Core Of Cen Research Articles

Signatures of leptophilic t -channel dark matter from active galactic nuclei

In this work, we study indirect photon signatures of leptophilic dark matter (DM) coming from Centaurus A (Cen A), where a DM density spike is believed to have survived to date contrary to the case of our galaxy. We consider a model where DM is a Majorana fermion which interacts with right-handed electrons via a scalar mediator. Assuming that the photons measured from the core of Cen A are coming from SM processes, we derive constraints on the average annihilation cross section which are 7 orders of magnitude stronger than the ones from measurements of the Galactic Center. Focusing on the allowed parameter space range, we calculate the flux of photons coming from the radiative DM-electron scattering in the active galactic nuclei (AGN) jets and the circular polarization asymmetry of these photons. We find that this flux is two orders of magnitude lower than the background but its circular polarization asymmetry can reach values close to 100%, indicating the need to experimentally exploit the high fraction of circular polarization in order to detect these interactions. Since the origin of the photons in the GeV-TeV range from Cen A is not completely clear and an exotic origin is compatible with the measurements as well, we also consider the scenario in which synchrotron radiation can only partially explain the photon flux and we fit the excess with signals coming from DM annihilation, finding a best fit for a DM candidate with a mass ${m}_{\stackrel{\texttildelow{}}{\ensuremath{\chi}}}=123\text{ }\text{ }\mathrm{GeV}$ and a coupling ${a}_{R}=0.018$.

The γ-ray spectrum of the core of Centaurus A as observed with H.E.S.S. and Fermi-LAT

Centaurus A (Cen A) is the nearest radio galaxy discovered as a very-high-energy (VHE; 100 GeV–100 TeV) γ-ray source by the High Energy Stereoscopic System (H.E.S.S.). It is a faint VHE γ-ray emitter, though its VHE flux exceeds both the extrapolation from early Fermi-LAT observations as well as expectations from a (misaligned) single-zone synchrotron-self Compton (SSC) description. The latter satisfactorily reproduces the emission from Cen A at lower energies up to a few GeV. New observations with H.E.S.S., comparable in exposure time to those previously reported, were performed and eight years of Fermi-LAT data were accumulated to clarify the spectral characteristics of the γ-ray emission from the core of Cen A. The results allow us for the first time to achieve the goal of constructing a representative, contemporaneous γ-ray core spectrum of Cen A over almost five orders of magnitude in energy. Advanced analysis methods, including the template fitting method, allow detection in the VHE range of the core with a statistical significance of 12σ on the basis of 213 hours of total exposure time. The spectrum in the energy range of 250 GeV–6 TeV is compatible with a power-law function with a photon index Γ = 2.52 ± 0.13stat ± 0.20sys. An updated Fermi-LAT analysis provides evidence for spectral hardening by ΔΓ ≃ 0.4 ± 0.1 at γ-ray energies above 2.8+1.0−0.6 GeV at a level of 4.0σ. The fact that the spectrum hardens at GeV energies and extends into the VHE regime disfavour a single-zone SSC interpretation for the overall spectral energy distribution (SED) of the core and is suggestive of a new γ-ray emitting component connecting the high-energy emission above the break energy to the one observed at VHE energies. The absence of significant variability at both GeV and TeV energies does not yet allow disentanglement of the physical nature of this component, though a jet-related origin is possible and a simple two-zone SED model fit is provided to this end.

Discovery of a new extragalactic population of energetic particles

We report the discovery of a statistically significant hardening in the Fermi-LAT $\gamma$-ray spectrum of Centaurus A's core, with the spectral index hardening from $\Gamma_{1}=2.73 \pm 0.02$ to $\Gamma_{1}=2.29 \pm 0.07$ at a break energy of ($2.6 \pm 0.3$) GeV. Using a likelihood analysis, we find no evidence for flux variability in Cen A's core lightcurve above or below the spectral break when considering the entire 8 year period. Interestingly, however, the first $\sim3.5$ years of the low energy lightcurve shows evidence of flux variability at the $\sim3.5 \sigma$ confidence level. To understand the origin of this spectral break, we assume that the low energy component below the break feature originates from leptons in Centaurus A's radio jet and we investigate the possibility that the high energy component above the spectral break is due to an additional source of very high energy particles near the core of Cen A. We show for the first time that the observed $\gamma$-ray spectrum of an Active Galactic Nucleus is compatible with either a very large localized enhancement (referred to as a spike) in the dark matter halo profile or a population of millisecond pulsars. Our work constitutes the first robust indication that new $\gamma$-ray production mechanisms can explain the emission from active galaxies and could provide tantalizing first evidence for the clustering of heavy dark matter particles around black holes.

The Energetic Particle Population in Centaurus A

AbstractWe report a significant hardening of theFermi-LAT gamma-ray spectrum from the core of Cen A at E > 2.4 GeV, suggesting there is a source of high energy particles in the core of Cen A which is in addition to the jet component. We show that the observed gamma-ray spectrum is compatible with either a spike in the dark matter halo profile or a population of millisecond pulsars. This work gives a strong indication of new gamma-ray production mechanisms in active galactic nuclei and could even provide evidence for the clustering of heavy dark matter particles around black holes.

Photo-disintegration of heavy nuclei at the core of Cen A

Fermi LAT has detected gamma ray emissions from the core of Cen A.More recently, a new component in the gamma ray spectrum from the core has been reported in the energy range of 4 GeV to tens of GeV.We show that the new component and the HESS detected spectrum of gamma rays from the core at higher energy have possibly a common origin in photo-disintegration of heavy nuclei. Assuming the cosmic rays are mostly Fe nuclei inside the core and their spectrum has a low energy cut-off at 52 TeV in the wind frame moving with a Doppler factor 0.25 with respect to the observer on earth, the cosmic ray luminosity required to explain the observed gamma ray flux above 1 GeV is found to be 1.5 × 1043 erg/sec.

EVIDENCE FOR A SECOND COMPONENT IN THE HIGH-ENERGY CORE EMISSION FROM CENTAURUS A?

We report on an analysis of \fermi data from four year of observations of the nearby radio galaxy Centaurus A (Cen A). The increased photon statistics results in a detection of high-energy ($>\:100$ MeV) $\gamma$-rays up to 50 GeV from the core of Cen A, with a detection significance of about 44$\sigma$. The average gamma-ray spectrum of the core reveals evidence for a possible deviation from a simple power-law. A likelihood analysis with a broken power-law model shows that the photon index becomes harder above $E_b \simeq 4$ GeV, changing from $\Gamma_1=2.74\pm0.03$ below to $\Gamma_2=2.09\pm0.20$ above. This hardening could be caused by the contribution of an additional high-energy component beyond the common synchrotron-self Compton jet emission. A variability analysis of the light curve with 15-, 30-, and 60-day bins does not provide evidence for variability for any of the components. Indications for a possible variability of the observed flux are found on 45-day time scale, but the statistics do not allow us to make a definite conclusion in this regards. We compare our results with the spectrum reported by H.E.S.S. in the TeV energy range and discuss possible origins for the hardening observed.

Testing hadronic models of gamma ray production at the core of Cen A

Pierre Auger experiment has observed a few cosmic ray events above 55 EeV from the direction of the core of Cen A. These cosmic rays might have originated from the core of Cen A. High energy gamma ray emission has been observed by HESS from the radio core and inner kpc jets of Cen A. We are testing whether pure hadronic interactions of protons or heavy nuclei with the matter in the core region or photo-disintegration of heavy nuclei can explain the cosmic ray and high energy gamma ray observations from the core of Cen A. The scenario of $p-\gamma$ interactions followed by photo-pion decay has been tested earlier by Sahu et al. (2012) and found to be consistent with the observational results. In this paper we have considered some other possibilities (i) the primary cosmic rays at the core of Cen A are protons and the high energy gamma rays are produced in $p-p$ interactions,(ii) the primary cosmic rays are Fe nuclei and the high energy gamma rays are produced in $Fe-p$ interactions and (iii) the primary cosmic rays are Fe nuclei and they are photo-disintegrated at the core. The daughter nuclei de-excite and high energy gamma rays are produced.

Free-free absorption in the nucleus of Cen A: evidences from 7-mm emission variability

The existence of free-free absorption in the nucleus of Cen A was suggested by Tingay & Murphy in 2001 as an explanation for the large value of the spectral index between 2.2 and 5 GHz. In this paper we present further evidence for this absorption, based on the observations of short time-scale variability at mm wavelengths and their lack of correlation with X-ray emission, as expected if they were both originated in the same physical process. To explain the short-term variability, we assumed that the mm-wave emission is produced near the base of the inhomogeneous parsec-scale jet that has a bulk velocity of about 0.5c. The presence of an ionized media between the jet and the observer will produce the observed 7-mm flux density variability, as different jet features move behind the absorbing material. We estimated that a region with 5 x 10 6 electrons cm -3 and 1.5 x 10 15 cm radius can explain the intensity and the duration of the mm-flux variations, but the corresponding column density would not be enough to absorb significantly the X-ray flux, explaining the lack of correlation between the radio and X-ray variability. The upper limit for the cloud size inferred for the absorbing region shows that the media surrounding the core of Cen A must be clumpy.

Radio Galaxies: Supernova to Synchrotron

One of the earliest surprises yielded by the science of radio astronomy was that galaxies emit radio waves. The processes that produce visible light the nuclear burning in stars were self-evident in galaxies. Radio waves hadn't been expected there. Yet some galaxies emit as much energy in radio as they do in light. Furthermore, the radio-emitting matter of galaxies generally extends a long distance beyond the visible portion of the galaxy. This extension raises rather serious problems of organization and dynamics. Centaurus A was the first radio galaxy discovered arid remains the nearest known radio galaxy, about 10 million light-years away. Its shape is typical of the breed. Visibly, Centaurus A is a bright, compact ellipse with a dark lane or band of dust across its middle. In radio Cen A has two teardrop-shaped lobes extending at right angles to the dust band and reaching far beyond the bright disk to a distance of 50 or 60 kiloparsecs (160,000 to 200,000 light-years). The question is what makes these lobes and what maintains them? David De Young of Kitt Peak National Observatory now presents a theory that answers those questions at least in part. Since the beginning of radio astronomy it has been known that the radio emissions of Cen A and other radio galaxies are synchrotron emission, the radiation of energetic electrons spiraling around in a magnetic field. So the question becomes how to ensure a continuous supply of energetic electrons. De Young begins with recent observations made at Cerro Tololo Inter-American Observatory in Chile that all is not dark in the radio lobes. Sensitive observation can see groups of massive young stars there. De Young postulates that in the core of Cen A is a large rotating mass of gas. Some of this gas continually spews out along the poles of rotation. As it moves through the inner part of the galaxy, it picks up material, debris ejected from stars, and drags it out into the lobes. In the lobes stars like the ones observed can be formed from this mixture of primordial and processed matter. Some of these stars will be so massive that they last only a short time, say 10 million years, before they explode as supernovas. Supernovas yield lots of energetic electrons. The sequence continues for eons: gas from the core, new stars, supernovas, energetic electrons. What remains to be explained is why the gas is ejected from the core in the first place. -D. E. Thomsen 0