arXiv subjects

Roland M. Crocker

Publications and source records attributed to Roland M. Crocker.

67 records · Page 4Linked to original sources

arXiv2010

Gamma-Rays and the Far-Infrared-Radio Continuum Correlation Reveal a Powerful Galactic Centre Wind

We consider the thermal and non-thermal emission from the inner 200 pc of the Galaxy. The radiation from this almost star-burst-like region is ultimately driven dominantly by on-going massive star formation. We show that this region's radio continuum (RC) emission is in relative deficit with respect to the expectation afforded by the Far- infrared-Radio Continuum Correlation (FRC). Likewise we show that the region's gamma-ray emission falls short of that expected given its star formation and resultant supernova rates. These facts are compellingly explained by positing that a powerful (400-1200 km/s) wind is launched from the region. This wind probably plays a number of important roles including advecting positrons into the Galactic bulge thus explaining the observed ~kpc extension of the 511 keV positron annihilation signal around the GC. We also show that the large-scale GC magnetic field falls in the range ~100-300 microG and that - in the time they remain in the region - GC cosmic rays do not penetrate into the region's densest molecular material.

astro-ph.GA↗

arXiv2010

Radio and gamma-ray constraints on dark matter annihilation in the Galactic center

We determine upper limits on the dark matter (DM) self-annihilation cross section for scenarios in which annihilation leads to the production of electron--positron pairs. In the Galactic centre (GC), relativistic electrons and positrons produce a radio flux via synchroton emission, and a gamma ray flux via bremsstrahlung and inverse Compton scattering. On the basis of archival, interferometric and single-dish radio data, we have determined the radio spectrum of an elliptical region around the Galactic centre of extent 3 degrees semi-major axis (along the Galactic plane) and 1 degree semi-minor axis and a second, rectangular region, also centered on the GC, of extent 1.6 degrees x 0.6 degrees. The radio spectra of both regions are non-thermal over the range of frequencies for which we have data: 74 MHz -- 10 GHz. We also consider gamma-ray data covering the same region from the EGRET instrument (about GeV) and from HESS (around TeV). We show how the combination of these data can be used to place robust constraints on DM annihilation scenarios, in a way which is relatively insensitive to assumptions about the magnetic field amplitude in this region. Our results are approximately an order of magnitude more constraining than existing Galactic centre radio and gamma ray limits. For a DM mass of m_χ=10 GeV, and an NFW profile, we find that the velocity-averaged cross-section must be less than a few times 10^-25 cm^3 s^-1.

hep-ph↗

arXiv2010

A New Lower Limit to the Galactic Centre's Magnetic Field

The amplitude of the magnetic field surrounding the Galactic Centre (GC) on large scales (> 100 pc) has been uncertain by two orders of magnitude for several decades: different analyses report fields as weak as ~6 microG on the one hand and 1 mG on the other. Here I report on our recent work which shows that the field on 400 pc scales has a firm lower limit of about 50 microG. To obtain this result we compiled existing (mostly single dish) radio data to construct the spectrum of the GC region on these size scales. This spectrum is a broken power law with a down-break (most conservatively) attributable to a transition from bremsstrahlung to synchrotron cooling of the in-situ cosmic-ray electron population. The lower limit on the magnetic field arises through the consideration that the synchrotron-emitting electrons should not produce too much gamma-ray emission given existing constraints from the EGRET instrument.

astro-ph.GA↗

arXiv2010

A lower limit of 50 microgauss for the magnetic field near the Galactic Centre

The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. On a scale of approximately 100 pc fields of approximately 1000 microG have been reported, implying a magnetic energy density more than 10,000 times stronger than typical for the Galaxy. Alternatively, the assumption of pressure equilibrium between the various phases of the Galactic Centre interstellar medium (including turbulent molecular gas; the contested "very hot" plasma; and the magnetic field) suggests fields of approximately 100 microG over approximately 400 pc size scales. Finally, assuming equipartition, fields of only approximately 6 microG have been inferred from radio observations for 400 pc scales. Here we report a compilation of previous data that reveals a down-break in the region's non-thermal radio spectrum (attributable to a transition from bremsstrahlung to synchrotron cooling of the in situ cosmic-ray electron population). We show that the spectral break requires that the Galactic Centre field be at least 50 microG on 400 pc scales, lest the synchrotron-emitting electrons produce too much gamma-ray emission given existing constraints. Other considerations support a field of 100 microG, implying that > 10% of the Galaxy's magnetic energy is contained in only < 0.05% of its volume.

astro-ph.GA↗

arXiv2008

Neutrinos and Gamma Rays from Galaxy Clusters

The next generation of neutrino and gamma-ray detectors should provide new insights into the creation and propagation of high-energy protons within galaxy clusters, probing both the particle physics of cosmic rays interacting with the background medium and the mechanisms for high-energy particle production within the cluster. In this paper we examine the possible detection of gamma-rays (via the GLAST satellite) and neutrinos (via the ICECUBE and Auger experiments) from the Coma cluster of galaxies, as well as for the gamma-ray bright clusters Abell 85, 1758, and 1914. These three were selected from their possible association with unidentified EGRET sources, so it is not yet entirely certain that their gamma-rays are indeed produced diffusively within the intracluster medium, as opposed to AGNs. It is not obvious why these inconspicuous Abell-clusters should be the first to be seen in gamma-rays, but a possible reason is that all of them show direct evidence of recent or ongoing mergers. Their identification with the EGRET gamma-ray sources is also supported by the close correlation between their radio and (purported) gamma-ray fluxes. Under favorable conditions (including a proton spectral index of 2.5 in the case of Abell 85, and sim 2.3 for Coma, and Abell 1758 and 1914), we expect ICECUBE to make as many as 0.3 neutrino detections per year from the Coma cluster of galaxies, and as many as a few per year from the Abell clusters 85, 1758, and 1914. Also, Auger may detect as many as 2 events per decade at ~ EeV energies from these gamma-ray bright clusters.

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arXiv2007

Exploring the High-Energy Cosmic Ray Spectrum with a Toy Model of Cosmic Ray Diffusion

We introduce a static toy model of the cosmic ray (CR) universe in which cosmic ray propagation is taken to be diffusive and cosmic ray sources are distributed randomly with a density the same as that of local L* galaxies, $5 \times 10^{-3}$ Mpc$^{-3}$. These sources "fire" at random times through the history of the universe but with a set expectation time for the period between bursts. Our toy model model captures much of the essential CR physics despite its simplicity and, moreover, broadly reproduces CR phenomenology for reasonable parameter values and without extreme fine-tuning. Using this model we investigate -- and find tenable -- the idea that the Milky Way may itself be a typical high-energy cosmic ray source. We also consider the possible phenomenological implications of the magnetic CR horizon for the overall cosmic ray spectrum observed at Earth. Finally, we show that anisotropy studies should most profitably focus on cosmic rays detected at energies above the so-called GZK cut-off, $\sim 6 \times 10^{19}$ eV.

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arXiv2007

Radio Synchrotron Emission from Secondary Leptons in the Vicinity of Sgr A*

A point-like source of ~TeV gamma-rays has recently been seen towards the Galactic center by HESS and other air Cerenkov telescopes. In recent work (Ballantyne et al. 2007), we demonstrated that these gamma-rays can be attributed to high-energy protons that (i) are accelerated close to the event horizon of the central black hole, Sgr A*, (ii) diffuse out to ~pc scales, and (iii) finally interact to produce gamma-rays. The same hadronic collision processes will necessarily lead to the creation of electrons and positrons. Here we calculate the synchrotron emissivity of these secondary leptons in the same magnetic field configuration through which the initiating protons have been propagated in our model. We compare this emission with the observed ~GHz radio spectrum of the inner few pc region which we have assembled from archival data and new measurements we have made with the Australia Telescope Compact Array. We find that our model predicts secondary synchrotron emission with a steep slope consistent with the observations but with an overall normalization that is too large by a factor of ~ 2. If we further constrain our theoretical gamma-ray curve to obey the implicit EGRET upper limit on emission from this region we predict radio emission that is consistent with observations, i.e., the hadronic model of gamma ray emission can, simultaneously and without fine-tuning, also explain essentially all the diffuse radio emission detected from the inner few pc of the Galaxy.

astro-ph↗

arXiv2007

The Cosmic Ray Distribution in Sagittarius B

The HESS instrument has observed a diffuse flux of ~ TeV gamma rays from a large solid angle around the Galactic center (GC). This emission is correlated with the distribution of gas in the region suggesting that the gamma rays originate in collisions between cosmic ray hadrons (CRHs) and ambient matter. Of particular interest, HESS has detected gamma rays from the Sagittarius (Sgr) B Molecular Cloud Complex. Prompted by the suggestion of a hadronic origin for the gamma rays, we have examined archival 330 and 74 MHz Very Large Array radio data and 843 MHz Sydney University Molonglo Sky Survey data covering Sgr B, looking for synchrotron emission from secondary electrons and positrons (expected to be created in the same interactions that supply the observed gamma rays). Intriguingly, we have uncovered non-thermal emission, but at a level exceeding expectation. Adding to the overall picture, recent observations by the Atacama Pathfinder Experiment telescope show that the cosmic ray ionization rate is ten times greater in the Sgr B2 region of Sgr B than the local value. Lastly, Sgr B2 is also a very bright X-ray source. We examine scenarios for the spectra of CRHs and/or primary electrons that would reconcile all these different data. We determine that (i) a hard (~ E^-2.2), high-energy (> TeV) population CRHs is unavoidably required by the HESS gamma ray data and (ii) the remaining broad-band, non-thermal phenomenology is explained either by a rather steep (~ E^-2.9) spectrum of primary electrons or a (~ E^-2.7) population of CRHs. No single, power-law population of either leptons or hadrons can explain the totality of broadband, non-thermal Sgr B phenomenology.

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arXiv2007

A Possible Link Between the Galactic Center HESS Source and Sgr A*

Recently, HESS and other air Cerenkov telescopes have detected a source of TeV gamma-rays coincident with the Galactic center. It is not yet clear whether the gamma-rays are produced via leptonic or hadronic processes, so it is important to consider possible acceleration sites for the charged particles which produce the gamma-rays. One exciting possibility for the origin of these particles is the central black hole, Sgr A*, where the turbulent magnetic fields close to the event horizon can accelerate protons to TeV energies. Using a realistic model of the density distribution in a 6 pc x 6 pc x 6pc cube at the Galactic center, we here calculate the trajectories followed by these TeV protons as they gyrate through the turbulent medium surrounding Sgr A*. Diffusing out from the black hole, the protons produce TeV gamma-rays via pi^0 decay following a collision with a proton in the surrounding medium. After following over 222,000 such trajectories, we find that the circumnuclear ring around Sgr A* can reproduce the observed 0.1-100 TeV HESS spectrum and flux if the protons are injected into this medium with an effective power-law index of 0.75, significantly harder than the observed photon index of 2.25. The total energy in the steady-state 1-40 TeV proton population surrounding Sgr A* is inferred to be approx 5x10^{45} ergs. Only 31% of the emitted 1-100 TeV protons encounter the circumnuclear torus, leaving a large flux of protons that diffuse outward to contribute to the Galactic ridge emission observed by HESS on scales of >~ 1 degree.

astro-ph↗

arXiv2005

Neutrinos from the Galactic Center in the Light of its Gamma-Ray Detection at TeV Energy

We re-evaluate the event rate expected in km^3-scale detectors for neutrinos from the direction of the Galactic Center (GC) in light of recent spectral measurements obtained by the HESS instrument for ~TeV gamma-radiation from this direction. In the most plausible scenario the re-evaluated event rate is smaller than that previously calculated--and here re-calculated--on the basis of EGRET data. However, the GC TeV gamma-ray detections by the Whipple, CANGAROO, and HESS instruments, together with the strong indications for an overabundance of cosmic rays coming from the GC at EeV energies, strengthen the expectation for a detectable, TeV-PeV GC neutrino signal from proton-proton interactions in that region. If the TeV gamma-ray--EeV cosmic ray anisotropy connection is correct, this signal will be detectable within a year and half for km^3-scale neutrino detectors in the Northern Hemisphere at super-TeV energies and, significantly, should also be detectable in 1.6 years by the South Polar IceCube detector at energies > 10^14 eV. The GC neutrino signal should also produce a detectable signal from neutrino showering and resonant W^- production by anti-electron-neutrinos in the volume of a km^3-scale detector.

astro-ph↗

arXiv2004

The AGASA/SUGAR Anisotropies and TeV Gamma Rays from the Galactic Center: A Possible Signature of Extremely High-energy Neutrons

Recent analysis of data sets from two extensive air shower cosmic ray detectors shows tantalizing evidence of an anisotropic overabundance of cosmic rays towards the Galactic Center (GC) that ``turns on'' around $10^{18}$ eV. We demonstrate that the anisotropy could be due to neutrons created at the Galactic Center through charge-exchange in proton-proton collisions, where the incident, high energy protons obey an $\sim E^{-2}$ power law associated with acceleration at a strong shock. We show that the normalization supplied by the gamma-ray signal from EGRET GC source 3EG J1746-2851 -- ascribed to pp induced neutral pion decay at GeV energies -- together with a very reasonable spectral index of 2.2, predicts a neutron flux at $\sim 10^{18}$ eV fully consistent with the extremely high energy cosmic ray data. Likewise, the normalization supplied by the very recent GC data from the HESS air-Cerenkov telescope at TeV energies is almost equally-well compatible with the $\sim 10^{18}$ eV cosmic ray data. Interestingly, however, the EGRET and HESS data appear to be themselves incompatible. We consider the implications of this discrepancy. We discuss why the Galactic Center environment can allow diffusive shock acceleration at strong shocks up to energies approaching the ankle in the cosmic ray spectrum. Finally, we argue that the shock acceleration may be occuring in the shell of Sagittarius A East, an unusual supernova remnant located very close to the Galactic Center. If this connection between the anisotropy and Sagittarius A East could be firmly established it would be the first direct evidence for a particular Galactic source of cosmic rays up to energies near the ankle.

astro-ph↗

arXiv2004

Neutrino Interferometry In Curved Spacetime

Gravitational lensing introduces the possibility of multiple (macroscopic) paths from an astrophysical neutrino source to a detector. Such a multiplicity of paths can allow for quantum mechanical interference to take place that is qualitatively different to neutrino oscillations in flat space. After an illustrative example clarifying some under-appreciated subtleties of the phase calculation, we derive the form of the quantum mechanical phase for a neutrino mass eigenstate propagating non-radially through a Schwarzschild metric. We subsequently determine the form of the interference pattern seen at a detector. We show that the neutrino signal from a supernova could exhibit the interference effects we discuss were it lensed by an object in a suitable mass range. We finally conclude, however, that -- given current neutrino detector technology -- the probability of such lensing occurring for a (neutrino-detectable) supernova is tiny in the immediate future.

hep-ph↗

arXiv2001

Discovering Long Wavelength Neutrino Oscillations in the Distorted Neutrino Spectrum of Galactic Supernova Remnants

We investigate the muon neutrino event rate in km$^3$ neutrino telescopes due to a number of galactic supernova remnants expected on the basis of these objects' known $γ$-ray signals. We evaluate the potential of these neutrino signals to exhibit evidence of the sub-dominant neutrino oscillations expected in various neutrino mixing schemes including pseudo-Dirac scenarios and the Exact Parity Model. With ten years' data, neutrino signals from Sgr A East should either discover or exclude neutrino oscillations governed by a $δm^2$ parameter in the range $10^{-12}$ to $10^{-15}$ eV$^2$. Such a capability is not available to terrestrial or solar system neutrino experiments.

astro-ph↗