Direction Of Centaurus Research Articles

Study of the PeV neutrino, γ-rays, and UHECRs around the lobes of Centaurus A

Pierre Auger observatory reported the distribution of arrival directions of the highest energy cosmic rays. These events were collected in 10 years of operations with declinations between -90$^\circ$ and +45$^\circ$. The IceCube neutrino telescope reported the detection of 82 extraterrestrial neutrinos in the High-Energy Starting Events catalog. The highest-energy neutrino event (IC35) reported in this catalog had an energy of 2004$^{+236}_{-262}$ TeV and its reconstruction was centered at RA=$208.4^\circ$ and DEC=$-55.8^\circ$ (J2000). Being Centaurus A the nearest radio-loud active Galactic nucleus and one of the potential candidates for accelerating cosmic rays up to $\sim\, 10^{20}$ eV, we show that the ultra-high-energy cosmic rays (UHECRs) around the direction of Centaurus A (15$^\circ$ radius) could be accelerated inside the giant lobes. Studying the composition of UHECRs through the photo-disintegration processes, the maximum energy that these can reach in the giant lobes and the average deflecting angles that nuclei undergo due to Galactic and extragalactic magnetic fields, it is shown that the most promise candidates of UHECR composition are light nuclei such as carbon/nitrogen nuclei. Considering the interactions of the relativistic carbon nuclei with photon fields (extragalactic background light and synchrotron) and materials (inside and outside the lobes), we do not find enough evidence to associate the IC35 event with the UHECRs reported by Pierre Auger observatory.

SEARCHES FOR ANISOTROPIES IN THE ARRIVAL DIRECTIONS OF THE HIGHEST ENERGY COSMIC RAYS DETECTED BY THE PIERRE AUGER OBSERVATORY

We analyze the distribution of arrival directions of ultra-high energy cosmic rays recorded at the Pierre Auger Observatory in 10 years of operation. The data set, about three times larger than that used in earlier studies, includes arrival directions with zenith angles up to $80^\circ$, thus covering from $-90^\circ$ to $+45^\circ$ in declination. After updating the fraction of events correlating with the active galactic nuclei (AGNs) in the V\'eron-Cetty and V\'eron catalog, we subject the arrival directions of the data with energies in excess of 40 EeV to different tests for anisotropy. We search for localized excess fluxes and for self-clustering of event directions at angular scales up to $30^\circ$ and for different threshold energies between 40~EeV and 80~EeV. We then look for correlations of cosmic rays with celestial structures both in the Galaxy (the Galactic Center and Galactic Plane) and in the local Universe (the Super-Galactic Plane). We also examine their correlation with different populations of nearby extragalactic objects: galaxies in the 2MRS catalog, AGNs detected by Swift-BAT, radio galaxies with jets and the Centaurus~A galaxy. None of the tests shows a statistically significant evidence of anisotropy. The strongest departures from isotropy (post-trial probability ${\sim}1.4$\%) are obtained for cosmic rays with $E>58$~EeV in rather large windows around Swift AGNs closer than 130~Mpc and brighter than $10^{44}$~erg/s (18$^\circ$ radius) and around the direction of Centaurus~A (15$^\circ$ radius).

CORRELATION OF γ-RAY AND HIGH-ENERGY COSMIC RAY FLUXES FROM THE GIANT LOBES OF CENTAURUS A

The spectral energy distribution of giant lobes shows one main peak detected by the Wilkinson Microwave Anisotropy Probe at low energy of $10^{-5}$ eV and a faint $\gamma$-ray flux imaged by Fermi Large Area Telescope at energy $\geq$ 100 MeV. On the other hand, Pierre Auger Observatory associated some ultra-high-energy cosmic rays with the direction of Centaurus A and IceCube reported 28 neutrino-induced events in a TeV - PeV energy range, although none of them related with this direction. In this work we describe the spectra for each of the lobes, the main peak with synchrotron radiation, and the high-energy emission with $pp$ interactions. Obtaining a good description of the main peak, we deduce the magnetic fields, electron densities and the age of the lobes. Describing successfully the $\gamma$-ray emission by pp interactions and considering as targets those thermal particles in the lobes with density in the range $10^{-10}$ to $10^{-4}\, {\rm cm}^{-3}$, we calculate the number of ultra-high-energy cosmic rays. Although $\gamma$-spectrum is well described with any density in the range, only when $10^{-4}\, {\rm cm}^{-3}$ is considered, the expected number of events is very similar to that observed by Pierre Auger Observatory, otherwise we obtain an excessive luminosity. In addition, correlating the $\gamma$-ray and neutrino fluxes through pp interactions we calculate the number of high-energy neutrinos expected in IceCube. Our analysis indicates that neutrinos above 1 TeV cannot be produced in the lobes of Centaurus A, which is consistent with the results recently published by IceCube Collaboration.

On the influence of galactic magnetic fields on the arrival direction distribution of ultra-high-energy cosmic rays

We study the influence of galactic magnetic field (GMF) on both the correlation between ultra-high-energy cosmic rays (UHECRs) and active galactic nuclei (AGNs) and the clustering of UHECRs in the direction of Centaurus A (Cen A), both of which are observed in the arrival direction distribution of 69 UHECRs with energy E ≥ 55 EeV detected by the Pierre Auger Observatory. If UHECRs are dominated by protons, the correlation and the clustering are not affected significantly. However, if UHECRs are dominated by iron nuclei, the influence of the GMF becomes significant, and the correlation and the clustering may be fake due to the GMF while the arrival direction distribution outside of the GMF is compatible with the isotropy.

ON THE EXCESS OF ULTRA-HIGH ENERGY COSMIC RAYS IN THE DIRECTION OF CENTAURUS A

A posteriori anisotropy study of ultra-high energy cosmic rays (UHECRs) with the Pierre Auger Observatory (PAO) has shown evidence of excess of cosmic ray particles above 55 EeV within $18^{\circ}$ of the direction of the radio galaxy Centaurus A. However, the origin of the excess remains elusive. We simulate the propagation of different species of particles coming from the direction of Centaurus A in the Galactic magnetic fields, and find that only particles of nuclear charge $Z\la 10$ can avoid being deflected outside of the $18^{\circ}$ window of Centaurus A. On the other hand, considering the increasingly heavy composition of UHECRs at the highest energies measured by PAO, a plausible scenario for cosmic rays from the direction of Centaurus A can be found if they consist of intermediate-mass nuclei. The chemical composition of cosmic rays can be further constrained by lower-energy cosmic rays of the same rigidity. We find that cosmic ray acceleration in the lobes of Centaurus A is not favored, while acceleration in the stellar winds that are rich in intermediate-mass nuclei, could meet the requirement. This suggests that the observed excess may originate from cosmic ray accelerators induced by stellar explosions in the star-forming regions of Centaurus A and/or the Centaurus cluster located behind Centaurus A.

HOW MANY ULTRA-HIGH ENERGY COSMIC RAYS COULD WE EXPECT FROM CENTAURUS A?

The Pierre Auger Observatory has associated a few ultra high energy cosmic rays with the direction of Centaurus A. This source has been deeply studied in radio, infrared, X-ray and $\gamma$-rays (MeV-TeV) because it is the nearest radio-loud active galactic nuclei. Its spectral energy distribution or spectrum shows two main peaks, the low energy peak, at an energy of $10^{-2}$ eV, and the high energy peak, at about 150 keV. There is also a faint very high energy (E $\geq$ 100 GeV) $\gamma$-ray emission fully detected by the High Energy Stereoscopic System experiment. In this work we describe the entire spectrum, the two main peaks with a Synchrotron/Self-Synchrotron Compton model and, the Very High Energy emission with a hadronic model. We consider p$\gamma$ and $pp$ interactions. For the p$\gamma$ interaction, we assume that the target photons are those produced at 150 keV in the leptonic processes. On the other hand, for the pp interaction we consider as targets the thermal particle densities in the lobes. Requiring a satisfactory description of the spectra at very high energies with p$\gamma$ interaction we obtain an excessive luminosity in ultra high energy cosmic rays (even exceeding the Eddington luminosity). However, when considering pp interaction to describe the $\gamma$-spectrum, the obtained number of ultra high energy cosmic rays are in agreement with Pierre Auger observations. Moreover, we calculate the possible neutrino signal from pp interactions on a Km$^3 $ neutrino telescope using Monte Carlo simulations.

Effect of electron-photon cascading on the observed energy spectra of extragalactic sources of ultra-high-energy -rays

Air showers produced by neutral particles, presumably γ-rays, have now been detected from Cygnus X-3 at TeV energies and above 1015 eV. Photons of energy above 1014 eV will suffer pair-production interactions with photons of the microwave background, and the γ-ray flux from sources further than several kpc away will be attenuated. Recent observations of high-energy muons from Cygnus X-3 in underground nucleon decay detectors have led, together with arguments based on the attenuation of γ-rays, to the suggestion that the primary particles responsible for the air showers may not be γ-rays. The recent detection of ultra-high-energy ‘γ-rays’ from a source much further away, in the Large Magellanic Cloud, and an apparent excess of air showers above 1015 eV from the direction of Centaurus A is even more surprising. If, however, the intergalactic magnetic field is sufficiently low, electron–photon cascading in the microwave background may enable distant γ-ray sources such as those in other galaxies to be detected above 1015 eV energies even though the interaction length of γ-rays at 1015 eV is less than 10 kpc. An exact Monte Carlo cascade simulation has been performed to investigate the propagation of γ-rays over large distances to see what effect this will have on the observed spectra of extragalactic sources. The implications of these results for future observations are discussed.

On the Absence of Detectable Formaldehyde in the Direction of Centaurus A

On the Absence of Detectable Formaldehyde in the Direction of Centaurus A