arXiv subjects

Andrew M. Taylor

Publications and source records attributed to Andrew M. Taylor.

At least 37 records · Page 2Linked to original sources

arXiv2021

Particle acceleration in radio galaxies with flickering jets: GeV electrons to ultrahigh energy cosmic rays

Variability is a general property of accretion discs and their associated jets. We introduce a semi-analytic model for particle acceleration and radio jet/lobe evolution and explore the effect of Myr timescale jet variability on the particles accelerated by an AGN jet. Our work is motivated by the need for local powerful ultrahigh energy cosmic ray (UHECR) sources and evidence for variability in AGN and radio galaxies. Our main results are: i) UHECR and nonthermal radiative luminosities track the jet power but with a response set by the escape and cooling times, respectively; ii) jet variability produces structure in the electron, synchrotron and UHECR spectra that deviates from that produced for a constant jet power - in particular, spectral hardening features may be signatures of variability; iii) the cutoff in the integrated CR spectrum is stretched out due to the variation in jet power (and, consequently, maximum CR energy). The resulting spectrum is the convolution of the jet power distribution and the source term. We derive an approximate form for a log-normal distribution of powers; iv) we introduce the idea of $\sim 10$ GeV 'proxy electrons' that are cooling at the same rate that UHECRs of rigidity 10 EV are escaping from the source, and determine the corresponding photon frequencies that probe escaping UHECRs. Our results demonstrate the link between the history of an astrophysical particle accelerator and its particle contents, nonthermal emission and UHECR spectrum, with consequences for observations of radio galaxies and UHECR source models.

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arXiv2021

Ultrahigh-energy dipole and beyond

Recent data from the Pierre Auger Observatory has revealed the presence of a large-scale dipole in the arrival direction distribution of ultra-high energy cosmic rays (UHECR). In this work, we build up an understanding of the diffusive origin of such a dipolar behavior as well as its dependency on energy and astrophysical source assumptions such as extragalactic magnetic field strength and cosmic ray composition. We present a novel analytical approach for calculating the angular distribution of CR coming from a single source and discuss the regimes in which the steady-state dipole result is expected. We also present a semianalytical method for calculating the evolution with energy of the resultant dipole for an ensemble of sources. We show that a local source allows for a strong growth of the dipole with energy over a large energy range. The possibility of a transition from a dipolar to non-dipolar regime at the highest energies and its implications for the source density, magnetic field intensity, and cosmic ray composition are discussed.

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arXiv2020

Revisiting the distance to the nearest UHECR source: Effects of extra-galactic magnetic fields

We update the constraints on the location of the nearest UHECR source. By analyzing recent data from the Pierre Auger Observatory using state-of-the-art CR propagation models, we reaffirm the need of local sources with a distance less than 25-100 Mpc, depending on mass composition. A new fast semi-analytical method for the propagation of UHECR in environments with turbulent magnetic fields is developed. The onset of an enhancement and a low-energy magnetic horizon of cosmic rays from sources located within a particular distance range is demonstrated. We investigate the distance to the nearest source, taking into account these magnetic field effects. The results obtained highlight the robustness of our constrained distances to the nearest source.

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arXiv2019

IceCube Neutrinos from Hadronically Powered Gamma-Ray Galaxies

In this work we use a multi-messenger approach to determine if the high energy diffuse neutrino flux observed by the IceCube Observatory can originate from $γ$-ray sources powered by Cosmic Rays interactions with gas. Typical representatives of such sources are Starburst and Ultra-Luminous Infrared Galaxies. Using the three most recent calculations of the non-blazar contribution to the extragalactic $γ$-ray background measured by the Fermi-LAT collaboration, we find that a hard power-law spectrum with spectral index $α\leq 2.12$ is compatible with all the estimations for the allowed contribution from non-blazar sources, within 1$σ$. Using such a spectrum we are able to interpret the IceCube results, showing that various classes of hadronically powered $γ$-ray galaxies can provide the dominant contribution to the astrophysical signal above 100 TeV and about half of the contribution to the energy flux between 10-100 TeV. With the addition of neutrinos from the Galactic plane, it is possible to saturate the IceCube signal at high energy. Our result shows that these sources are still well motivated candidates.

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arXiv2019

The Effects of the Galactic Magnetic Field on UHECR From Local Sources

We summarise the results found following a study of the effects that Galactic magnetic fields can have on the propagation of cosmic rays from local extragalactic sources. This study focuses on the coherent structures in the Jansson-Farrar Galactic magnetic field model, namely the: disk field, a toroidal field, and an x-field components. The phenomena of Galactic magnetic field shadowing, source deflection, and Galactic magnetic field tunnel vision are all noted. Attention throughout this study is placed on particles with rigidity around $10^{18.5}$ EV, believed to dominate the cosmic ray spectrum above the ankle. The Galactic magnetic field component predominantly responsible for giving rise to each of these different effects was determined.

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arXiv2019

A hadronuclear interpretation of a high-energy neutrino event coincident with a blazar flare

Although many high-energy neutrinos detected by the IceCube telescope are believed to have anextraterrestrial origin, their astrophysical sources remain a mystery. Recently, an unprecedenteddiscovery of a high-energy muon neutrino event coincident with a multiwavelength flare from ablazar, TXS 0506+056, shed some light on the origin of the neutrinos. It is usually believed that ablazar is produced by a relativistic jet launched from an accreting supermassive black hole (SMBH).Here we show that the high-energy neutrino event can be interpreted by the inelastic hadronuclearinteractions between the accelerated cosmic-ray protons in the relativistic jet and the dense gasclouds in the vicinity of the SMBH. Such a scenario only requires a moderate proton power in thejet, which could be much smaller than that required in the conventional hadronic model whichinstead calls upon the photomeson process. Meanwhile, the flux of the multiwavelength flare fromthe optical to gamma-ray band can be well explained by invoking a second radiation zone in thejet at a larger distance to the SMBH. In our model, the neutrino emission lasts a shorter time thanthe multiwavelength flare so the neutrino event is not necessarily correlated with the flare but it is probably accompanied by a spectrum hardening above a few GeV.

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arXiv2018

Flux Distribution of Gamma-Ray Emission in Blazars: The Example of Mrk 501

Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light curves, and different models have been proposed to explain these data. Here, we collect some of the recent developments on this topic using the well-known blazar Mrk 501 as example of complex and interesting aspects coming from its flux distribution in different energy ranges and at different timescales. The observational data we refer to are those collected in a complementary manner by Fermi-LAT over multiple years, and by the First G-APD Cherenkov Telescope (FACT) telescope and the H.E.S.S. array in correspondence of the bright flare of June 2014.

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arXiv2018

Binary Neutron Star Merger Remnants as Sources of Cosmic Rays Below the "Ankle"

We investigate non-thermal electron and nuclei energy losses within the binary neutron star merger remnant produced by the event GW170817. The lack of a cooling feature within the detected synchrotron emission from the source is used to constrain the magnetic field at the mG level, assuming that this emission is electron synchrotron in origin, and that the accelerated spectrum in the electrons follows the form $dN/dE_e \propto E_e^{-2}$. The level of subsequent gamma-ray emission from the source is demonstrated to provide a further constraint on the source magnetic field strength. We also put forward alternative strong ($\sim$G) magnetic field scenarios able to support this emission. For such stronger fields, the photo-disintegration of non-thermal nuclei within the source is considered, and a bottleneck period of $\sim$5-30 days is found when this process peaks. We find that this class of source is in principle able to support the population of cosmic rays detected at Earth below the "ankle".

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arXiv2018

Energetic gamma-ray emission from solar flares

Recent advances in the $γ$-ray observations of solar flares by the \textit{Fermi} satellite, demand revisions in the hadronic $γ$-ray flux computation below 1 GeV. In this work we utilize recently updated pion production cross sections, along with an accurate description of low energy nuclear interactions. Applying these new interaction descriptions to model the \textit{Fermi} Large Area Telescope (LAT) solar flare data, we infer the primary particle spectral parameters. Application of this new cross section description leads to significantly different spectral parameters compared to those obtained previously. Furthermore, the inclusion of nuclei in these calculations leads to a primary spectrum that is generally harder than that required from proton only considerations. Lastly, the flare data at lower MeV energies, detected by the \textit{Fermi} Gamma-ray Burst Monitor (GBM), is demonstrated to provide additional low-energy spectral information.

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arXiv2017

Galactic Cosmic-Rays in a Breeze

We study a scenario in which the Fermi bubbles are formed through a Galactocentric outflow of gas and pre-accelerated cosmic-rays (CR). We take into account CR energy losses due to proton-proton interactions with the gas present in the bubbles, and calculate the associated gamma-ray emission. We find that CRs diffusing and advecting within a breeze outflow result in an approximately flat surface brightness profile of the gamma-ray emission, as observed by Fermi satellite. Finally, we apply similar outflow profiles to larger Galactocentric radii, and investigate their effects on the CR spectrum and boron-to-carbon ratio. Hardenings can appear in the spectrum, even in cases with equal CR diffusion coefficients in the disk and halo.

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arXiv2017

Extragalactic Observations with HESS: Past and Future

The present H.E.S.S. II era of the H.E.S.S. collaboration follows from the successful upgrade of the H.E.S.S. array, and the first published results obtained with this new instrument. Thanks to these achievements, a lower energy threshold with H.E.S.S. II than that obtained previously with H.E.S.S. I has been clearly demonstrated. The success of these developments has now opened up a whole new lower-energy window to the Universe. I here explore, from the extragalactic point of view, both what has been learnt so far from the H.E.S.S. observations through this new window, and postulate also on what else might still be seen beyond this. I will firstly reflect upon the AGN result highlights, obtained using the new H.E.S.S. II instrument. Specifically, the rise in detections of bright FSRQ AGN with H.E.S.S. II will be addressed [eg. PKS 0736 (z=0.19), PKS 1510 (z=0.36), and 3C 279 (z=0.54)]. Hand in hand with this recent progress, the benefits brought to our efforts to observe GRBs through both access to this new lower energy window, and the quick instrument response to ToO alerts, will be covered. Furthermore, the potential for the discovery of new transient phenomena in the H.E.S.S. II era will also be emphasised. Lastly, the question as to lessons learnt from previous H.E.S.S. I AGN results about the intrinsic source spectra, primarily focusing on the HBL class, will be considered.

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arXiv2017

The ionising effect of low energy cosmic rays from a class II object on its protoplanetary disk

We investigate the ionising effect of low energy cosmic rays (CRs) from a young star on its protoplanetary disk (PPD). We consider specifically the effect of $\sim3\,$GeV protons injected at the inner edge of the PPD. An increase in the ionisation fraction as a result of these CRs could allow the magnetorotational instability to operate in otherwise magnetically dead regions of the disk. For the typical values assumed we find an ionisation rate of $ζ_\mathrm{CR} \sim 10^{-17}\mathrm{s^{-1}}$ at $1\,$au. The transport equation is solved by treating the propagation of the CRs as diffusive. We find for increasing diffusion coefficients the CRs penetrate further in the PPD, while varying the mass density profile of the disk is found to have little effect. We investigate the effect of an energy spectrum of CRs. The influence of a disk wind is examined by including an advective term. For advective wind speeds between $1-100\mathrm{km\,s^{-1}}$ diffusion dominates at all radii considered here (out to 10$\,$au) for reasonable diffusion coefficients. Overall, we find that low energy CRs can significantly ionise the midplane of PPDs out to $\sim\,1\,$au. By increasing the luminosity or energy of the CRs, within plausible limits, their radial influence could increase to $\sim2\,$au at the midplane but it remains challenging to significantly ionise the midplane further out.

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arXiv2017

UHECR propagation from Centaurus A

In the light of the recently predicted isotopic composition of the kpc-scale jet in Centaurus A, we re-investigate whether this source could be responsible for some of the ultra-high energy cosmic rays detected by the Pierre Auger Observatory. We find that a nearby source like Centaurus A is well motivated by the composition and spectral shape, and that such sources should start to dominate the flux above ~ 4 EeV. The best-fitting isotopes from our modelling, with the maximum 56Fe energy fixed at 250 EeV, are of intermediate mass, 12C to 16O, while the best-fitting particle index is 2.3.

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arXiv2017

Cosmic Rays in a Galactic Breeze

Motivated by the discovery of the non-thermal Fermi bubble features both below and above the Galactic plane, we investigate a scenario in which these bubbles are formed through Galacto-centric outflow. Cosmic rays (CR) both diffusing and advecting within a Galactic breeze outflow, interacting with the ambient gas present, give rise to gamma-ray emission, providing an approximately flat surface brightness profile of this emission, as observed. Applying the same outflow profile further out within the disk, the resultant effects on the observable CR spectral properties are determined. A hardening in the spectra due to the competition of advective and diffusive propagation within a particular energy range is noted, even in the limiting case of equal CR diffusion coefficients in the disk and halo. It is postulated that this hardening effect may relate to the observed hardening feature in the CR spectrum at a rigidity of $\approx 200$ GV.

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arXiv2016

Indication of a Local "Fog" of Sub-Ankle UHECR

During their propagation through intergalactic space, ultrahigh energy cosmic rays (UHECRs) interact with the background radiation fields. These interactions give rise to energetic electron/positron pairs and photons which in turn feed electromagnetic cascades, contributing to the isotropic gamma-ray background (IGRB). The gamma-ray flux level generated in this way highly depends upon the UHECR propagation distance, as well as the evolution of their sources with redshift. Recently, the Fermi-LAT collaboration reported that the majority of the total extragalactic gamma-ray flux originates from extragalactic point sources. This posits a stringent upper limit on the IGRB generated via UHECR propagation, and subsequently constrains their abundance in the distant Universe. Focusing on the contribution of UHECR at energies below the ankle within a narrow energy band ($(1-4)\times 10^{18}$eV), we calculate the diffuse gamma-ray flux generated through UHECR propagation, normalizing the total cosmic ray energy budget in this band to that measured. We find that in order to not over-produce the new IGRB limit, a local "fog" of UHECR produced by nearby sources may exist, with a possible non-negligible contribution from our Galaxy. Following the assumption that a given fraction of the observed IGRB at 820GeV originates from UHECR, we obtain a constraint on the maximum distance for the majority of their sources. With other unresolved source populations still contaminating the new IGRB limit, and UHECR above the ankle invariably contributing also to this background, the results presented here are rather conservative.

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arXiv2015

AGN observations with a less than 100 GeV threshold using H.E.S.S. II

The recent addition of the 28 m Cherenkov telescope (CT5) to the H.E.S.S. array extended the experiment's sensitivity towards low energies. The lowest energy threshold is obtained using monoscopic observations with CT5, providing access to gamma-ray energies below 100 GeV. This is particularly beneficial for studies of Active Galactic Nuclei (AGN) with soft spectra and located at redshifts >= 0.5. Stereoscopic measurements with the full array (CT1-5) provide a better background rejection than CT5 Mono, at a cost of a higher threshold. We report on the analysis employing the CT5 data for AGN observations with a < 100 GeV threshold. In particular, the spectra of PKS 2155-304 and PG 1553+113 are presented.

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arXiv2015

Indications of Negative Evolution for the Sources of the Highest Energy Cosmic Rays

Using recent measurements of the spectrum and chemical composition of the highest energy cosmic rays, we consider the sources of these particles. We find that the data strongly prefers models in which the sources of the ultra-high energy cosmic rays inject predominantly intermediate mass nuclei, with comparatively few protons or heavy nuclei, such as iron or silicon. If the number density of sources per comoving volume does not evolve with redshift, the injected spectrum must be very hard ($α\simeq 1$) in order to fit the spectrum observed at Earth. Such a hard spectral index would be surprising and difficult to accommodate theoretically. In contrast, much softer spectral indices, consistent with the predictions of Fermi acceleration ($α\simeq 2$), are favored in models with negative source evolution. With this theoretical bias, these observations thus favor models in which the sources of the highest energy cosmic rays are preferentially located within the low-redshift universe.

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arXiv2015

A Unified Model of the Fermi Bubbles, Microwave Haze, and Polarized Radio Lobes: Reverse Shocks in the Galactic Center's Giant Outflows

The Galactic Center's giant outflows are manifest in three different, non-thermal phenomena: i) the hard-spectrum, γ-ray `Fermi Bubbles' emanating from the nucleus and extending to |b| ~ 50 degrees; ii) the hard-spectrum, total-intensity microwave (~ 20-40 GHz) `Haze' extending to |b| ~ 35 degrees in the lower reaches of the Fermi Bubbles; and iii) the steep spectrum, polarized, `S-PASS' radio (~ 2-20 GHz) Lobes that envelop the Bubbles and extend to |b| ~ 60 degrees. We find that the nuclear outflows inflate a genuine bubble in each Galactic hemisphere that has the classical structure, working outwards, of reverse shock, contact discontinuity, and forward shock. Expanding into the finite pressure of the halo and given appreciable cooling and gravitational losses, the contact discontinuity of each bubble is now expanding only very slowly. We find observational signatures in both hemispheres of giant, reverse shocks at heights of ~ 1 kpc above the nucleus; their presence ultimately explains all three of the non-thermal phenomena mentioned above. Synchrotron emission from shock-reaccelerated cosmic-ray electrons explains the spectrum, morphology, and vertical extent of the microwave Haze and the polarized radio Lobes. Collisions between shock-reaccelerated hadrons and denser gas in cooling condensations that form inside the contact discontinuity account for most of the Bubbles' γ-ray emissivity.

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