Detection of Giant pulses from pulsar PSR B0950 08

1
Detection of Giant pulses from pulsar PSR B095008

• Smirnova T.V.
• tania_at_prao.ru
• Pushchino Radio Astronomy Observatory of ASC
  FIAN

We investigated amplitude variations of subpulses
of PSR B095008 at 112 MHz at various longitudes
(phases) and detected very strong pulses
exceeding the amplitude of the mean profile by
more than one hundred times. Detected giant
pulses from this pulsar have the same signature
as giant pulses of other pulsars.
PSR B0950 is one of the strongest pulsars at the
meter frequencies, flux density S 2 Jy at f
102 MHz, the distance is R 262 pc, DM 2.97
pc/cm3 Strong linear polarization of emission PL
70 - 100 at low frequencies, microstructure
with tµ 200 µs Rotation measure RM 1.35
rad/m2 , DM 2.97 pc/cm3, period of Faradey
modulation at 112 MHz PF 15 MHz Strong
diffractive scintillation tdif gt 300 s and fdif
220 kHz at f 112 MHz.
Observations and data reduction We obtained data
on the Large Scanning Antenna (BSA telescope) of
the Pushchino Radio Astronomy Observatory at
111.846 MHz during 22 days of observation in June
July of 2009. Linearly polarized emission was
received. Receiver 461 channels with 5 KHz
bandwidth per channel, the full bandwidth ? 2.3
MHz. The time duration within one day is 3.3 min
(770 pulses), sampling 0.4096 ms. For
each observing session (day) we calculated the
peak amplitude A, the signal-to-noise ratio S/N
(A/sN) and the energy in the mean pulse by
summing the intensities within the mean profile,
the mean value of sN for individual pulses. We
see strong variations of the mean profile
amplitudes from day to day due to propagation
effects A/sN is changing up to 44 times during
22 days of observation. We used the following
relation to scale the pulse peak amplitudes for
different days in flux-density units
(Jy) A(t)Jy A(t) S k /lt A gt, where S 2
Jy at our frequency, k 14.9 is a coefficient
relating to the ratio of the peak amplitude to
pulse energy averaged over the pulsar period, and
ltAgt is the mean value of amplitudes A(t) for the
entire series of observation in relative units,
ltAgt 30 Jy. The mean profile obtained by
summing of 17 profiles (13000 pulses) with S/N gt
14 is shown in Fig. 1 by solid line. The profile
has three components, with a separation between
components 2 and 3, ? 6.2 ms. We put here also
the mean profile at 430 MHz (thin line) taken
from European Pulsar Network Data Archive
normalized to the same amplitude. The weak
precursor with two unresolved components in the
main pulse we see at 430 MHz. The frequency
dependence of the profile width in this range is
W0.5 f -0.35.
Fig.1. Mean pulse profiles at 112 MHz (thick
line) and at 430 MHz (thin line)
Fig. 2. Giant pulses at different longitudes of
mean profile. The mean profile was multiplied by
100.
Individual pulses Our analysis of individual
pulses consisted of determining the positions
(phases) and amplitudes for subpulses with a peak
amplitude which exceeded some level in units of
sN within each pulse. These amplitudes were used
then to construct the amplitude distribution
function at various longitudes of the mean pulse.
We detected very strong pulses at longitudes of
all three components. In Fig 2 we show giant
pulses and mean profiles multiplied by 100 for
two days of observation. Amplitude of the
strongest pulse by 120 times exceeds the
amplitude of mean profile for this day and in 508
times exceeds the amplitude of averaged for 17
days profile. Peak flux density of this pulse is
15240 Jy and energy of it is 81240 Jyms that
exceeds the mean pulse energy by a factor of 153.
We have rare but strong pulses at the
longitude of component one (precursor), their
amplitude can be up to 490 times more than
amplitude of mean profile at this longitude. We
see in this figure that when emission takes place
at the precursor it is absent at the longitude of
the main pulse (MP) and vice-versa. In common
emission becomes much weaker in MP when strong
pulses exist in precursor. In Fig.3 (solid line)
we show profile obtained by summing of weak
pulses with 6sN gt I gt 3sN at longitudes of MP.
The number of pulses in summing (233) is about
the same as for profile selecting only strong
pulses with I gt 10sN (262) in the range of
longitudes of the MP (shown by dash line). We see
emission in the precursor which means weak pulses
in MP dont influence it. The shape of profile
here has also two components and the same width
as for MP. The width of MP and separation between
components for profile from strong pulses are 1.5
times less than for profile from weak pulses.
Longitude analysis of intensity variations of
pulses with discrete of 0.4 ms and I gt 5sN is
shown in Fig. 4 modulation index m and the
number of pulses (upper plot) mean intensity ltIgt
(lower plot). We see strong modulation activity
at the longitudes of precursor. The number of
pulses here is 10 times less than in MP.
Fig. 3. The mean profile obtained from the weak
pulses with 6 sN gt I gt 3 sN realized at
longitudes of the main pulse (solid line). The
normalized mean profile from strong pulses is
shown by dash line.
Fig. 4. Longitude dependences modulation index
m and the number of pulses with I gt 5sN (upper
plot) mean intensity ltIgt(l) and mean profile
multiplied by 10 (bottom)
Cumulative probability distribution To exclude
influence of scintillation and polarization
effects on intensity variations from day to day
we did the following correction of pulse
intensities I(t) In(t) s 0N A0/(s nN
An), where s 0N and A0 are sigma noise and
amplitude of the mean profile for specific
reference day, index n corresponds to day number
n. Cumulative distribution function (CDF) of the
number of pulses with I gt 5sN taken place at
longitudes of MP from 17 days of observation is
shown in Fig. 5 in log-log scale. The common
number of pulses was 3385. Two lines are the
result of a least-squares fit. CDF has a power
law with a changing of slope from n -1.25
0.04 to n -1.84 0.07 for I gt 600 Jy. We also
build CDF using the same procedure but for pulses
within longitudes of component 1 (precursor) we
chose 5 days and detected pronounced intensity in
the mean profiles at these longitudes. The number
of pulses here was 30 times (119) less than for
MP. This CDF is shown in Fig.6. Data can be
fitted within a power law where n -1.5 0.1.
Due to limited statistics in this field we can't
conclude that slopes of steeper part of CDF for
MP and precursor are different they have an
agreement within 3 s error. Interpretation of
the pulse behavior in the frame of
induced-scattering model The studied properties
of the precursor strong modulation, absence of
emission in the MP in the presence of powerful
GPs in the precursor range, similarity of the
precursors shape with that of the mean profile
obtained for strong pulses, and increase the
emission intensity at low frequencies are well
explained by the mechanism proposed by Petrova
1, involving induced scattering of the MP
emission by relativistic particles of strongly
magnetized plasma in the pulsar magnetosphere. It
was shown in 1 that the scattering of the MP
emission occurs at larger heights than the
initial pulse. The scattered radiation is
directed away from the pulsar surface,
approximately along the local magnetic field,
and, because of aberration due to rotation of the
star, the scattered component appears as a
forward-shifted precursor. The intensities of the
incident (I1) and scattered (I2) emission are
redistributed, conserving of their total
intensity I I1 I2. The intensity
redistribution between the incident (MP) and
scattered emission is determined by eq. (11) from
1 I1 I(I10/I20)exp(-G)/(1(I10/I20)exp(-G)) I
2 I/(1(I10/I20)exp(-G)), where a subscript 0
refers to the initial intensities of the beam and
background emission due to spontaneous
scattering. Here G is an efficiency of the energy
transfer. For the parameters of PSR B095008 and
frequency 112 MHz , I10/I20 1010 ? 1012 for the
distances where scattering occurs r 108 ? 109
?m respectively. From observation we have I2/I1
lt 210-3 (absence of precursor), I1/I2 lt
5.610-3 (absence of MP), then substituting these
values (1) we have G lt 17 ? G gt 28 for these
cases. We adopt in the case of an absence of
emission the 2 sN level. The efficiency of
scattering G can also be estimated using eq. (13)
from 1. Substituting these equation parameters
of total radio luminosity, magnetic field at the
surface, spectral index, angular width of the MP
and the angle between the magnetic axis and the
tangent to the magnetic field line in the
scattering region at a distance r (for a dipole
field ? r/2rL, where rL 1.2109 cm is the
radius of the light cylinder) we got G
421(r108?m)-8. Agreement with the values of G
derived above requires r 1.5 108 cm (no
emission in the precursor) and r 1.4 108 cm
(no emission in the MP). The scattering can take
place at different levels as G changes, and
variations in G at a given level are due to
fluctuations of the secondary-plasma
parameters. Thus, the anticorrelation between the
intensities of the precursor and MP, together
with the similarity of their shapes, can be
understood as consequence of transfer of the MP
energy to the precursor. The relative
amplitude of the precursor considerably
increases, and it is three times stronger at 112
MHz than at 430 MHz. This increase in the
scattering efficiency is often the case with
decreasing frequency. The power law of GP
intensity distribution could be also explained by
the theory of induced scattering of the pulsar
emission 2.
Fig. 6. Cumulative distribution function for
pulses with I/sN gt 5 (longitude of component
one).
Fig. 5. Cumulative distribution function for
pulses with I/sN gt 5 (longitude of main pulse).

• Conclusions
• We detected that giant pulses have the same
  signature as other pulsars.
• We had shown that if giant pulses take place at
  longitudes of main pulse (MP) then emission at
  the longitude of precursor is absent and if GP
  appear at the longitude of precursor then
  emission in MP becomes weak or absent.
• Mean profile obtained by summing only strong
  pulses with S/N gt 10 have a width and separation
  between components of MP in 1.5 times less than
  from summing weak pulses with S/N lt 6. The shape
  of precursor is very similar to the shape of MP
  obtained from strong pulses.
• Intensity of individual pulses of PSR B095008 at
  112 MHz can exceed the peak flux density of the
  average pulse by hundreds times the strongest
  pulse has I 15240Jy. Rare but very strong
  pulses take place at the longitude of precursor
  (component one) with a peak flux density up to
  5750 Jy, amplitude of the strongest pulse exceeds
  by 508 times the amplitude of the mean profile at
  this longitude.
• Cumulative distribution function (CDF) for
  pulses in MP is described by a piece-wise power
  law with a changing of slope from n -1.25
  0.04 to n -1.84 0.07 for I gt 600 Jy. CDF for
  pulses at longitudes of component one has a
  power law with n -1.5 0.1. They have an
  agreement within 3 s error but theres little
  statistics for precursor.
• We have shown that the studied properties of the
  pulses of PSR B095008 can be well explained
  qualitatively in the model of induced scattering
  of the MP radiation by relativistic particles of
  strongly magnetized plasma in the pulsar
  magnetosphere. We estimated the level at which
  the proposed scattering of the MP emission takes
  place r 0.1rL and of the scattering
  efficiency parameter G.
• REFERENCES
• 1. S. A. Petrova, Mon. Not. R. Astron. Soc. 384,
  L1 (2008).
• 2. S. A. Petrova, Astron. Astrophys. 424, 227
  (2004).