About

Roger Blandford is the Luke Blossom Professor in the School of Humanities and Sciences at Stanford University, and a Professor of Physics and of Particle Physics and Astrophysics in the Physics Department. His research interests include theoretical astrophysics and cosmology, focusing on understanding the fundamental processes governing the universe. His work contributes to the broader field of astrophysics through theoretical investigations into cosmic phenomena, advancing knowledge in areas such as particle physics and astrophysical cosmology.

Research topics

• Physics
• Astrophysics
• Astronomy
• Optics

Selected publications

• A Hierarchical Shock Model of Ultra-high-energy Cosmic Rays

  The Astrophysical Journal · 2026-02-06 · 1 citations

  articleOpen accessSenior author

  Abstract We propose that a hierarchical shock model—including supernova remnant shocks, galactic wind termination shocks, and accretion shocks around cosmic filaments and galaxy clusters—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 3 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.

  PublisherDOI

• PKS 2131−021—Discovery of Strong Coherent Sinusoidal Variations from Radio to Optical Frequencies: Compelling Evidence for a Blazar Supermassive Black Hole Binary

  The Astrophysical Journal · 2025-05-14 · 11 citations

  articleOpen access

  Abstract Haystack and Owens Valley Radio Observatory observations recently revealed strong, intermittent, sinusoidal total flux-density variations that maintained their coherence between 1975 and 2021 in the blazar PKS 2131−021 ( z = 1.283). This was interpreted as possible evidence of a supermassive black hole binary (SMBHB). Extended observations through 2023 show a coherence over 47.9 yr, with an observed period P 15 GHz = (1739.8 ± 17.4) days. We reject, with p -value = 2.09 × 10 −7 , the hypothesis that the variations are due to random fluctuations in the red noise tail of the power spectral density. There is clearly a physical phenomenon in PKS 2131−021 producing coherent sinusoidal flux-density variations. We find the coherent sinusoidal intensity variations extend from below 2.7 GHz to optical frequencies, from which we derive an observed period P optical = (1764 ± 36) days. Across this broad frequency range, there is a smoothly varying monotonic phase shift in the sinusoidal variations with frequency. Hints of periodic variations are also observed at γ -ray energies. The importance of well-vetted SMBHB candidates to searches for gravitational waves is pointed out. We estimate the fraction of blazars that are SMBHB candidates to be >1 in 100. Thus, monitoring programs covering tens of thousands of blazars could discover hundreds of SMBHB candidates.

  PublisherDOI

• Atacama Cosmology Telescope: Observations of supermassive black hole binary candidates. Strong sinusoidal variations at 95, 147 and 225 GHz in PKS 2131$-$021 and PKS J0805$-$0111

  arXiv (Cornell University) · 2025-04-05

  preprintOpen access

  Large sinusoidal variations in the radio light curves of the blazars PKS J0805$-$0111 and PKS 2131$-$021 have recently been discovered with an 18-year monitoring programme at the Owens Valley Radio Observatory, making these systems strong supermassive black hole binary (SMBHB) candidates. The sinusoidal variations in PKS 2131$-$021 dominate its light curves from 2.7 GHz to optical frequencies. We report sinusoidal variations observed in both objects with the Atacama Cosmology Telescope (ACT) at 95, 147 and 225 GHz consistent with the radio light curves. The ACT 95 GHz light curve of PKS 2131$-$021 agrees well with the contemporaneous 91.5 GHz ALMA light curve and is comparable in quality, while the ACT light curves of PKS J0805$-$0111, for which there are no ALMA or other millimetre light curves, show that PKS 2131$-$021 is not an isolated case, and that this class of AGN exhibits the following properties: (a) the sinusoidal pattern dominates over a broad range of frequencies; (b) the amplitude of the sine wave compared to its mean value is monochromatic (i.e., nearly constant across frequencies); (c) the phase of the sinusoid phase changes monotonically as a function of frequency; (d) the sinusoidal variations are intermittent. We describe a physical model for SMBHB systems, the modified Kinetic Orbital model, that explains all four of these phenomena. Monitoring of ${\sim}8000$ blazars by the Simons Observatory over the next decade should provide a large number of SMBHB candidates that will shed light on the nature of the nanohertz gravitational-wave background.

  PublisherOA PDFDOI

• Ultra High Energy Cosmic Rays

  ArXiv.org · 2025-05-28

  preprintOpen accessSenior author

  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.

  PublisherOA PDFDOI

• A Hierarchical Shock Acceleration Model for Ultra-High-Energy Cosmic Rays and Multimessenger Signatures

  2025-09-23

  articleOpen accessSenior author

  Accretion shocks in the large-scale structure of the universe are promising sources of ultra-high-energy cosmic rays (UHECRs). In addition to accelerating UHECRs, these shocks should produce distinct multimessenger signatures, including synchrotron radio emission, gamma rays, and neutrinos. We investigate how a hierarchical shock acceleration framework—progressing from supernova remnant shocks to galactic wind termination shocks and ultimately to cosmic structure-formation shocks—naturally explains the full cosmic ray spectrum extending beyond the ankle. Using a hydrodynamic cosmological simulation, we compute the energy processed at cluster and filamentary shocks and predict the corresponding radio synchrotron emission. We find that microgauss magnetic fields at these shocks could explain both UHECRs and the diffuse radio synchrotron background below 10\,GHz. These fields may be amplified through a combination of early-Universe galactic outflows, plasma instabilities, and small-scale turbulent dynamo action. The model also predicts a correlation between UHECR anisotropy and large-scale cosmic structure, which can be tested with Auger and future radio and gamma-ray observatories. Our results motivate a multimessenger strategy to identify UHECR sources and test whether accretion shocks dominate their acceleration.

  PublisherOA PDFDOI

• Fermi detection of gamma-ray emission from the hot coronae of radio-quiet active galactic nuclei

  Nature Astronomy · 2025-06-20 · 6 citations

  articleOpen access
  PublisherOA PDFDOI

• Relativistic Jets and Winds in Radio-Identified Supermassive Black Hole Binary Candidates

  ArXiv.org · 2025-10-02

  preprintOpen access

  Supermassive black hole binary systems (SMBHBs) are thought to emit the recently discovered nHz gravitational wave background; however, not a single individual nHz source has been confirmed to date. Long-term radio-monitoring at the Owens Valley Radio Observatory has revealed two potential SMBHB candidates: blazars PKS 2131-021 and PKS J0805-0111. These sources show periodic flux density variations across the electromagnetic spectrum, signaling the presence of a good clock. To explain the emission, we propose a generalizable jet model, where a mildly relativistic wind creates an outward-moving helical channel, along which the ultra-relativistic jet propagates. The observed flux variation from the jet is mostly due to aberration. The emission at lower frequency arises at larger radius and its variation is consequently delayed, as observed. Our model reproduces the main observable features of both sources and can be applied to other sources as they are discovered. We make predictions for radio polarization, direct imaging, and emission line variation, which can be tested with forthcoming observations. Our results motivate future numerical simulations of jetted SMBHB systems and have implications for the fueling, structure, and evolution of blazar jets.

  PublisherOA PDFDOI

• GRB 221009A: The B.O.A.T. Burst that Shines in Gamma Rays

  The Astrophysical Journal Supplement Series · 2025-02-28 · 16 citations

  articleOpen access

  Abstract We present a complete analysis of Fermi Large Area Telescope (LAT) data of GRB 221009A, the brightest gamma-ray burst (GRB) ever detected. The burst emission above 30 MeV detected by the LAT preceded, by 1 s, the low-energy (<10 MeV) pulse that triggered the Fermi Gamma-Ray Burst Monitor (GBM), as has been observed in other GRBs. The prompt phase of GRB 221009A lasted a few hundred seconds. It was so bright that we identify a bad time interval of 64 s caused by the extremely high flux of hard X-rays and soft gamma rays, during which the event reconstruction efficiency was poor and the dead time fraction quite high. The late-time emission decayed as a power law, but the extrapolation of the late-time emission during the first 450 s suggests that the afterglow started during the prompt emission. We also found that high-energy events observed by the LAT are incompatible with synchrotron origin, and, during the prompt emission, are more likely related to an extra component identified as synchrotron self-Compton (SSC). A remarkable 400 GeV photon, detected by the LAT 33 ks after the GBM trigger and directionally consistent with the location of GRB 221009A, is hard to explain as a product of SSC or TeV electromagnetic cascades, and the process responsible for its origin is uncertain. Because of its proximity and energetic nature, GRB 221009A is an extremely rare event.

  PublisherDOI

• The Atacama Cosmology Telescope: Observations of supermassive black hole binary candidates

  Astronomy and Astrophysics · 2025-12-19

  articleOpen access

  Large sinusoidal variations in the radio light curves of the blazars PKS J0805–0111 and PKS 2131–021 have recently been discovered with an 18-year monitoring programme at the Owens Valley Radio Observatory, making these systems strong supermassive black hole binary (SMBHB) candidates. The sinusoidal variations in PKS 2131–021 dominate its light curves from 2.7 GHz to optical frequencies. We report sinusoidal variations observed in both objects with the Atacama Cosmology Telescope (ACT) at 95, 147, and 225 GHz consistent with the radio light curves. The ACT 95 GHz light curve of PKS 2131–021 agrees well with the contemporaneous 91.5 GHz ALMA light curve and is comparable in quality, while the ACT light curves of PKS J0805–0111, for which there are no ALMA or other millimetre light curves, show that PKS 2131–021 is not an isolated case, and that this class of AGN exhibits the following properties: (a) the sinusoidal pattern dominates over a broad range of frequencies; (b) the amplitude of the sine wave compared to its mean value is monochromatic (i.e. nearly constant across frequencies); (c) the phase of the sinusoid phase changes monotonically as a function of frequency; (d) the sinusoidal variations are intermittent. We describe a physical model for SMBHB systems, the modified Kinetic Orbital model, that explains all four of these phenomena. The monitoring of ∼8000 blazars by the Simons Observatory over the next decade should provide a large number of SMBHB candidates that will shed light on the nature of the nanohertz gravitational-wave background.

  PublisherDOI

• Ultrahigh-Energy Cosmic Rays

  Annual Review of Astronomy and Astrophysics · 2025-08-18 · 6 citations

  articleOpen accessSenior author

  Ultrahigh-energy cosmic rays (UHECRs) are charged particles with energies between ∼10 18 eV and ∼3 × 10 20 eV ∼ 50J. 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. Much theoretical and observational progress has occurred in the 60 years since the discovery of UHECRs to determine their nature and identify their sources: ▪ The highest-energy UHECRs appear to be heavy nuclei with rigidity extending up to ∼10 EV. ▪ A significant (6.9σ) dipole anisotropy has been measured, but our poor understanding of Galactic magnetic fields makes it hard to interpret. ▪ The UHECR luminosity density is ∼10 44 erg Mpc −3 year −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 UHECRs.

  PublisherDOI

Recent grants

• New Challenges to Astrophysical Particle Acceleration

  NSF · $222k · 2012–2015

• Gravitational Optics, Dark Matter, and the Evolution of Faint Galaxies

  NSF · $471k · 2004–2008

• Cosmological Applications of Gravitational Lensing

  NSF · $501k · 2008–2014

Frequent coauthors

• I. A. Grenier

  Centre National de la Recherche Scientifique

  2536 shared

• J. M. Casandjian

  Université Paris Cité

  1747 shared

• L. Tibaldo

  Université de Toulouse

  1708 shared

• A. Reimer

  1664 shared

• E. Nuss

  Laboratoire Univers et Particules de Montpellier

  1642 shared

• F. Piron

  Laboratoire Univers et Particules de Montpellier

  1598 shared

• O. Reimer

  1596 shared

• J. Cohen-Tanugi

  Université Clermont Auvergne

  1579 shared

Labs

• Undergraduate Physics MajorsPI

Education

• Ph.D., Physics

  Princeton University

  1977

• B.A., Physics

  Harvard University

  1972