Transcript Пробный доклад

Detection of Cosmic Ray Protons by
Radio Recombination Lines of Hydrogen
G. T. Smirnov, R. L. Sorochenko
Pushchino Radio Astronomy Observatory
Cosmic rays in the Galaxy
Cosmic rays (CR) play an important role in physical and chemical processes taking
place in the cold ISM outside HII regions. This is related to hydrogen ionization by
CR protons which influences the temperature and dynamics of the ISM as well as its
chemical composition.
Chemical reactions with ions take place more rapidly than reactions between neutral
particles. For this reason abundance of some molecules containing hydrogen and
oxygen depends on abundance of Н+ ions and accordingly on rate of ionization of
hydrogen.
The cross sections for hydrogen ionization by protons decrease with increasing
energy. At the same time low-energy protons are more strongly absorbed in the ISM.
As a result the main contribution to ISM ionization is due to protons with energies 1
<E < 10 MeV.
Ionization rate of interstellar hydrogen by CR
CR with such low energies cannot be detected on the Earth because they are
deflected by the solar wind and associated magnetic field. Their intensity and
accordingly the rate of ISM ionization by CR, ζH, has been estimated using various
indirect methods:
–
–
–
Extrapolation of the CR spectrum E > 1 GeV observed on Earth to MeV energies.
Extrapolation to MeV energies of the CR spectrum E > 70 MeV measured on the Pioneer
and Voyager spacecrafts moved to a distance of 60 AU from the Sun.
On abundance of the molecules formed with participation of Н+ and Н2+.
The values of ζH obtained by these methods strongly differ up to 1 or 2 orders of
magnitude.
It seams promising to use for determination of ζH radio recombination lines (RRL)
which represent an effective tool for investigating the ISM.
Cosmic rays and RRL
When H+ ions recombine with electrons highly excited atoms of hydrogen are formed
and radio recombination lines are emitted. Detection of hydrogen RRL from cold ISM
would testify on proton component of CR and would allow a direct derivation of
ionization rate of interstellar hydrogen ζН.
However numerous attempts to detect RRL of hydrogen from cold ISM made for a
number of lines from H300α to H540α were unsuccessful. Liszt (2003) claimed that the
hydrogen RRL could never been observed since H+ ions would be neutralized by
negatively charged molecules of polycyclic aromatic hydrocarbon (PAH).
We made a new attempt to receive RRL of hydrogen having improved significantly
sensitivity and observing technique.
The experiment scheme
There are two molecular clouds in Perseus arm at VLSR =-48 and -38 km/s in front of Cassiopeia A. A photo
dissociation region (PDR) is created on surfaces of the clouds by external background UV radiation (λ>912 A).
Carbon is fully ionized while hydrogen is neutral in the PDR. But hydrogen can be partially ionized by CR and we
will search for hydrogen RRL from this PDR.
More than 10 times increase in sensitivity
Maser amplification of RRL.
Te = 50 K
ne=0.15 cm-3
Hydrogen
Physical conditions in the PDR (Te=50 K, ne=0.15
cm-3) were derived from RRL of carbon observed
in a wide frequency range . Based on calculated
level populations of highly excited states we
identified the optimum quantum number n ~250 or
frequency range ~400 MHz where maser
amplification is maximal. In spite of this
amplification the optical depth of the hydrogen
lines is expected to be very small: τH ~10−4.
Confident detection of such weak lines required
the creation of rather sensitive instrumentation. To
get high sensitivity 8 RRL in 2 polarizations were
planned to receive simultaneously.
The new spectral complex 400 MHz at RT-22
Feed for 2 polarizations.
2-channel HF box.
2-channel video converter 0-40 MHz.
16382 channel digital spectrum analyzer.
Basic specifications of the new spectral complex
Input frequency range 400-440 MHz
Simultaneous reception of 2 polarizations
Eight independent 500 kHz bands
Spectral resolution 1 kHz
On-line cleaning of interferences with linear frequency modulation
Cleaning of interferences
The 400 MHz receiver band is heavily
contaminated by interferences. Red line spectrum of a 500 kHz band influenced by a
interference from a radar. Any search for weak
lines would be hopeless.
We were forced to introduce into the spectrum
analyzer a specially designed hardware which
detect the radar interferences and eliminate it
in real time. Blue line – the similar spectrum
with the on-line cleaning of the radar
interference. Weak lines can be registered after
long integration time.
TL/TCas
First detection of hydrogen RRL from cold ISM
2.0x10
-3
1.5x10
-3
1.0x10
-3
5.0x10
-4
The final RRL spectrum toward Cassiopeia A
averaged over 6 transitions (248α-253α) and 2
polarizations. The equivalent integration time is
2500 hours (if it were a single line in a single
polarization). The carbon line profile consists of
2 known components at VLSR=-48 km s-1 and
VLSR=-38 km s-1.
C248-C253
0.0
-5.0x10
-4
3.0x10
-4
2.0x10
-4
1.0x10
-4
TL/TCas
-80
-60
-40
-20
0
20
The profile of hydrogen RRL shows 2 features
exactly at the same velocities as that of carbon.
The component at VLSR=-48 km s-1 is detected
quite confidently. The component at VLSR=-38
km s-1 is also visible. However, the signal to
noise ratio is low and the detection is only
tentative.
H248-H253
0.0
-1.0x10
-4
-2.0x10
-4
-80
-60
-40
-20
VLSR, km/s
0
20
Ionization rate of hydrogen
 H nH   2ne nH

3.3 1015  2 ne  H  H bn  n C
H 
Te0.5
 C  C bn  n H
Ionization rate of hydrogen derived from the RRL components
at VLSR = -48 km s-1 is ζH =(1±0.25)10-16 s-1.
Ionization rate by X-rays
5.0
3 3pc
5
5 pc
1010pc
V,km
km/s
s-1
ΔV,
4.5
4.0
20
20 pc
3.5
The 2 clouds are near Cas A and are subject to X-rays from the
supernova remnant. To estimate ionization rate by X-rays we
must know the distance between the clouds and Cas A .
The distance can be determined using width of carbon RRL.
The radio emission of Cas A stimulates transitions between
highly excited levels thus reducing their lifetimes and producing
radiation broadening of RRL.
The observed widths of the –48 km s-1 feature are consistent
with pure Doppler broadening 3.5 km s-1. Absence of any
additional radiation broadening means that the cloud is no less
than 20 pc from Cas A. At this distance ζH.X =10-19 s-1.
Ionization rate by the diffuse Galactic X-ray ζH.Xdiff =2·10-19 s-1.
3.0
160
180
200
220
nn
240
260
280
Thus the measured ionization rate ζH =10-16 s-1can not be
accounted for by X-rays.
Ionization rate by cosmic rays
Technique
Line of sight
ζН (10-16 s-1)
Cas A
1.0±0.25
Present paper
Ground based
The Galaxy
0.07
Spitzer, 1968
Spacecrafts
The Galaxy
RRL
OH
Н3 +
ζ Per
ό Per
0.33
0.9±0.3
0.25
3.7
3.7±2.2
1.9
7.8
4.8
<7.5
References
0.3-0.4
Webber, 1998
1.0 ±0. 4
Harquist, 1978
van Dichoeck,1986
Federman, 1996
ζ Oph
0.25
1.9 ± 0.7
<2.2
3.0
McCall, 2003
Indriolo et al. 2007
CR from Cassiopeia A
For a long time it has been considered that supernovae produce CR. The production of
relativistic electrons has been clearly established from the non-thermal radio and X-ray
emission observed from supernova remnants (SNR). However, evidences, which prove
that SNR are sources of relativistic protons, the main constituent of cosmic rays, are not
still reliable.
Can the ionization rate we measured be due to protons from Cas A? Protons with
energies of 1-10 MeV, which make the main contribution to the hydrogen ionization, cannot
reach the cloud (VLSR=-48 km s-1) in 330 years (the age of the supernova remnant) since it
is more than 20 pc away from Cas A. Thus Cas A can not contribute to the measured
value. We conclude that the measured value is due to background CR protons.
Keohane et al, (1996) suggested that the cloud at –38 km s-1 is very close to Cas A. If
the distance is <10 pc the higher value of ζH may be expected due to contribution from
supernova. This is a task for future more sensitive RRL observations.
γ-rays from Cassiopeia A
Recently Fermi-LAT and VERITAS have detected gamma-rays from Cas A. The gamma-ray
spectrum is consistent with π0 decay testifying that Cas A is a source of CR protons.
Obtaining direct information from RRL observations about such protons would be valuable for
studies of generation of CR in supernovae.
Conclusions
The first detection of hydrogen RRL from the cool ISM demonstrates the
possibility of determining the ionization rate of hydrogen by CR employing this
direct method.
The value ζH = (1 ± 0.25) • 10−16 s−1 derived from RRL observations toward
Cassiopeia A is the ionization rate of interstellar hydrogen by background CR
protons.
It would be of considerable interest to observe carbon and hydrogen RRL in
the cloud at −38 km/s.