Download Cosmic lasers and Photon-correlation Spectroscopy (C.Germana, D

Claudio Germanà and Dainis Dravins
INAF Observatory of Padua
Lund Observatory
1. Laser Emission in astrophysical sources
2. Photon-Correlation Spectroscopy:
Resolving narrow spectral lines
3. Signal – to – Noise ratio
Energy level populations
described by Boltzmann’s
statistics
Medium acts as an absorber
Population inversion
Medium acts as an amplifier
”Light amplification by stimulated emission of radiation”
LASER
Lasers may be observed if:
1) Population inversion is feasible
2) Pumping mechanism for population inversion
3) Structures allow amplification
(e.g., clouds)
...laser emission might be observed in:
Fe II and O I lines in η Carinae
(Johansson & Letokhov 2004, 2005)
Wolf-Rayet stars
He II He I lines
(Varshni & Nasser
1975,1986)
Mass – loosing stars
S. Johansson & V.S. Letokhov
Astrophysical lasers operating in optical Fe II lines in stellar ejecta of Eta Carinae
Astron.Astrophys. 428, 497 (2004)
Model of a compact gas condensation near η Car with its Strömgren boundary
between photoionized (H II) and neutral (H I) regions
S. Johansson & V. S. Letokhov
Laser Action in a Gas Condensation in the Vicinity of a Hot Star
JETP Lett. 75, 495 (2002) = Pis’ma Zh.Eksp.Teor.Fiz. 75, 591 (2002)
at 9997 Å
A microsecond “bottle-neck”
creates a population inversion in
the 3 → 2 transition of Fe II
S. Johansson & V.S. Letokhov
Astrophysical lasers and nonlinear optical effects in space
New Astron. Rev. 51, 443 (2007)
...how to confirm Laser emission?
Expected extremely narrow
linewidth < 1 mÅ (0.1 pm)
(Johansson & Letokhov 2004)
by Dravins et al. 2007
Spectral resolution  100 million!!
What about a spectral line?
Electric field emitted from one atom which undergoes collisions:
E n(t)= E0 cos(ω0t + φn (t))
φn (t) is a Gaussian (chaotic process)
Total electric field from the system of n atoms (Loudon 1973):
a(t) is a Gaussian
... signal in Fourier’s notation...
exp(iωt)
Fourier component
E(t)TOT thermal light
a(t) ≠ cost (Gaussian)
E(t)TOT laser light
a(t) ≈ cost
...spectral line profile...
a(t)≠ cost (Gaussian)
a(t) ≈ cost
...FWHM and time scale of intensity fluctuations
Fourier’s temporal domain
Fourier’s energy domain
Photon (intensity) – correlation Spectroscopy
Intensity interferometry
Narrabri stellar intensity interferomter (R.Hanbury Brown, R.Q.Twiss et al., University of Sydney)
Required Telescope diameters
1/ 2
 T 

S / N  n1  c  A    |  | 
 2 0 
2
S/N 3
has been set
S/N for laser spectral lines
If there is laser emission, the coherence time of light
is three or
more orders of magnitude greater and so the S/N.
The required telescope diameter is smaller!!