron energy than the radio and X-ray spectra, suggesting it has a distinct, hadronic origin. The non-detection of gamma rays below 10 GeV implies additional constraints on the relativistic electron spectrum. A hadronic interpretation of the observed gamma rays is strongly supported. This observation confirms the long-sought connection between Galactic PeVatrons and SNRs. Moreover, it suggests that G106.3+2.7 could be the brightest member of a new population of SNRs whose gamma-ray energy flux peaks at TeV energies. Such a population may contribute to the cosmic-ray knee and be revealed by future very high energy gamma-ray detectors.

astro-ph.HE↗

arXiv2022

Jets, Disks and Winds from Spinning Black Holes: Nature or Nurture?

A brief summary is given of an alternative interpretation of the Event Horizon Telescope observations of the massive black hole in the nucleus of the nearby galaxy M87. It is proposed that the flow is primarily powered by the black hole rotation, not the release of gravitational energy by the infalling gas. Consequently, the observed millimetre emission is produced by an "ergomagnetosphere" that connects the black hole horizon to an "ejection disk" from which most of the gas supplied at a remote "magnetopause" is lost through a magnetocentrifugal wind. It is argued that the boundary conditions at high latitude on the magnetopause play a crucial role in the collimation of the relativistic jets. The application of these ideas to other types of source is briefly discussed.

astro-ph.HE↗

arXiv2022

Ergomagnetosphere, Ejection Disc, Magnetopause in M87. I Global Flow of Mass, Angular Momentum, Energy and Current

We interpret the 1.3mm VLBI observations made by the Event Horizon Telescope of the black hole in M87. It is proposed that, instead of being a torus of accreting gas, the observed annular ring is a rotating, magnetically-dominated ergomagnetosphere that can transmit electromagnetic angular momentum and energy outward to the disc through a combination of large scale magnetic torque and small scale instabilities. It is further proposed that energy can be extracted by magnetic flux threading the ergosphere through the efficient emission of long wavelength electromagnetic disturbances onto negative energy orbits, when the invariant $B^2-E^2$ becomes negative. In this way, the spinning black hole and its ergosphere not only power the jets but also the ejection disc so as to drive away most of the gas supplied near the Bondi radius. This outflow takes the form of a MHD wind, extending over many decades of radius, with a unidirectional magnetic field, that is collimated by the infalling gas across a magnetopause. This wind, in turn, collimates the relativistic jets and the emission observed from the jet sheath may be associated with a return current. A model for the global flow of mass, angular momentum, energy and current, on scales from the horizon to the Bondi radius, is presented and discussed.

astro-ph.HE↗

arXiv2021

MP3 White Paper 2021 -- Research Opportunities Enabled by Co-locating Multi-Petawatt Lasers with Dense Ultra-Relativistic Electron Beams

Novel emergent phenomena are expected to occur under conditions exceeding the QED critical electric field, where the vacuum becomes unstable to electron-positron pair production. The required intensity to reach this regime, $\sim10^{29}\,\mathrm{Wcm^{-2}}$, cannot be achieved even with the most intense lasers now being planned/constructed without a sizeable Lorentz boost provided by interactions with ultrarelativistic particles. Seeded laser-laser collisions may access this strong-field QED regime at laser intensities as low as $\sim10^{24}\,\mathrm{Wcm^{-2}}$. Counterpropagating e-beam--laser interactions exceed the QED critical field at still lower intensities ($\sim10^{20}\,\mathrm{Wcm^{-2}}$ at $\sim10\,\mathrm{GeV}$). Novel emergent phenomena are predicted to occur in the "QED plasma regime", where strong-field quantum and collective plasma effects play off one another. Here the electron beam density becomes a decisive factor. Thus, the challenge is not just to exceed the QED critical field, but to do so with high quality, approaching solid-density electron beams. Even though laser wakefield accelerators (LWFA) represent a very promising research field, conventional accelerators still provide orders of magnitude higher charge densities at energies $\gtrsim10\,\mathrm{GeV}$. Co-location of extremely dense and highly energetic electron beams with a multi-petawatt laser system would therefore enable seminal research opportunities in high-field physics and laboratory astrophysics. This white paper elucidates the potential scientific impact of multi-beam capabilities that combine a multi-PW optical laser, high-energy/density electron beam, and high-intensity x rays and outlines how to achieve such capabilities by co-locating a 3-10 PW laser with a state-of-the-art linear accelerator.

physics.plasm-ph↗

arXiv2020

Coming of Age of the Standard Model

Cosmology now has a standard model - a remarkably simple description of the universe, its contents and its history. A symposium held last September in Cambridge, UK, gave this model a 'health check' and discussed fascinating questions that lie beyond it.

astro-ph.CO↗

arXiv2020

GeV-TeV Counterparts of SS 433/W50 from Fermi-LAT and HAWC Observations

The extended jets of the microquasar SS 433 have been observed in optical, radio, X-ray, and recently very-high-energy (VHE) $γ$-rays by HAWC. The detection of HAWC $γ$-rays with energies as great as 25 TeV motivates searches for high-energy $γ$-ray counterparts in the Fermi-LAT data in the 100 MeV--300 GeV band. In this paper, we report on the first-ever joint analysis of Fermi-LAT and HAWC observations to study the spectrum and location of $γ$-ray emission from SS~433. Our analysis finds common emission sites of GeV-to-TeV $γ$-rays inside the eastern and western lobes of SS 433. The total flux above 1 GeV is $\sim 1\times10^{-10}\,\rm cm^{-2}\,s^{-1}$ in both lobes. The $γ$-ray spectrum in the eastern lobe is consistent with inverse-Compton emission by an electron population that is accelerated by jets. To explain both the GeV and TeV flux, the electrons need to have a soft intrinsic energy spectrum, or undergo a quick cooling process due to synchrotron radiation in a magnetized environment.

astro-ph.HE↗

arXiv2019

Black Holes as Cosmic Dynamos

An introduction is given to a meeting on the role of massive and stellar black holes in powering non-thermal activity in a rich variety of cosmic sources. Relevant properties of magnetized, spinning black holes are summarized and their observational expression, within galactic nuclei, in terms of radio loudness and Fanaroff-Riley class, is briefly described. The dependence of the accretion mode on the rate and manner of the mass supply beyond the black hole sphere of influence is also discussed. It is argued that hydromagnetic outflows from accretion disks are generally expected over as many as six decades of radius and that they may be the source of emission line gas. These outflows collimate the relativistic jets which are probably generated in an electromagnetic form but become hydromagnetic as they entrain gas through boundary layers where most of the initial nonthermal emission occurs. It is proposed that the particle acceleration close to the hole emphasizes the proton channel which allows secondary pairs to be created at far higher energies than is possible from direct acceleration. These pairs radiate synchrotron gamma-rays which can escape along the jet because the outflow effectively shields them from pair-producing, soft photons. Jets are subject to helical instabilities which can tangle their magnetic field and may destroy them. The jet should become plasma-dominated through intermittent, "magnetoluminescent" untangling of the field which causes nonthermal emission all along its length. Powerful jets remain supersonic out to the "hot spots" at the extremities of the source; weaker jets become subsonic plumes or bubbles. The prospects for learning much more about the nature and operation of jets over the next decade are excellent.

astro-ph.HE↗

arXiv2019

Data-Driven Reconstruction of Gravitationally Lensed Galaxies using Recurrent Inference Machines

We present a machine learning method for the reconstruction of the undistorted images of background sources in strongly lensed systems. This method treats the source as a pixelated image and utilizes the Recurrent Inference Machine (RIM) to iteratively reconstruct the background source given a lens model. Our architecture learns to minimize the likelihood of the model parameters (source pixels) given the data using the physical forward model (ray tracing simulations) while implicitly learning the prior of the source structure from the training data. This results in better performance compared to linear inversion methods, where the prior information is limited to the 2-point covariance of the source pixels approximated with a Gaussian form, and often specified in a relatively arbitrary manner. We combine our source reconstruction network with a convolutional neural network that predicts the parameters of the mass distribution in the lensing galaxies directly from telescope images, allowing a fully automated reconstruction of the background source images and the foreground mass distribution.

astro-ph.IM↗

arXiv2018

Relativistic Jets in Active Galactic Nuclei

The nuclei of most normal galaxies contain supermassive black holes, which can accrete gas through a disk and become active. These Active Galactic Nuclei, AGN, can form jets which are observed on scales from AU to Mpc and from meter wavelengths to TeV gamma ray energies. High resolution radio imaging and multi-wavelength/messenger campaigns are elucidating the conditions under which this happens. Evidence is presented that: AGN jets are formed when the black hole spins and the accretion disk is strongly magnetized, perhaps on account of gas accretingat high latitude beyond the black hole sphere of influence; AGN jets are collimated close to the black hole by magnetic stress associated with a disk wind; higher power jets can emerge from their galactic nuclei in a relativistic, supersonic and proton-dominated state and they terminate in strong, hot spot shocks; lower power jets are degraded to buoyant plumes and bubbles; jets may accelerate protons to EeV energies which contribute to the cosmic ray spectrum and which may initiate pair cascades that can efficiently radiate synchrotron gamma rays; jets were far more common when the universe was a few billion years old and black holes and massive galaxies were growing rapidly; jets can have a major influence on their environments, stimulating and limiting the growth of galaxies. The observational prospects for securing our understanding of AGN jets are bright.

astro-ph.HE↗

arXiv2018

Detection of polarized gamma-ray emission from the Crab nebula with Hitomi Soft Gamma-ray Detector

We present the results from the Hitomi Soft Gamma-ray Detector (SGD) observation of the Crab nebula. The main part of SGD is a Compton camera, which in addition to being a spectrometer, is capable of measuring polarization of gamma-ray photons. The Crab nebula is one of the brightest X-ray / gamma-ray sources on the sky, and, the only source from which polarized X-ray photons have been detected. SGD observed the Crab nebula during the initial test observation phase of Hitomi. We performed the data analysis of the SGD observation, the SGD background estimation and the SGD Monte Carlo simulations, and, successfully detected polarized gamma-ray emission from the Crab nebula with only about 5 ks exposure time. The obtained polarization fraction of the phase-integrated Crab emission (sum of pulsar and nebula emissions) is (22.1 $\pm$ 10.6)% and, the polarization angle is 110.7$^o$ + 13.2 / $-$13.0$^o$ in the energy range of 60--160 keV (The errors correspond to the 1 sigma deviation). The confidence level of the polarization detection was 99.3%. The polarization angle measured by SGD is about one sigma deviation with the projected spin axis of the pulsar, 124.0$^o$ $\pm$0.1$^o$.

astro-ph.HE↗