Planned Cherenkov Telescope Array Research Articles

Novae as Tevatrons: prospects for CTA and IceCube

The discovery of novae as sources of ~GeV gamma-rays highlights the key role of shocks and relativistic particle acceleration in these transient systems. Although there is evidence for a spectral cut-off above energies ~1-100 GeV at particular epochs in some novae, the maximum particle energy achieved in these accelerators has remained an open question. The high densities of the nova ejecta (~10 orders of magnitude larger than in supernova remnants) render the gas far upstream of the shock neutral and shielded from ionizing radiation. The amplification of the magnetic field needed for diffusive shock acceleration requires ionized gas, thus confining the acceleration process to a narrow photo-ionized layer immediately ahead of the shock. Based on the growth rate in this layer of the hybrid non-resonant cosmic ray current-driven instability (considering also ion-neutral damping), we quantify the maximum particle energy, Emax, across the range of shock velocities and upstream densities of interest. We find values of Emax ~ 10 GeV - 10 TeV, which are broadly consistent with the inferred spectral cut-offs, but which could also in principle lead to emission extending to higher energies >100 GeV accessible to atmosphere Cherenkov telescopes, such as the planned Cherenkov Telescope Array (CTA). Detecting TeV neutrinos with IceCube in hadronic scenarios appears to be more challenging, although the prospects are improved if the shock power during the earliest, densest phases of the nova outburst is higher than is implied by the observed GeV light curves, due to downscattering of the gamma-rays by electrons within the ejecta. Novae provide ideal nearby laboratories to study magnetic field amplification and the onset of cosmic ray acceleration, because other time-dependent sources (e.g. radio supernovae) typically occur too distant to detect as gamma-ray sources.

A self-consistent interpretation of the GeV–TeV emission from a distant blazar PKS 1424+240

We propose a scenario for self-consistent interpretation of GeV - TeV spectrum of a distant blazar PKS 1424+240. In this scenario, ultra-high energy (UHE) protons are assumed to exist in the blazar jet and produce gamma rays through synchrotron emission emitted by relativistic protons and pair cascades (resulted from $p\gamma$ interaction); meanwhile, some of these UHE protons may escape from the jet and are injected into intergalactic space. Therefore, we assume that UHE cosmic rays (CR) originate from relativistic protons in the jet and use energy-independent escape timescale to obtain UHECR injection spectrum. Both contributions of gamma rays injected by the source and secondary gamma rays produced in interactions of UHECRs emitted by the blazar with photon background during their propagation through intergalactic space are calculated. Our results show that this scenario is able to reproduce the GeV-TeV spectrum of PKS 1424+240 self-consistently in a broad range of redshifts $0.6<z<1.3$. The required relativistic jet power $L_p\simeq3\times10^{46}\ \rm erg\ s^{-1}$ only moderately depends on the assumed source redshift, while the proton escape timescale (and the injected UHECR luminosity) strongly depends on $z$. We compare the integral TeV fluxes predicted in our scenario with the sensitivity of the planned Cherenkov Telescope Array (CTA), and discuss the implications to future observations.

Significance in gamma ray astronomy with systematic errors

The influence of systematic errors on the calculation of the statistical significance of a γ-ray signal with the frequently invoked Li and Ma method is investigated. A simple criterion is derived to decide whether the Li and Ma method can be applied in the presence of systematic errors. An alternative method is discussed for cases where systematic errors are too large for the application of the original Li and Ma method. This alternative method reduces to the Li and Ma method when systematic errors are negligible. Finally, it is shown that the consideration of systematic errors will be important in many analyses of data from the planned Cherenkov Telescope Array.

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DISENTANGLING HADRONIC AND LEPTONIC CASCADE SCENARIOS FROM THE VERY-HIGH-ENERGY GAMMA-RAY EMISSION OF DISTANT HARD-SPECTRUM BLAZARS

Recent data from the \emph{Fermi} Large Area Telescope have revealed about a dozen distant hard-spectrum blazars that have very-high-energy (VHE; $\gtrsim 100$ GeV) photons associated with them, but most of them have not yet been detected by imaging atmospheric Cherenkov telescopes. Most of these high-energy gamma-ray spectra, like those of other extreme high-frequency peaked BL Lac objects, can be well explained either by gamma rays emitted at the source or by cascades induced by ultra-high-energy cosmic rays, as we show specifically for KUV 00311$-$1938. We consider the prospects for detection of the VHE sources by the planned Cherenkov Telescope Array (CTA) and show how it can distinguish the two scenarios by measuring the integrated flux above $\sim 500$ GeV (depending on source redshift) for several luminous sources with $z \lesssim 1$ in the sample. Strong evidence for the origin of ultra-high-energy cosmic rays could be obtained from VHE observations with CTA. Depending on redshift, if the often quoted redshift of KUV 00311-1938 ($z = 0.61$) is believed, preliminary H.E.S.S. data favor cascades induced by ultra-high-energy cosmic rays. Accurate redshift measurements of hard-spectrum blazars are essential for this study.

Erratum: Relevance of axionlike particles for very-high-energy astrophysics [Phys. Rev. D84, 105030 (2011)

Many extensions of the Standard Model predict the existence of ALPs, which are very light spin-zero bosons with a two-photon coupling. Photon-ALP oscillations occur in the presence of an external magnetic field, and ALPs can lead to observable effects on the measured photon spectrum of astrophysical sources. An intriguing situation arises when blazars are observed with the Cherenkov Telescopes H.E.S.S., MAGIC, CANGAROO III and VERITAS. The extragalactic background light (EBL) produced by galaxies during cosmic evolution gives rise to a source dimming which becomes important in the VHE band. This dimming can be considerably reduced by photon-ALP oscillations in the large-scale magnetic fields, and the resulting blazar spectra become harder than expected. We find that for ALPs lighter than 5 x 10^{-10} eV the photon survival probability is larger than predicted by conventional physics above a few hundred GeV. This is a clear-cut prediction which can be tested with the planned Cherenkov Telescope Array and HAWC. Moreover, we offer a new interpretation of the VHE blazars detected so far, according to which the large spread in the values of the observed spectral index is mainly due to the wide spread in the source distances rather than to large variations of their internal physical properties.

Gamma-rays from nebulae around binary systems containing energetic rotation-powered pulsars

We consider nebulae which are created around binary systems containing rotation powered pulsars and companion stars with strong stellar winds. It is proposed that the stellar and pulsar winds have to mix at some distance from the binary system, defined by the orbital period of the companion stars and the velocity of the stellar wind. The mixed pulsar-stellar wind expands with a specific velocity determined by the pulsar power and the mass loss rate of the companion star. Relativistic particles, either from the inner pulsar magnetosphere and/or accelerated at the shocks between stellar and pulsar winds, are expected to be captured and isotropized in the reference frame of the mixed wind. Therefore, they can efficiently comptonize stellar radiation producing GeV-TeV $\gamma$-rays in the inverse Compton process. We calculate the $\gamma$-ray spectra expected in such scenario for the two example binary systems: J1816+4510 which is the redback type millisecond binary and LS 5039 which is supposed to contain energetic pulsar. It is concluded that the steady TeV $\gamma$-ray emission from J1816+4510 should be on the 100 hr sensitivity limit of the planned Cherenkov Telescope Array, provided that $\epsilon\sim 10%$ of the rotational energy lost by the pulsar is transferred to TeV electrons. On the other hand, the comparison of the predicted steady TeV $\gamma$-ray emission, expected from $\gamma$-ray binary LS 5039, with the observations of the TeV emission in a low state, reported by the H.E.S.S. Collaboration, allows us to put stringent upper limit on the product of the part of the hemisphere in which the mixed pulsar-stellar wind is confined, $\Delta_{\rm mix}$, and the energy conversion efficiency, $\epsilon$, from the supposed pulsar to the TeV electrons injected in this system, $\Delta_{\rm mix}\cdot \epsilon <1%$.

Gamma rays from supernova remnants

The recent galactic plane surveys with space- and ground-based detectors revealed a number of high and very-high energy γ-ray sources associated with young and middle-aged supernova remnants (SNRs). These results imply effective production of relativistic particles, most likely through the process of diffusive shock acceleration. The interpretation of γ-ray data from several prominent representatives of young SNRs within the so-called ‘hadronic models’ demands hard proton spectra extending to 100TeV, and total energy released in accelerated protons and ions WCR⩾1050erg. This can be treated as a support of the SNR paradigm of galactic cosmic rays. However, the hadronic models are not free of pitfalls, and pose in fact non-trivial challenges. Moreover, in many cases γ-ray data can be successfully explained also by the inverse Compton scattering of directly accelerated electrons. Future deep spectroscopic and morphological studies of SNRs with the planned Cherenkov Telescope Array (CTA) over four decades in energy, from approximately 30GeV to 300TeV, promise a breakthrough regarding the identification of radiation mechanisms. The extension of studies beyond the shells of SNRs will be another important objective aimed at the extraction of information about the highest energy particles which have left remnants at the early epochs of their evolution. These particles, which potentially carry an answer to a key question whether SNRs operate as PeVatrons, can be traced via γ-ray emission outside the remnants. Finally, the expected significant increase of the number of γ-ray emitting SNRs by CTA should allow compelling population studies – a key issue for the proof of the SNR origin of galactic cosmic rays.

Importance of axion-like particles for very-high-energy astrophysics

Several extensions ol the Standard Model predict the existence ol Axion-Like Particles (ALPs), very light spin-zero bosons with a two-photon coupling. ALPs can give rise to observable effects in very-high-energy astrophysics. Above roughly 100 GeV the horizon of the observable Universe progressively shrinks as the energy increases, due to scattering of beam photons off background photons in the optical and infrared bands, which produces e+ e− pairs. In the presence of large-scale magnetic fields photons emitted by a blazar can oscillate into ALPs on the way to us and back into photons before reaching the Earth. Since ALPs do not interact with background photons, the effective mean free path of beam photons increases, enhancing the photon survival probability. While the absorption probability increases with energy, photon-ALP oscillations are energy-independent, and so the survival probability increases with energy compared to standard expectations. We have performed a systematic analysis of this effect, interpreting the present data on very-high-energy photons from blazars. Our predictions can be tested with presently operating Cherenkov Telescopes like H.E.S.S., MAGIC, VERITAS and CANGAROO III as well as with detectors like ARGO-YBJ and MILAGRO and with the planned Cherenkov Telescope Array and the HAWC γ-ray observatory. ALPs with the right properties to produce the above effects can possibly be discovered by the GammeV experiment at FERMILAB and surely by the planned photon regeneration experiment ALPS at DESY.

High energy γ-ray emission from compact galactic sources in the context of observations with the next generation Cherenkov Telescope Arrays

The observational progress in the γ-ray astronomy in the last few years has led to the discovery of more than a thousand sources at GeV energies and more than a hundred sources at TeV energies. A few different classes of compact objects in the Galaxy have been established. They show many unexpected features at high energies the physics of which remains mainly unknown. At present it is clear that detailed investigation of these new phenomena can be performed only with the technical equipment which offer an order of magnitude better sensitivity, and a few times better energy, angular and time resolution in the broad energy range staring from a few tens of GeV up to a few hundreds TeV. Such facilities can be realized by the next generation of instruments such as the planned Cherenkov Telescope Array (CTA).The aim of this report is to summarize up to date observational results on the compact galactic sources in the GeV–TeV γ-ray energy range, discuss their theoretical implications, and indicate which hypothesis considered at present might be verified with the next generation of telescopes. We point out which of the observational features of the γ-ray sources are important to investigate with special care with the planned CTA in order to throw new light on physical processes involved. Their knowledge should finally allow us to answer the question on the origin of energetic particles in our Galaxy.