3.2 Models of VHE Emission from J1745–290
3.2.2 Leptonic Models
There are also models that propose leptonic origins of the gamma-ray emission from the GC. The leptonic mechanism with the potential for creation of the highest energy gamma rays is inverse Compton (IC) scattering, where a high-energy electron scatters off a low energy photon, imparting much energy to the photon. The other mechanisms, synchrotron radiation and bremsstrahlung, do not create photons of high enough energy to dominate the VHE range. The cross-section for IC scattering is given by the Klein-Nishina formula, Equation 2.44. This means higher energy emission should be highly K-N suppressed when the radiation fields have higher temperatures. For example, if the temperature of the photon field exceeds 100 K, emission above 10 TeV will be highly suppressed. The flux of IC emission provides us with information different from hadronic models. The rate of IC processes depends on the population of high energy electrons and the density of ambient light near the GC. This ambient light could come from stars, cosmic microwave background, or other sources.
Soon after the first detection of the GC by H.E.S.S., a plerion, or wind nebula model of leptonic emission was proposed (Atoyan & Dermer, 2004). This model utilizes an advection- dominated accretion flow (ADAF). Electrons are accelerated to approximately 100 MeV in the inner 20Rs of the black hole by second-order Fermi processes. These processes are driven by the magnetohydrodynamics of the turbulent magnetized corona, which is a sea of plasma similar to what surrounds the Sun. These electrons emit synchrotron radiation in the radio /
sub-mm range. Closer to the center of the black hole, the orbit is said to be more stable, and instabilities and shocks will result in higher energies due to first-order Fermi acceleration. The synchrotron radiation from these higher energy electrons is responsible for the X-ray flares, such as those observed by Chandra, XMM-Newton, and INTEGRAL. The ADAF also drives a sub-relativistic magnetohydrodynamic wind, whose termination shock accelerates electrons much in the same way as a pulsar wind nebula, about 1014.5m from the SMBH. These electrons, after injection, create steady-state X-ray emission and interact with the FIR dust radiation through IC scattering, bumping these photons to TeV energies.
In another leptonic model proposed by Kusunose & Takahara (2012), nonthermal electrons are accelerated by NIR and X-ray flares. These electrons accumulate in a region with a size of about 1016m and magnetic fields weaker than 1 × 10−4G, then IC scatter off of soft photons emitted by stars and dust.
One shortcoming of the various leptonic models is that most of them cannot explain the MeV/GeV emission and the TeV emission simultaneously. This could mean that there are just different regions of emission with different properties; there is too much positional uncertainty in Fermi -LAT, H.E.S.S., and VERITAS to say that the emission is coming from a single location. Models such as the curvature radiation-inverse Compton model (Aharonian & Neronov, 2005a) also require ordering of the electric and magnetic fields in the inner few Rs near the central SMBH. Purely leptonic models are able to explain both the X-ray and gamma-ray emission, and are theoretically better at explaining Fermi -LAT emission than any hadronic models (Malyshev et al., 2015).
Perhaps the most likely alternative to Sgr A* for an astrophysical source of TeV emission from J1745–290 is the energetic PWN G359.95-0.04. This source is too close to Sgr A* to be eliminated as a candidate based on its location, with its tail being a mere 4 arcseconds from the SMBH and its extent less than 5 arcseconds away. Furthermore, its quiescent state flux of X-ray emission is quadruple that of Sgr A*. The PWN could be a source of high energy electrons and positrons that are ejected in the wind powered by the spin down energy of the pulsar. These high-energy electrons inverse-Compton scatter off of ambient photons. Wang et al. (2006) claim that far infrared radiation (FIR) photons, such as stellar
photons reprocessed by dust, are better targets than the ultraviolet (UV) photons, which are suppressed by the Klein-Nishina effect. PWNe are not known to exhibit variable behavior, so observation of variability would disfavor G359.95-0.04 as the central emitter.
Hinton & Aharonian (2007) also discuss a relationship between G359.95-0.04 and the TeV emission from HESS J1745–290. They found that the X-ray spectrum softened as the distance increased from the pulsar’s head. This could mean that the electrons are cooled by synchrotron radiation or Thomson scattering, rather than by IC scattering in the Klein–Nishina (K-N) regime. Based on the flux of the H.E.S.S. emission for E & 1 TeV, they estimate the PWN to have a magnetic field strength of & 100 µG and an average FIR density of about 5000 eV/cm3. They predict an electron injection spectrum that follows dNdE ∝ E−αe−E/E0 with a spectral
index of 2 and cutoff energy of 100 TeV. The injection source should have a total power of 6.7 × 1035erg/s and an age of 10,000 years according to this model.
They also discuss the possible relationship of J1745–290 to the INTEGRAL source IGR J1745.6-2901.
Another alternative model that could explain both the Fermi -LAT and TeV central excesses is a population of millisecond pulsars (MSPs) postulated by Bednarek & Sobczak, 2013. This MSP population at the GC could be the result of a past merger of globular clusters in the central parsec of the Galaxy. The Fermi -LAT and VHE emission would be due to the MSPs themselves and inverse-Compton scattered electrons accelerated in the wind regions of the MSPs, respectively. Given 10 % energy efficiency for HE lepton creation, thousands of MSPs would be required to exist within the central 10 pc region of the GC. In Bartels et al., 2016, this hypothesis is supported, and it is claimed it can explain all of the Fermi -LAT excess. In Bartels et al., 2018, they further this claim, adding that the binaries born from MSPs could create the 511 keV line emission.
All of the leptonic models discussed in this section are able to match the TeV spectrum observed (Archer et al., 2014). One weakness of the models proposed in Atoyan & Dermer, 2004 and Hinton & Aharonian, 2007 is that they do not match Fermi -LAT (Chernyakova et al., 2011) spectral points. It is possible, though, that the Fermi -LAT spectrum contains
signal from other sources because of its relatively poor angular resolution. Mori et al. (2015) analyze the spectral energy distribution of X-rays in the 20–40 keV range using Chandra data of IGR J17456–2901. They find it is consistent with a model that includes emission from G359.95–0.04 and the central hard X-ray emission (CHXE). Because the CHXE is not expected to emit gamma rays, they conclude G359.95–0.04 is the most likely X-ray counterpart of J1745–290.