Transcript Modeling the Spectral Energy Distributions and Variability

Diffuse Emission and Unidentified Sources
Plane of the Milky Way
(diffuse emission)
3C279 (quasar)
Geminga
(pulsar)
Crab
(SNR)
PSR 1951+32
EGRET, E > 100 MeV
(pulsar)
3C454.3 (quasar)
Vela (pulsar)
PKS 0528+134
(quasar)
More than half of all gamma-ray sources are still unidentified!
Diffuse g-Ray Emission
Dominant diffuse g-ray production mechanisms:
Interactions of cosmic-rays (highly relativistic
particles in space) with the interstellar medium
(ISM) and/or the interstellar radiation field.
1. pcr pISM → p p + p0 ;
p0 → 2 g
2. Bremsstrahlung of cosmic-ray electrons
3. Compton scattering of cosmic-ray electrons off
the interstellar radiation field (infrared/optical
light from stars)
But also: Sum of the contributions from many
faint, individual, unresolved sources
Cosmic Rays
Charged high-energy
particles (electrons,
protons, heavier
nuclei), but also
photons (g-rays) in
space
F(E) ~ E-2.7
Galactic origin
(supernovae)
~ 1015 eV
Energies:
MeV - >1020 eV
(ultra-high energy
cosmic rays =
UHECRs)
F(E) ~ E-3.1
Extragalactic origin
(AGN, GRBs?)
Ultra-High-Energy Cosmic Rays
The Greisen-Zatsepin-Kuzmin cutoff
The Universe is pervaded with a thermal “afterglow” of the Big
Band: The Cosmic Microwave Background (CMB) radiation:
Blackbody at T ≈ 2.7 K.
UHECR nuclei with energies E > 3x1019 eV interact with the CMB:
(pg → pp0 or pg → np+)
 Protons with E > 3x1019 eV from cosmological distances lose
much of their energy on their way to us
 UHECRs at E > 3x1019 eV must come from within ~ 100 Mpc.
The resulting cutoff (“GZK cutoff”) in the UHECR spectrum has
recently been measured by the Auger collaboration.
Spectrum of Diffuse g-Ray Emission
p0 decay
Bremsstrahlung
Compton scattering
Unresolved extragalactic sources
The Structure of the Milky Way
75,000 light years
Disk
Most gas and dust
is concentrated in
the nuclear bulge Sun
and the spiral arms.
Nuclear Bulge
Halo
Open Clusters
(newly born stars)
Globular Clusters
(old stars)
The Structure of the Milky Way
Distribution of stars
and neutral hydrogen
Distribution of dust
Sun
Bar
Ring
The Problem of Identifying
g-ray Sources
EGRET error
contours
Pulsar
Black Hole XRay Binary
What’s the source
of the g-ray
emission?
Need more information
(broadband spectrum;
variability)
Unidentified g-Ray Sources (UIDs)
Out of 270 sources in the EGRET catalog (sources of
> 100 MeV g-rays), 170 are unidentified!
Also, about two dozen TeV g-ray sources (detected by
HESS, MAGIC) are unidentified.
Almost all within Galactic latitude |l| < 30o
=> Almost certainly of Galactic origin
The Nature of UIDs
Unidentified sources show a variety of different properties:
They are certainly not one homogeneous source class.
Possible identifications:
• Background AGN (→ Variability!)
• Supernova remnants (→ Non-variable, extended)
• Pulsars (→ Pulsed emission; hard spectrum)
• Pulsar wind nebulae (→ non-variable, extended)
• X-ray/g-ray binaries (→ periodic [orbital] variability)
• O/B Associations (young, very massive stars with
strong stellar winds) (→ non-variable, extended)
High-Latitude Sources
Comparison of spectral and variability properties
Number of Sources
Complete
EGRET
catalog
High-Lat.
UIDs
Spectral Index
Variability Index
AGN
Pulsars
High-Latitude Sources
Number of Sources
Similar flux – number diagrams as AGN
Flux (> 100 MeV)
Most low-latitude sources are non-variable. Several
previously unidentified TeV sources could be identified with
pulsar wind nebula; most remain unidentified.
Examples:
1) The UID 3EG J1837-0423
•
•
•
•
Located only 1o off the Galactic plane
High peak g-ray flux for only 3.5 days
Never detected before or afterwards
Gamma-ray spectrum with photon index -2.1
Strong variability; hard photon spectrum
→ Background blazar?
Should be detectable in radio/infrared
for normal blazar properties
2) HESS J1303-631
• TeV g-ray source 0.5o
north of a known gray pulsar
• Less variable than
the pulsar.
HESS J1303-631
PSR B1259-63
HESS J1303-631
None of the nearby pulsars
is powerful enough to power
the TeV source
Possible Counterparts
Radio and X-ray sources near HESS J1303-631
HESS J1303-631
Related to a powerful
stellar wind (WR124)?