Diffuse Emission and Unidentified Sources 3C279 (quasar)

Plane of the Milky Way (diffuse emission)

PSR 1951+32 (pulsar)

Vela (pulsar)

Geminga (pulsar)

Crab (SNR)

PKS 0528+134 (quasar)3C454.3 (quasar)

More than half of all gamma-ray sources are still unidentified!

EGRET, E > 100 MeV

Diffuse -Ray EmissionDominant diffuse -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 + 0 ; 0 → 2

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 RaysCharged high-energy particles (electrons,

protons, heavier nuclei), but also

photons (-rays) in space

Energies:

MeV - >1020 eV

(ultra-high energy cosmic rays =

UHECRs)

F(E) ~ E-2.7

F(E) ~ E-3.1

Galactic origin (supernovae)

Extragalactic origin (AGN, GRBs?)

~ 1015 eV

Ultra-High-Energy Cosmic RaysThe Greisen-Zatsepin-Kuzmin cutoff

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 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:

(p → p0 or p → n+)

The resulting cutoff (“GZK cutoff”) in the UHECR spectrum has recently been measured by the Auger collaboration.

Spectrum of Diffuse -Ray Emission

0 decay

Bremsstrahlung

Compton scattering

Unresolved extragalactic sources

The Structure of the Milky Way

75,000 light years

Disk

Nuclear Bulge

HaloSun

Globular Clusters (old stars)

Open Clusters (newly born stars)

Most gas and dust is concentrated in the nuclear bulge

and the spiral arms.

The Structure of the Milky Way

Distribution of dust

Sun

RingBar

Distribution of stars and neutral hydrogen

The Problem of Identifying -ray Sources

EGRET error contours

Pulsar

Black Hole X-Ray Binary

What’s the source of the -ray emission?

Need more information (broadband spectrum;

variability)

Unidentified -Ray Sources (UIDs)Out of 270 sources in the EGRET catalog (sources of

> 100 MeV -rays), 170 are unidentified!

Also, about two dozen TeV -ray sources (detected by HESS, MAGIC) are unidentified.

Almost all within Galactic latitude |l| < 30o

=> Almost certainly of Galactic origin

The Nature of UIDs

Possible identifications:

• Background AGN (→ Variability!)

• Supernova remnants (→ Non-variable, extended)

• Pulsars (→ Pulsed emission; hard spectrum)

• Pulsar wind nebulae (→ non-variable, extended)

• X-ray/-ray binaries (→ periodic [orbital] variability)

• O/B Associations (young, very massive stars with strong stellar winds) (→ non-variable, extended)

Unidentified sources show a variety of different properties: They are certainly not one homogeneous source class.

Comparison of spectral and variability properties

High-Latitude Sources

Complete EGRET catalog

High-Lat. UIDs AGN Pulsars

Nu

mb

er

of S

ourc

es

Spectral Index

Variability Index

Most low-latitude sources are non-variable. Several previously unidentified TeV sources could be identified with

pulsar wind nebula; most remain unidentified.

Similar flux – number diagrams as AGN

High-Latitude Sources

Flux (> 100 MeV)

Nu

mb

er

of S

ourc

es

Examples:1) The UID 3EG J1837-0423• Located only 1o off the Galactic plane• High peak -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 -ray source 0.5o north of a known -ray pulsar

• Less variable than the pulsar.

PSR B1259-63

HESS J1303-631

HESS J1303-631None of the nearby pulsars is powerful enough to power the TeV source

Radio and X-ray sources near HESS J1303-631

Possible Counterparts

HESS J1303-631

Related to a powerful stellar wind (WR124)?