The Origin of Cosmic Rays: What Can GLAST Say?

Jonathan F. Ormes, Seth Digel, Igor V. Moskalenko^1^, and Alexander
Moiseev,

*Goddard Space Flight Center, Greenbelt, Maryland*

and Roger Williamson

*Stanford University*

on behalf of the GLAST collaboration

**Abstract.** Gamma rays in the band from 30 MeV to 300 GeV, used in
combination with direct measurements and with data from radio and X-ray
bands, provide a powerful tool for studying the origin of Galactic
cosmic rays. Gamma-ray Large Area Space Telescope (GLAST) with its fine
10-20 arcmin angular resolution will be able to map the sites of
acceleration of cosmic rays and their interactions with interstellar
matter. It will provide information which is necessary to study the
acceleration of energetic particles in supernova shocks, their transport
in the interstellar medium and penetration in the molecular clouds.

# ![](media/image1.pct){width="2.058333333333333in" height="2.8in"}

# Introduction {#introduction .INTRODUCTION}

The identification of the sites of cosmic-ray acceleration is one of the
main unsolved problems in Galactic cosmic-ray astrophysics. We know from
radio \[1\] and recent X-ray observations \[2\] of synchrotron radiation
from supernova remnants that electrons are accelerated to TeV energies.
Direct evidence for the acceleration of protons is not yet in hand.
There is a considerable theoretical literature that quantifies how this
acceleration takes place in the turbulent magnetic fields associated
with the shock waves generated by supernova explosions propagating into
the interstellar medium \[3\]. With the Gamma-ray Large Area Space
Telescope (GLAST), proposed for launch in 2005, we have the possibility
of detecting γ rays from the freshly accelerated cosmic-ray nuclei at
their acceleration site. The optimal candidate sites will be those where
the shock waves collide with either swept up interstellar matter or
nearby clouds. The most commonly discussed site is individual supernova
remnants \[4\] however multiple supernovas inside superbubbles have also
been considered \[5\].

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

^1^NRC Resident Research Associate visiting GSFC from Inst. for Nucl.
Phys., Moscow State U.

# Fig. 1: The Silicon GLAST instrument

# While the great majority of the cosmic rays are nuclei, not electrons, most of the electromagnetic signatures of cosmic rays reflects electrons as seen in synchrotron radiation or inverse Compton (IC) interactions of photons with electrons. This includes even the highest energy band, photons around 1 TeV. To identify the site where the bulk of the energy is pumped into relativistic particles we need to find a signature of accelerated nucleons. Such a signature will be the spectrum reflective of neutral-pion decay from collisions of freshly-accelerated nuclei with nearby gas and dust[^1]. Depending of the conditions in the medium the neutral-pion decay γ  rays will be evident as a spectral feature in the range 50 MeV to a few GeV, while above this range their spectral index will match the index of the cosmic rays at the site. This range is ideal for studies with GLAST. 

# GLAST capabilies & SNR

The GLAST telescope we have proposed is shown in Figure 1. Its
capabilities are summarized in Figure 2. At 1 GeV, the effective area,
including inefficiencies due to photon conversion and background
rejection, will be more than 1 m^2^, and our single photon angular
resolution will be 0.4 degrees. Supernova remnants can be observed well
off axis (up to 60 degrees) without loss of angular resolution and the
2.4 sr solid angle allows any individual source to be observed at a 20%
duty cycle. Thus, supernova remnants can be observed in the normal
sky-scanning mode of GLAST operation. Minimal tails in the point-spread
function (the 95% containment angle is typically 2.5 times the 68%
containment angle) optimize the capability for mapping the structure
of γ -ray emission. The challenge will be to find sources with an
angular scales large enough to be mapped with angular resolution
sufficient to separate the extended shell emission from that of the
central compact source, nominally a pulsar.

![](media/image2.pct){width="2.2868055555555555in"
height="2.2291666666666665in"}

+-----------------+-------------+-----------------+-------------------+
| # TABLE 1. Cand |             |                 |                   |
| idate supernova |             |                 |                   |
+-----------------+-------------+-----------------+-------------------+
| **Supernova**   | *           | **Estimated**   | **Angular**       |
|                 | *Distance** |                 |                   |
| **remnant**     |             | **Age (years)** | **Size (arcmin)** |
|                 | **(kpc)**   |                 |                   |
+-----------------+-------------+-----------------+-------------------+
| RXJ1713.7-3946  | 6           | ??              | 70                |
+-----------------+-------------+-----------------+-------------------+
| γ Cygni         | 1.8         | 7000            | 60                |
+-----------------+-------------+-----------------+-------------------+
| IC443           | 1.5         | 5000            | 45                |
+-----------------+-------------+-----------------+-------------------+
| SN1006          | \~3         | 1000            | 30                |
+-----------------+-------------+-----------------+-------------------+
| CasA            | 2.8         | 300             | 5                 |
+-----------------+-------------+-----------------+-------------------+
| Kepler          | 4.4         | 400             | 3                 |
+-----------------+-------------+-----------------+-------------------+

The obvious candidates for this search are those SNR for which EGRET
reported a finite flux \[6\]. We also include RXJ1713.7-3946 for which
TeV emission has recently been reported \[7\]. They are listed in Table
1 along with relevant parameters. To predict the signal GLAST might
detect for a typical case, we have modeled the γ rays from γ Cygni
taking the total flux as measured by EGRET and dividing it into two
separate components with 60% of the flux from the pulsar \[8\].

Fig. 2: Performance characteristics of
GLAST![](media/image3.pct){width="2.86875in"
height="3.9791666666666665in"}

Fig. 3: Simulated observation of γ Cygni

The result of this simulation[^2] is shown in Figure 3. Note that the
shell component is clearly resolved from the central point source.

Assuming this source (γ Cygni) is as modeled we expect to spatially
resolve the shell source and to be able to measure the spectrum \[9\]
shown in Figure 4. The locally observed cosmic-ray nuclei have spectra
proportional to E^-2.7^. This spectrum is the natural result of shock
models which predict harder source spectra, E^-δ^ with spectral
exponents δ in the range 2.0 - 2.3 combined with the energy dependent
losses by diffusion. The IC and bremsstrahlung components can be
deconvolved and subtracted to determine the π^o^ contribution. The
asymptotic spectral exponent at energies above 10 GeV should be
indicative of the source spectrum of freshly-accelerated cosmic rays.

# Diffuse continuum Galactic gamma-rays![](media/image4.pct){width="2.6944444444444446in" height="2.6215277777777777in"}

Once accelerated, the cosmic rays quickly diffuse away from their
sources, mix with those from other sources and diffuse out of the
Galaxy. Most theoretical treatments of cosmic-ray transport from their
sources to the Earth assume the result of this process is a steady flux
of cosmic rays throughout the Galaxy with sources in equilibrium with
losses. With GLAST we have a chance to probe the spatial structure on a
scale that can begin to test this hypothesis.

Of the many possible observables of cosmic-ray transport through the
interstellar medium to earth (e.g. Be, B isotopes, positrons,
antiprotons) the least exploited to date has been γ  rays. The diffuse γ
-ray emission in any direction is a measure of the point by point
product of the cosmic-ray intensity and the matter density in the
interstellar medium. The secondary antiprotons and positrons are
produced in the same collisional processes. The EGRET team has modeled
this diffuse γ-ray emission \[10\] on 0.5 degree scale. Their models
distribute the cosmic rays proportionally to the matter in the Galaxy on
scales of order 1 kpc.

One of the most puzzling findings from EGRET is the excess in the
Galactic diffuse γ -ray emission above 1 GeV \[11\]. Independent
confirmation and extension toward the higher energies would help to
understand this excess. Is the spectrum of cosmic-ray nuclei \[12\] or
electrons \[13\] elsewhere in the Galaxy different from what we measure
locally? If so, what are the corresponding consequences for all
cosmic-ray physics? The range of such possibilities have been recently
discussed in the literature \[14\].

![](media/image5.png){width="2.6645833333333333in"
height="2.6743055555555557in"}

The self-consistent approach to this problem developed by Strong et al.
\[14\] models the diffusive propagation of nucleons and electrons.
Figure 5 shows their predictions of the various contributing components.
Their conclusion, reflected in the figure, is that the harder electron
spectrum in the Galaxy is more likely. But their solution is not unique
and requires confirmation with the additional data that GLAST will
provide. Measurements of high-energy antiproton and positron spectra
provide a useful constraint on the nucleon spectrum, while the electron
spectrum can be different from place to place due to the large electron
energy losses (though radio measurements of synchrotron emission provide
a constraint for some electron energies). These hypotheses can be tested
with GLAST by measuring the spectrum of Galactic diffuse γ -ray emission
above few GeV. First, one can distinguish between the inverse Compton
and neutral-pion decay components, and, second, the latitude and
longitude γ -ray profiles are different for gas-related nucleon and
broader electron components.

Another result of the studies \[15\] is that the inverse Compton (IC)
scattering, especially at high latitudes, plays a more important role
than previously thought. The anisotropic distribution of photons above
the Galactic plane will result in greater (up to 40%) flux from IC in
the halo. Evidence for a large γ -ray halo has been found in an analysis
of the EGRET data \[16\]. This affects our estimates of the
extragalactic component, the intensity of cosmic rays in the Galaxy at
large and the size of the Galactic cosmic-ray containment halo. The
importance of high latitude IC can be tested with GLAST by measuring γ
-ray profiles at high energies.

# molecular clouds and the extragalactic diffuse gamma-ray emission

A longstanding question in cosmic-ray astrophysics has been the extent
to which cosmic rays can penetrate into clouds and interact with
material there. The issue is one of the length scale of equipartition of
energy between cosmic rays, thermal motion of interstellar gas and
energy in magnetic fields. Equipartition seems to hold on the kpc scale,
but what about scales below 1 kpc? GLAST will be

able to study the γ  -ray emission from molecular clouds, super-bubble
walls and other interstellar concentrations of matter on an angular
scale heretofore impossible reaching the tens of arc minute range. At a
distance of 1 kpc, spatial scales of 3-10 parsec will be resolved.

A major objective of GLAST will be to resolve the high latitude diffuse
emission \[17\] and find, or at least greatly reduce, the fraction of
that flux which is due to unresolved point sources. A reduced residual
Galactic component would place a new limit on the contribution from the
IC process in the halo and would have implications for cosmic ray
propagation in the Galaxy. High-energy antiproton and positron
measurements will provide an important constraint on the spectrum of
protons and heavier nuclei and by implication on the electron (e^-^)
spectrum. This helps to resolve the issues of whether or not the local
electron spectrum is characteristic of the Galaxy as a whole and its
diffusive evolution. It therefore addresses the question of whether or
not the high-energy diffuse Galactic excess flux found by EGRET requires
an increased high latitude IC component.

# conclusion

Having improved understanding of the role of cosmic rays is essential
for the study of many topics in γ -ray astronomy. The results of the
studies discussed here will have important consequences for both
Galactic structure and the diffuse extragalactic γ -ray emission. It is
worth noting that complete understanding of the Galactic emission is
essential also for the study of the dark matter in the Universe.

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[^1]: ^2^ P+P-\>π^o^+X and π^o^ ⇒ 2γ

[^2]: We assumed that 60% of the γ−ray emission from

    γ Cygni comes from a suspected pulsar \[7\]. The spectrum of
    the γ Cygni region is taken from Ref. 6.