EGRET unidentified sources and gamma-ray pulsarsI. CGRO mission and the instrument EGRET and it’s scientific goals

II. Simple introduction of EGRET sources

III. Why are we interested in EGRET sources? What is the relation of EGRET source and gamma-ray pulsars? ex: 3EG J1835+5918

IV. What is the direction of our research? The next Geminga?

CGRO (Compton Gamma-Ray Observatory) Missions

* A study of discrete objects

* A search sites of nucleosynthesis - the process for building up heavy elements, and other gamma-ray lines emitted in astrophysical processes.

* The exploration of the Galaxy in gamma rays in order to study the effects of cosmic rays

* A study of the nature of other galaxies

* A search for cosmological effects

* Observation of gamma-ray bursts

* Mapping of distribution of diffuse gamma-ray line emission to determine its origin.

Scientific Instruments of CGRO

1. BATSE (30KeV-1.9MeV)

2. OSSE (0.05-10MeV)

3. Comptel (1-30MeV)

4. EGRET (20MeV-30GeV) :

Goals: study of the high energy transfers in neutron stars, other galactic objects, and active galaxies, the galactic and extragalactic high-energy gamma-ray diffuse radiation, energetic solar phenomena, cosmic rays and supernovae, and the high-energy gamma-ray emission of the gamma ray bursts of yet unknown origin.

October 5, 2001

-The source positions are plotted in Galactic coordinates with color-coded classifications. -As indicated, the EGRET instrument has now identified more than sixty members of a new class of active galaxies known as blazars.-These objects are characterized by extremely variable gamma-ray emission, and emit most of their energy in the gamma-ray regime.

Oct, 2002

History

1st EGRET catalogue: 127 (1EG)

2nd EGRET catalogue: 129 (2EG)

Supplement to 2nd EGRET catalogue: 157 (2EGS)

3th EGRET catalogue: 271 (3EG)

Why are we interested in EGRET sources?

1. Rotation-powered pulsars likely dominate the Galactic γ-ray source population. (Halpern & Ruderman 1993, Helfand 1994, Yadidaroglu & Romani 1997)

2. The unidentified γ-ray sources may include a significant fraction of the nearby neutron star population that has yet to be accounted for and may provide a useful windows into their physics. (Gehrels et al 2000, Grenier 2000, Harding & Zhang 2001)

γ-ray sources will most easily be identified as soft blackbody X-ray Sources without optical counterparts.

(D.J. Thompson et al, 1999)

(Many firsts of Geminga) (P.A. Caraveo et al, 1997)

An Example: 3EG J1835+5918Also Known as GRO J1837+59, 2EG J1835+5919, GEV J1835+5921

* First discovered at photon energies above 100 MeV abroad CGRO in 1991

* First report on GRO J1837+59 (Nolan et al. 1994) include only data from EGRET observations between 1991 and 1993

* Listed in the 2nd EGRET catalogue of γ-ray point sources. (Thompson et al. 1995) * Appearance of 3th EGRET catalogue (E >100MeV) and the GeV source compilation (E > 1GeV). (Lamb & Macomb 1997; Reimer et al. 1997)

* 12 individual observations

Features of 3EG J1835+5918* The source remains the brightest unidentified EGRET source outside the galactic

plane.

* The energy requirements of γ-ray production demand powerful sources; therefore ordinary stars and normal galaxies can be rule out as γ-ray sources.

* The absence of any counterpart for this source locate at 25˚ off Galactic plane.

* Pulsed radio emission with the Greenbank telescope at 770 MHz towards J1835+5918 yielded only 1 mJy flux density upper limit.

* fx/fv > 6000; 250pc< d <800pc .

* Deep optical imaging with the HST Space Telescope Imaging Spectrograph CCD failed to detect this source to a limit of V>28.5.

* Compatible with a non-variable source of an average flux of (E >100MeV)

127109.5 scm

*A single power law spectral index of 1.73 0.07 is consist with 1σfor all individual spectra.

Indication of spectral variability

Spectrum of 3EG J1835+5918 With the consistency of the individual

throughout the EGRET observations established, we co-added the data fromcycles 1 to 7 in order to determine the best overall spectrum of 3EG J1835+5918.A single power-law fit appears inadequatefor this source.

The spectrum resembles the γ-ray spectraof known γ-ray pulsars like Vela or Geminga (Thompson et al. 1997).

The hard spectral index, as determined to be -1.7 0.06 between 70 MeV and 4GeV.

X-ray observation of 3EG J1835+5918

RX J1836.2+5925 is characterized by an obvious lack of radio emission, indetectibilityby means of an UV-excess identification technique, lack of optical counterpart up toV≈23 mag. in the V and B bands, and location well inside the 68 per cent likelihood teststatistic contours of 3EG J1835+5918.

The high Fγ/Fradio value seems to rule out a blazar origin.

The lack of optical counterparts up to V≈23 mag. and radio emission for RX J1836.2+5925is a further characteristic signature for isolated, radio-quiet neutron stars. (Caraveo, Bignami & T

rumper 1996)

Next Geminga??

1. 3EG J1835+5918/RX J1836.2+5925??

2. 3EG J2020+4017/RX J2020.2+4026??

3. 3EG J0010+7309/RX J0007.0+7302??

Is there any difference between these EGRET sources and Geminga?

What will be our next step?