High Energy Astrophysics in the Milky Way

As mentioned earlier in this chapter, the Crab Nebula was eventually detected by the Whipple Cherenkov telescope in 1989 (Weekes et al. [1989]). This was the first ever detected source of VHEγ-rays in outer space. Photons of lower energies from the direc-tion of the Crab Nebula were detected much earlier, starting with records of observadirec-tions of a suddenly appearing bright spot on the sky that was visible (probably even during daylight) with the naked eye in 1054. Today this sudden appearance of a ’guest star’ is known to be the result of the violent end of the life of a massive star whose core collapsed to a rapidly rotating∼30 km diameter neutron star (see Longair [2011]). The magnetic field flux of the progenitor star is conserved in the core collapse and thus the magnetic field is increased by a huge factor. The rotational axis of a neutron star is typically mis-aligned with the magnetic field axis and the rotating magnetic dipoles become sources of intensive radiation that can be detected by a fixed observer as pulsed emission. Still

8Recently there has been one measurement that claimed that the local dark matter density is compatible with zero based on a very similar method (Bidin et al. [2012]). It is, however, now generally accepted that the cited measurement made one wrong assumption on the dependence of the radial rotational velocity around the galactic center as a function of the distance to the galactic disc (see Bovy and Tremaine [2012]). If this assumption is corrected the measurement results in a local dark matter density of (0.3±0.1) GeV/cm³which is within errors compatible with the standard value for the local dark matter density of 0.3 to 0.7 GeV/cm³ (see Weber and de Boer [2010])

discussed is the exact location of the source of the pulsed emission for which different models exist. In case of the neutron star in the center of the Crab Nebula, i.e. the Crab pulsar, pulsed emission detected every 33 ms on earth with different telescopes that observe towards the Crab Nebula over a wide frequency band from radio toγ-rays.

Recently even pulsedγ-rays up to∼400 GeV (VERITAS Col. [2011]) have been detected from the Crab pulsar which is very challenging for theoretical models of the generation of pulsed emission because the assumed spatial origin of the pulsed emission is typically thought to be in regions of very intensive photon fields whereγ-rays of several 100 GeV are expected to undergo an electron-positron pair conversion. Apart from the pulsed emission, the region in the vicinity of the Crab pulsar, i.e. the Crab Nebula, is since the Whipple detection known to be a source of (un-pulsed)γ-rays. By now (January 2013)⁹ 23 similar pulsar wind nebulae (PWNe) are found to emit VHEγ-rays (E >100 GeV).

All but one of the detected PWNe are within the galaxy, mostly in the vicinity of the galactic plane. The one extragalactic PWN detected with Cherenkov telescopes is found in the large Magellanic cloud (H.E.S.S. Col. [2012]). In total 105 VHEγ-ray sources are detected, 84 of them are classified within the known types of VHE γ-ray emitters and 21 sources are yet unidentified. Out of the 84 identified sources, 47 are within the Milky Way.

The second most abundant galactic source type after PWNe are remnants of supernova explosions (SNRs) which consist of the expanding material ejected in a supernova. In contrast to PWNe, which are powered by the loss of rotational power (’spin down’) of a pulsar, SNRs are powered by an initial supernova. In total 17 SNRs are currently detected with VHEγ-rays. All other seven detected and classified galactic VHE γ-ray sources are either star forming regions (3) or binary systems (4).

Each source class that is found to emit VHEγ-rays is interesting to be studied for differ-ent reasons, an overview is given in Hinton and Hofmann [2009]. The work discussed in this thesis is not focused on the investigation of a specific source or source class. Instead, sources of spatially very extended VHEγ-ray emission are searched for. Among the 21 not yet classified sources one is of special interest in this context. Figure 1.1 shows a region in the vicinity of the Milky Way center where VHE γ-rays have been detected.

The emission is spatially correlated with molecular gas regions which are shown as white contours in fig. 1.1. The correlation between molecular gas and VHEγ-ray emission is an evidence for a hadronic production mechanism, i.e. the VHE emission is supposed to be produced by the interaction of hadrons (f.i. protons) with molecular gas which mostly produces pions of which about one third is neutral and decays into two γ-rays.

An alternative explanation for the production of VHE γ-rays is that electrons scatter low energy photon fields (star light, cosmic microwave background or even synchrotron photons emitted by the electrons themself) in an inverse Compton process. This is, however, less preferred because the intensity of theγ-ray emission should be increasing with increasing low energy photon density which is not correlated with the molecular gas density in an inverse Compton scenario. Thus it seems likely that the ’diffuseγ-ray

9See the ’default catalog’ of TeV astronomy at http://tevcat.uchicago.edu which lists all published VHE γ-ray sources. This catalog is referred to whenever a source is called ’detected’ in this section.

1.2 The Milky Way

Figure 1.1: Diffuse emission in the vicinity of the galactic center region as detected with H.E.S.S. (H.E.S.S. Col. [2005b]). Shown is aγ-ray excess map (color scale).

White contours indicating the density of molecular hydrogen gas as traced by CS emission. A correlation between the observed γ-ray excess and the molecular gas density is inferred which hints towards a hadronic origin of the emission.

emission’ in the vicinity of the galactic center ridge mapped in fig. 1.1 is powered by a population of very energetic protons. It is not yet clear where this population of protons gains its energy. What seems clear is that no individual source in the galactic center region nor the ensemble of sources is likely to be the accelerator of the proton population (Wommer et al. [2008]).

On the other hand, there is also the hadronic component of the cosmic radiation that constantly hits the earth atmosphere with a GeV to TeV energy spectrum given in eq.

1.1. It is generally assumed that hadronic cosmic rays in the GeV to TeV energy range are produced within the Milky Way in multiple isolated sources (possibly SNRs) because the magnetic field of the Milky Way confines them within the galaxy. Additionally, cos-mic rays are supposed to be moving in random directions without preference for any special direction far away from their origin as supported by the nearly isotropic cosmic ray flux on earth. This can be well explained by stochastic scattering of cosmic rays on turbulent galactic magnetic fields (see f.i. Fatuzzo et al. [2010]) and described by the energy dependent diffusion of cosmic rays within the Milky Way. A simple model (Fatuzzo et al. [2010]) with an energy dependent diffusion constant (D ∼E^δ) predicts that the cosmic ray spectrum far away from their origin (i.e. with a distance much larger than the diffusion length) is given by Φ(E) ∼ E^{−(α+1.5δ)} when particles are in-jected with an energy spectrum∼E^−α. The spectral slope of the flux spectrum far away from the cosmic ray sources does not depend on the location and it is thus assumed that the spectral slope of cosmic rays is the same as the slope of the cosmic ray spectrum measured on earth everywhere within the Milky Way on large scales. It is thus not surprising on a first glimpse that the vicinity of the galactic center, where the density of molecular gas is among the highest in the galactic disc, VHEγ-rays are produced in hadronic interactions. However, the measured spectral index of the diffuse emission in the galactic center ridge (−2.3) is incompatible with the spectral index of hadronic TeV cosmic rays measured on earth (−2.7) and thus the large scale cosmic ray population within the Milky Way can be ruled out as the source of thisγ-ray emission.

Although the γ-ray emission detected in the galactic center ridge is unlikely to be pro-duced by the same population of CRs that hit the earth atmosphere it is very likely that VHE γ-rays are produced by interactions of this CR population with gas in the Milky Way disc. This effect, the ’galactic diffuse emission’, has yet not been detected at TeV energies with Cherenkov telescopes but its detection ’would be extremely valu-able’ as highlighted in the 2008 white paper for the future of ground-based TeV γ-ray astronomy of the American Physical Society (Buckley et al. [2008]). Later in this thesis, this topic will be investigated, primarily because this effect is a possible foreground for the detection of particle dark matter self annihilation in the Milky Way with Cherenkov telescopes as will become clear later.