Extragalactic Magnetic Fields Research Articles

The Large-scale Anisotropy and Flux (de)magnification of Ultrahigh-energy Cosmic Rays in the Galactic Magnetic Field

We calculate the arrival direction distribution of ultrahigh-energy cosmic rays (UHECRs) with a new suite of models of the Galactic magnetic field (GMF), assuming sources follow the large-scale structure of the Universe. Compared to previous GMF models, the amplitude of the dipole component of the UHECR arrival flux is significantly reduced. We find that the reduction is due to the accidentally coinciding position of the peak of the extragalactic UHECR flux and the boundary of strong flux demagnification due to the GMF toward the central region of the Galaxy. This serendipitous sensitivity of UHECR anisotropies to the GMF model will be a powerful probe of the source distribution as well as Galactic and extragalactic magnetic fields. Demagnification by the GMF also impacts the visibility of some popular source candidates.

Isotropy of Cosmic Rays beyond 10^{20} eV Favors Their Heavy Mass Composition.

We report an estimation of the injected mass composition of ultrahigh energy cosmic rays (UHECRs) at energies higher than 10EeV. The composition is inferred from an energy-dependent sky distribution of UHECR events observed by the Telescope Array surface detector by comparing it to the Large Scale Structure of the local Universe. In the case of negligible extragalactic magnetic fields (EGMFs), the results are consistent with a relatively heavy injected composition at E∼10 EeV that becomes lighter up to E∼100 EeV, while the composition at E>100 EeV is very heavy. The latter is true even in the presence of highest experimentally allowed extragalactic magnetic fields, while the composition at lower energies can be light if a strong EGMF is present. The effect of the uncertainty in the galactic magnetic field on these results is subdominant.

Atacama Large Aperture Submillimeter Telescope (AtLAST) science: Gas and dust in nearby galaxies

Understanding the physical processes that regulate star formation and galaxy evolution are major areas of activity in modern astrophysics. Nearby galaxies offer unique opportunities to inspect interstellar medium (ISM), star formation (SF), radiative, dynamic and magnetic ( B → ) physics in great detail from sub-galactic (kpc) scales to sub-cloud (sub-pc) scales, from quiescent galaxies to starbursts, and from field galaxies to overdensities. In this case study, we discuss the major breakthroughs in this area of research that will be enabled by the Atacama Large Aperture Submillimeter Telescope (AtLAST), a proposed 50-m single-dish submillimeter telescope. The new discovery space of AtLAST comes from its exceptional sensitivity, in particular to extended low surface brightness emission, a very large 2° field of view, and correspondingly high mapping efficiency. This paper focuses on four themes which will particularly benefit from AtLAST: 1) the LMC and SMC, 2) extragalactic magnetic fields, 3) the physics and chemistry of the interstellar medium, and 4) star formation and galaxy evolution. With ~ 1000 − 2000 hour surveys each, AtLAST could deliver deep dust continuum maps of the entire LMC and SMC fields at parsec-scale resolution, high-resolution maps of the magnetic field structure, gas density, temperature and composition of the dense and diffuse ISM in ~ 100 nearby galaxies, as well as the first large-scale blind CO survey in the nearby Universe, delivering molecular gas masses for up to 106 galaxies (3 orders of magnitude more than current samples). Through such observing campaigns, AtLAST will have a profound impact on our understanding of the baryon cycle and star formation across a wide range of environments.

Ultra-high-energy Cosmic-Ray Sources Can Be Gamma-Ray Dim

Ultra-high-energy cosmic rays (UHECRs), accelerated hadrons that can exceed energies of 1020 eV, are the highest-energy particles ever observed. While the sources producing UHECRs are still unknown, the Pierre Auger Observatory has detected a large-scale dipole anisotropy in the arrival directions of cosmic rays above 8 EeV. In this work, we explore whether resolved gamma-ray sources can reproduce the Auger dipole. We use various Fermi Large Area Telescope catalogs as sources of cosmic rays in CRPropa simulations. We find that in all cases, the simulated dipole has an amplitude significantly larger than that measured by Auger, even when considering large extragalactic magnetic field strengths and optimistic source weighting schemes. Our result implies that the resolved gamma-ray sources are insufficient to account for the population of sources producing the highest-energy cosmic rays, and there must exist a population of UHECR sources that lack gamma-ray emission or are unresolved by the current-generation gamma-ray telescopes.

Constraints on UHECR Sources and Extragalactic Magnetic Fields from Directional Anisotropies

A dipole anisotropy in ultra–high-energy cosmic ray (UHECR) arrival directions, of extragalactic origin, is now firmly established at energies E > 8 EeV. Furthermore, the UHECR angular power spectrum shows no power at smaller angular scales than the dipole, apart from hints of possible individual hot or warm spots for energy thresholds ≳40 EeV. Here we exploit the magnitude of the dipole and the limits on smaller-scale anisotropies to place constraints on two quantities: the extragalactic magnetic field (EGMF) and the number density of UHECR sources or the volumetric event rate if UHECR sources are transient. We also vary the bias between the extragalactic matter and the UHECR source densities, reflecting whether UHECR sources are preferentially found in over- or underdense regions, and find that little or no bias is favored. We follow Ding et al. (2021) in using the CosmicFlows-2 density distribution of the local universe as our baseline distribution of UHECR sources, but we improve and extend that work by employing an accurate and self-consistent treatment of interactions and energy losses during propagation. Deflections in the Galactic magnetic field are treated using either the full JF12 magnetic field model, with both random and coherent components, or just the coherent part, to bracket the impact of the GMF on the dipole anisotropy. This large-scale structure model gives good agreement with both the direction and magnitude of the measured dipole anisotropy and forms the basis for simulations of discrete sources and the inclusion of EGMF effects.

Effect of extragalactic magnetic field on cascade gamma-ray emission

The intensity of diffuse cascade gamma-ray emission produced in electromagnetic cascades when ultra-high energy cosmic rays propagate in extragalactic space is discussed. It is obtained that in the range of ~107—109 eV diffuse cascade gamma-ray spectra weakly depends on extragalactic magnetic field value. Thus, there is no need to refine extragalactic magnetic field models to estimate the intensity of cascade emission in this energy range.

Effect of the Extragalactic Magnetic Field on Cascade Gamma-Ray Emissions

Effect of the Extragalactic Magnetic Field on Cascade Gamma-Ray Emissions

Deep Synoptic Array Science: Implications of Faraday Rotation Measures of Fast Radio Bursts Localized to Host Galaxies

Faraday rotation measures (RMs) of fast radio bursts (FRBs) offer the prospect of directly measuring extragalactic magnetic fields. We present an analysis of the RMs of 10 as yet nonrepeating FRBs detected and localized to host galaxies with robust redshift measurements by the 63-antenna prototype of the Deep Synoptic Array (DSA-110). We combine this sample with published RMs of 15 localized FRBs, nine of which are repeating sources. For each FRB in the combined sample, we estimate the host-galaxy dispersion measure (DM) contributions and extragalactic RM. We find compelling evidence that the extragalactic components of FRB RMs are often dominated by contributions from the host-galaxy interstellar medium (ISM). Specifically, we find that both repeating and as yet nonrepeating FRBs show a correlation between the host DM and host RM in the rest frame, and we find an anticorrelation between extragalactic RM (in the observer frame) and redshift for nonrepeaters, as expected if the magnetized plasma is in the host galaxy. Important exceptions to the ISM origin include a dense, magnetized circumburst medium in some repeating FRBs, and the intracluster medium of host or intervening galaxy clusters. We find that the estimated ISM magnetic-field strengths, , are characteristically ∼1–2 μG larger than those inferred from Galactic radio pulsars. This suggests either increased ISM magnetization in FRB hosts in comparison with the Milky Way, or that FRBs preferentially reside in regions of increased magnetic-field strength within their hosts.

First constraints on the strength of the extragalactic magnetic field from γ-ray observations of GRB 221009A

ABSTRACT The extragalactic magnetic field (EGMF) could be probed with γ-ray observations of distant sources. Primary very high-energy γ-rays from these sources absorb on extragalactic background light photons, and secondary electrons/positrons from the pair production acts create cascade γ-rays. These cascade γ-rays could be detected with space γ-ray telescopes such as Fermi-LAT. The γ-ray burst GRB 221009A was an exceptionally bright transient well suited for intergalactic γ-ray propagation studies. Using publicly available Fermi-LAT data, we obtain upper limits on the spectrum of delayed emission from GRB 221009A during the time windows of 10, 30, and 90 d after the burst, and compare these with model spectra calculated for various EGMF strengths B, obtaining constraints on B. We show that the values of B between 10−20 and 10−18 G are excluded.

Extragalactic electromagnetic cascades and cascade gamma-ray emission in magnetic fields of various strength

We discuss the magnetic field influence on diffuse gamma-ray emission from extragalactic electromagnetic cascades initiated by ultra-high energy cosmic rays. Regions in space vary considerably in field strength: it is possibly of 10−12 G and lower in voids, of ∼10−6 G inside galaxies, galactic clusters and groups, of ∼10−7 G around them, and of ∼10−8–10−9 G in filaments. Structures having fields higher than in voids occupy comparatively small fraction of the Universe, so they affect weakly on cascade emission. Still knowledge of this influence may be relevant studying large-scale component of the extragalactic magnetic field and to the search for exotic particles, as in the latter case contribution of all components to extragalactic gamma-ray background should be known, one of which is cascade emission. To study magnetic field effect we simulate particle propagation in homogeneous magnetic field of ∼10−6, 10−9, and 10−12 G and lower. It is found that in fields of ∼10−9 G and lower the spectra of diffuse cascade gamma-rays at energies E ≤ 1017 eV coincide. Thus no specific models of EGMF are required to study contribution of cascade emission in the extragalactic gamma-ray background at E ≤ 1017 eV. In the case of uniform field of 10−6 G (which seems to be unrealistic), this inference is valid in the energy range of ∼107109 eV. Results obtained can be also used studying large-scale component of the extragalactic magnetic field.

Nearby active galactic nuclei and starburst galaxies as sources of the measured UHECRs anisotropy signal

The Pierre Auger and the Telescope Array observatories have measured independent and statistical significant anisotropy in the arrival direction of ultra-high-energy cosmic rays (UHECR). Three hotspot regions with relative excess of events and a dipole signal have been identified in different regions of the sky and energy ranges. In this paper, we investigate the conditions under which these anisotropy signal could be generated by nearby (<23 Mpc) active galactic nuclei (AGN) and/or starburst galaxies (SBG). We studied a wide range of possibilities including injected nuclei (p, He, N, Si, and Fe), three UHECR luminosity proxies and three extragalactic magnetic field models. The results shows that both local AGN and SBG are needed to describe all the anisotropy signal. The contribution of AGN to hotspots and to the generation of the dipole is dominant in most cases. SBG is required only to explain the hotspot measured by the Telescope Array Observatory.

Exploring the Emission Mechanisms of Mrk 180 with Long-term X-Ray and γ-Ray Data

Markarian (Mrk) 180 is a BL Lacertae object located at a redshift of 0.045 and is a potential candidate for high-energy cosmic-ray acceleration. We have analyzed the Fermi Large Area Telescope (Fermi-LAT) γ-ray data of Mrk 180 collected over a period of 12.8 yr and found no significant enhancement in the flux from the long-term γ-ray light curve. We have also analyzed Swift X-ray, ultraviolet, and optical, and X-ray Multi-Mirror Mission (XMM-Newton) data to construct the multiwavelength spectral energy distribution (SED). The SED has been modeled with one-zone pure leptonic and lepto-hadronic scenarios to explain the underlying physics of multiwavelength emission. The pure leptonic model and the two lepto-hadronic models, viz., (i) line-of-sight interactions of ultrahigh-energy cosmic rays (UHECRs; E ≳ 1017 eV) with the cosmic background radiation and (ii) the interactions of relativistic protons with the cold protons in the jet, have been compared in our work. Moreover, an earlier study has associated Mrk 180 with the Telescope Array (TA) hotspot of UHECRs at E > 57 EeV. This speculation motivates us to check whether ultrahigh-energy protons and iron nuclei can reach the Earth from Mrk 180. After comparing the results of our simulation with the current observational data, we find that Mrk 180 is unlikely to be a source of the UHECR events contributing to the TA hotspot for conservative strengths of extragalactic magnetic fields.

Impact of the finite life-time of UHECR sources

The observational data on ultrahigh energy cosmic rays (UHECR), in particular their mass composition, show strong indications for extremely hard spectra of individual mass groups of CR nuclei at Earth. In this work, we show that such hard spectra can be the result of the finite life-time of UHECR sources, if a few individual sources dominate the UHECR flux at the highest energies. In this case, time delays induced by deflections in the turbulent extragalactic magnetic field as well as from the diffusive or advective escape from the source environment can suppress low-energy CRs, leading to a steepening of the observed spectrum. Considering radio galaxies as the main source of UHECRs, we discuss the necessary conditions that few individual sources dominate over the total contribution from the bulk of sources that have been active in the past. We provide two proof-of-principle scenarios showing that for a turbulent extragalactic magnetic field with a strength B and a coherence length l coh, the life-time of a source at a distance d src should satisfy t act ∼ (B/1 nG)2 (d src/10 Mpc)2 (l coh/1 Mpc) Myr to obtain the necessary hardening of the CR spectrum at Earth.

UHECR anisotropy and extragalactic magnetic fields with the Telescope Array

We study the energy-dependent distribution of ultra-high energy cosmic ray arrival directions with respect to luminous matter in the local Universe. We use a specially designed test statistic (TS) that is robust to uncertainties of the galactic magnetic field. We generate realistic mock UHECR sets assuming various injected compositions, and different strengths of the extragalactic magnetic field (EGMF). Applying the TS to both mock sets and the Telescope Array Surface Detector data we constrain, for a given EGMF strength, the UHECR injected mass composition at energies above 10 EeV. At highest energies, higher than 100 EeV, only heavy composition — iron or at least silicon — is compatible with the data, irrespective of the EGMF strength. We then compare the obtained results with the direct Telescope Array fluorescence measurements of the UHECR mass composition. Requiring that the TA composition measurements are compatible with the arrival direction distribution allows us to constrain the parameters of the EGMF. It appears that light composition, measured by TA at around 10 EeV, is compatible with the arrival directions if the EGMF has strength of order 1 nG.

CRPropa high statistics simulations for UHECR anisotropy studies

Despite the great progress made by modern cosmic ray observatories, the origin and acceleration mechanism of ultra-high-energy cosmic rays (UHECRs) remains unsolved problems to this day. However, there is experimental evidence for an anisotropic component in the UHECR arrival direction at energies greater than few EeV. The search for UHECR sources is further complicated by two main factors: during extragalactic propagation UHECRs interact with background photon fields (like the CMB and the EBL) and, since they are electrically charged, they are deflected by extragalactic and galactic magnetic fields (EGMF and GMF). Moreover, the strength, structure and origin of the EGMF are still not well known, causing reconstructing the deflection of UHECRs to be a non-trivial task. In this work we consider several EGMF models obtained from constrained MagnetoHydroDynamics (MHD) simulations of our local Universe to study the propagation of UHECRs through such a structured environment. We simulate propagation, interactions and observation of UHECRs by using the Monte Carlo code CRPropa3. We also take into account the effect of the GMF by adopting a lensing procedure of the arrival UHECR sky map. We explore several combination of source distribution, EGMF structure and mass composition. As a reference, we also simulate scenarios without the EGMF and with a statistically homogeneous field. We use our simulation results to compute UHECR observables, such as the energy spectrum, the angular power spectrum and the arrival direction map (before and after the GMF) in order to obtain constraints on possible combinations of source distributions and EGMF models.

Tracing Magnetic Fields in IC 1954 with Velocity Gradient

Magnetic fields, especially the magnetic braking effect, are crucial in transporting fuel materials to the galactic center and powering nuclear starbursts. To access the magnetic fields directly associated with molecular gas, we use the novel velocity gradient technique (VGT). By applying VGT to the high-resolution CO (2-1) emission lines obtained from the PHANGS-ALMA survey, we present the magnetic field orientation map for the galaxy IC 1954. With the advent of next-generation radio and spectroscopic observations from SKA and ngVLA, we discuss the promise of studying extragalactic magnetic fields at pc-scale.

Implications of multiwavelength spectrum on cosmic-ray acceleration in blazar TXS 0506+056

Context.The MAGIC collaboration has recently analyzed data from a long-term multiwavelength campaign of theγ-ray blazar TXS 0506+056. In December 2018 it was flaring in the very high-energy (VHE;E &gt; 100 GeV)γ-ray band, but no simultaneous neutrino event was detected.Aims.We modeled the observed spectral energy distribution (SED) using a one-zone leptohadronic emission.Methods.We estimated the neutrino flux through the restriction from the observed X-ray flux on the secondary radiation due to the hadronic cascade, initiated by protons with energyEp ≲ 0.1 EeV. We assumed that ultra-high-energy cosmic rays (UHECRs;E ≳ 0.1 EeV), with the same slope and normalization as the low-energy spectrum, are accelerated in the jet but escape efficiently. We propagate the UHE protons in a random turbulent extragalactic magnetic field (EGMF).Results.The leptonic emission from the jet dominates the GeV range, whereas the cascade emission from CR interactions in the jet contributes substantially to the X-ray and VHE range. The line-of-sight cosmogenicγ-rays from UHECRs produce a hardening in the VHE spectrum. Our model prediction for neutrinos from the jet is consistent with the 7.5-year flux limit by IceCube and shows no variability during the MAGIC campaign. Therefore, we infer that the correlation between GeV-TeVγ-rays and neutrino flare is minimal. The luminosity in CRs limits the cosmogenicγ-ray flux, which in turn bounds the RMS value of the EGMF to ≳10−5nG. The cosmogenic neutrino flux is lower than the IceCube-Gen2 detection potential for 10 yr of observation.Conclusions.Very high-energyγ-ray variability should arise from increased activity inside the jet; thus, detecting steady flux at multi-TeV energies may indicate UHECR acceleration. Upcomingγ-ray imaging telescopes, such as the CTA, will be able to constrain the cosmogenicγ-ray component in the SED of TXS 0506+056.

Influence of Extragalactic Magnetic Fields on Extragalactic Cascade Gamma-Ray Emission

We discuss the influence of extragalactic magnetic fields on the intensity of gamma-ray emission produced in electromagnetic cascades from ultra-high energy cosmic rays propagating in extragalactic space. Both cosmic rays and cascade particles propagate mostly out of galaxies, galactic clusters, and large-scale structures, as their relative volume is small. Therefore, their magnetic fields weakly affect emission produced in cascades. Yet, estimates of this influence can be useful in searching for dark matter particles when components of extragalactic gamma-ray background should be known, including cascade gamma-ray emission. To study magnetic field influence on cascade emission, we calculated cosmic particle propagation in fields of ~10−6 and 10−12 G (the former is typical inside galaxies and clusters and the latter is common in voids and outside galaxies and clusters). The calculated spectra of cascade gamma-ray emissions are similar in the range of ~107–109 eV, so analyzing cascade emission in this range it is not necessary to specify models of an extragalactic magnetic field.

Constraints on the extragalactic magnetic field strength from blazar spectra based on 145 months of Fermi-LAT observations

ABSTRACT Properties of the extragalactic magnetic field (EGMF) outside of clusters and filaments of the large-scale structure are essentially unknown. The EGMF could be probed with γ-ray observations of distant (redshift z &amp;gt; 0.1) blazars. TeV γ-rays from these sources are strongly absorbed on extragalactic background light photons; secondary electrons and positrons produce cascade γ-rays with the observable flux dependent on EGMF parameters. We put constraints on the EGMF strength using 145 months of Fermi-LAT observations of the blazars 1ES 1218+304, 1ES 1101−232, and 1ES 0347−121, and imaging atmospheric Cherenkov telescope observations of the same sources. We perform a series of full direct Monte Carlo simulations of intergalactic electromagnetic cascades with the elmag 3.01 code and construct a model of the observable spectra inside the point spread functions of the observing instruments for a range of EGMF strengths. We compare the observed spectra with the models for various values of the EGMF strength B and calculate the exclusion statistical significance for every value of B. We find that the values of the EGMF strength B ≤ 10−17 G are excluded at a high level of the statistical significance Z &amp;gt; 4σ for all the four options of the intrinsic spectral shape considered (power law, power law with exponential cutoff, log-parabola, and log-parabola with exponential cutoff). The value of B = 10−16 G is not excluded; it is still a viable option of the EGMF strength. These results were obtained for the case of steady sources.

Sensitivity of Cross-Correlation Studies by Using Xmax Information

The investigation about cosmic-ray sources through the study of the arrival direction of the particles is completely dependent on primary mass compositions. This is due to heavier particles having an average larger deflection during propagation in galactic and extragalactic magnetic fields. Recent result from the Auger Collaboration made it possible to obtain information on the chemical composition of the cosmic rays on an event-by-event basis in addition to data from the surface detector. Here, we have investigated the gain in detection efficiency of standard cross-correlation using the Landy–Szalay estimator. Our sample contained only events with the highest Xmax values (corresponding to the lightest charged particles, for which the deflections in magnetic fields are smaller on average). We obtained results showing that that gain depends on the proton fraction ranging from a factor of ∼2 to ∼4.