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Polarization observations of four southern pulsars at 1560 MHz
Chin.Astron.Astrophys.(l991)15/4,423-430 A translation of Acta Astrophys.Sin. (1991)11/3,227-233
0 Pergaaon Press plc Printed in Great Britain 02751062/91$10.00+.00
~~I~TI~ O~~VATI~S OF FWR SOUTHERN PULSARSAT 2560 MHz* Xin_ji'vz~3 R. N. Manchester3 A. G. Lyne’ ‘Centre of Astronomy and Astrophysics, CCAST (World Laboratory] ‘Department of Geophysics, Peking University 3Division of Radiophysics, CSIRO, Australia ‘NRAL. University of Manchester, Jodreli Bank, UK
w
ABSTRACT We present some interesting results from the mean pulse polarization observations of four southern pulsars made at the Australian National Radio Astronozty Observatory, Parkes, using the 64-s telescope in June and July, 1988. The 2 x 16 x 5 MHz filter system fros Jodrell Bank has proved excellent in de-dispersing the pulse signals and measuring their polarization properties. We give the data for the four pulsars in some detail and discuss their spectral behaviour.
Key words:
Pulsars-polarization
1. INTRODUCTION Polarization of mean pulse of pulsars is one of the basic topics in pulsar observational study. The Bean pulse often has a strong linear polarization with stable characteristics. This means that the sagnetic field of a pulsar is also very stable. Measurement of the polarization properties is particularly important in the study of polar cap models. Such studies as the question whether the radiating cone has an elliptical section [l-31, the sagnetic inclination (4-71, the evaluation of the position paraseters Q and /I~ where the sightline sweeps past the radiating cone 15, 81, the evolution of the pulsars [7, 91, the widths of drifting pulses and subpulses [lo, Ill, and the derivation of accurate formula for radio lusinoaity [8, 121 become sore than superficial only after adopting some polarization parameters. For some pulsars, the position angle of linear polarization shows eudden 90° jumps as the pulsar longitude changes, or the intensity of circular polarization show sudden changes in the direction of polarization at certain longitudes. For some pulsars, strong de-polarization is present in certain parts of their mean pulse profile. Observations of two orthogonal polarization modes and of the phenomenon of de-polarization and the study of the mechanisms have anounted to 1 Progras supported by National Educational Commission Astronomical Section Plan for Observations Abroad and the National Natural Science Foundation. Original version received 1990 May 15; passed by Referee November 14; revised version received 1991 March 5.
424
WU Xin-ji et al.
topics of great concern 113, 141. Multi-frequency observation not only provides spectral characteristics of the pulsar emission, it also provides information on the three-dimensional structure of the radiating cone. This is because according to the available theoretical models different frequencies are emitted in different locations, the higher frequency regions are closer to the surface of the neutron star than the lower frequency regions. Therefore, the separations between the components in the mean pulse profile, the polarization properties, the frequency dependence of the spectrur etc. have always been important topics for the statistical and theoretical studies. Rankin (1983, [15]) analysed the multi-frequency properties of intensity and polarization of mean pulse for a set of pulsars and arrived at the conclusion that the central portion (core) of the radiating cone has strong emission and so changed the traditional view of hollow cone; this is one of the important results in the observational and statistical studies of pulsars over the last ten years. In view of the importance of the polarization properties of pulsars in our knowledge of their emission properties and theoretical models, recent years have seen great strides being made in polarization observation in the northern hemisphere [5J. In the southern hemisphere, however, such observation has been at a halt and there was no such observation between 1977 and the present one reported here. Of the 155 pulsars discovered in the second Molonglo survey, only a few have polarization aeasured. Because of its geographical advantage and of the excellent work of its astronomers, Australia occupies a special position in the observational study of pulsars. Of the pulsars discovered, one half are contributed by Australia. In polarization observation, results at 400, 630 and 1612 MHz were successively published in 1977, 1978 and 1980 [IS-181. There were 18 pulsars at 400 MHz, 43 at 630 MHs and 38 at 1612 MHz. Of these, 9, 23 and 22, repectively are south of latitude -30°. They belong to 25 different pulsars. The resson why the southern polarization observation lagged behind is that the 64-B Parkes radio telescope had only a single channel receiver system, had no de-dispersing capability and could observe only the stronger pulsars. For our present observation we used a newly and successfully designed de-dispersing filter system, which raised greatly the sensitivity. We obtained polarization data on 47 sourthern pulsars, which is the largest s-pie obtained so far for the southern sky. Of the 47 pulsars, 17 were measured for the first time for polarization, and 21, for polarization at high frequency. In this paper we present the results of four representatives with first-time high frequency polarization data,
2. ANALYSIS AND TREATMENTOF OBSERVATIONS AND DATA Our observation was carried out on the Parkes 64-metre radio telescope of the National Radio Astronomy Observatory, Australia. The dates were 1988 June 1 and 2, July 14, 15 and 18. The Parkes polarimeter was as shown in the block diagram by Hamilton [16]. But the present observation differed from the previous ones in two respects. The pre-amplifier was cooled, so the systen noise temperature was lower, the equivalent flux when pointing at cool sky
FourSouthern Pulsars
425
was 66Jy. The second difference was that we used the new 2 x 16 x 5HRz muiti-channel fiiter system, recently successfully designed at Jodrell Rank. Each channel has a bandwidth of 5 M?Ie. The polsriueter h8s four outlets giving the Stokes parauetera U and V, the two components of the intensity IX, I,. The total bandwidth of each route is 80 MHz, evenly slloted to 16 filters. After detection, a multi-channel l-bit digital system samples the signal from esch filter over 1.07 l s intervals and writes the record on magnetic tape. Because we selected a high observing frequency (156O~s), the effect of dispersion by interstellar medium on pulses with a bandwidth of 5I4Hs is very small and allows the observation of pulsars with larger dispersions. The radio source 1934-63 (with known flux of 16Jy) was used for C8libr8ting the Because of differing periods, the comparison source of the receiver. number of recorded d8t8 points within a period is different for different pulsars, so 8 standardized treatsent is needed. For the longer-period pulsars, the data within each period are grouped into 256 groups; for the shorter-period pulsars, we take the actual number of sample points, when that number resulting fros the 1.07 ss scupling is less than 256. A key step in the data analysis is the period folding of the output of each of the 64 channels to yield high S/N ratio mean einglechannel results. Fig. la gives the actual exaaple of PSR 1356-60. Channel numbers l-16 refer to the parameter IX, 17-32, 1,. 33-48 to the parameter U, and 49-64, the paraueter V. Because of the effect of dispersion by interstellar sedium, the pulse arrives at different times at different frequencies. Take IX 8s example, Channel 16 has a central frequency of 1597.5MRs while Channel 1, one of 1522.5MHa. The high frequency pulse arrives first, the low frequency pulse arrives later , so we find a regular time lag. From the data of Pig. 1 we can ueaeure the dispersion measure DM. The time lag At over different frequencies caused dispersion in the interstellar space is AC -
DM
1.2 x
IO_‘v’
Av
(1)
At is in ss, o and Au are in MHz and DM is in pc/cu3. The difference in frequency between two neighbouring channels is Av = 5MRz and it is easy to calculate the lag of the first 15 of the 16 channels relative to the last. The process of de-dispersing ia very simple: we only have to add to the data after period folding for each channel the appropriate time lag and then add together the results for the 16 channels. Fig. lb shows the results for the four Stokes pammeters I (=Ix t Iy), 8 = (Ix - Iy), U, V for PPSR 156-60, labelled I, 2, 3, 4, respectively. The final results are the total intensity I, the effective linear polarization L = (eZ t ti)“‘, the effective circular polarization V (positive for left, negative for right circular polarization) and the position angle of the Linear polarization IP= (1/2)arctan (U/O). Figs.2~5 give the variation with longitude of the four pammeters for the four pulsars, the solid line is I, the dashed line is L, the dotted line is V and the curve in the upper part of the diagrau is u. The error box given in the left hand bottom
426
WU Xin-ji et al.
0’ pdse
Fig.
phase
g 0.2
I 0.4
0.6 Pulse phase
’
0.8
-
1 a) Results after period folding of the four parameters IX (Channels l-161, I, (17-321, II (33-46), Y (47-64) of PSR 1356-60. b) Mean results of the four Stokes parameters I, Q, (I, V of PSR 1356-60.
corner represent, in the vertical direction, the uncertainty (20) in the total intensity caused by random noise and, in the horizontal direction, the effect of time lag caused by interstellar dispersion. The error in the position angle of the linear polarization is caused by random noise and the error in the position of the antenna illuminator and is represented by the error bar. The effective linear polarization is defined to be always positive and it includes the effect of noise. To solve this problem we should subtract a value representing the noise level of L which can be equated with the mean L value outside the mean pulse. The value of L shown in the figure is the value after subtracting this noise level.
3. THE RESULTS Figs. 2-5 give the results of the first time high frequency polarization measurement of four pulsars. These include typical objects with single peak, double peak, and broad and complex multiple peak pulses as well as the relatively rare intermediate pulse. It is now broadly accepted that the pulsar radiating cone consists of three parts: a central part (core), a weak inner annulus and a strong outer annulus and the intensity distribution is not continuous over the cone annuli [16]. Because the sight line sweeps different parts of the cone, so we observe different profiles of the mean pulse. Very broad profiles are caused by small
Four Southern Pulsars
42-l
*‘..
0
50 Lcqitude(deg.)
Fig.
1
toirl
inrcnsity-
snd
Mean
pulse
poiiiion
*ngfe per)
profiler
100 LonKitude(deg.)
(lower
Fig.
curvesc
linear----.
circuirr
..-a..)
of linear
component
(up-
.f
*-.
3
angle
Mean
pulse
of Iincar
profiler
compoocor (Key,
and
posician
of PSR 1055-52
rce Fig.
2)
PSK 0148-06.
180
-
120 60 [ 150
100
M
L.
01 0
Fig.
angle
50
4
Mean
pulse
of linear
100
profiler
componcnr
(Key,
se< Fig.
-I .?
and
poririon
aI PSR
13S6-60.
2.)
0
Fig. angle
100
300
200 ~ngitud~deg.)
5
Mean of linear (Key,
pulrr
profiler
somponenc see Fig.
and of
2.)
poritian
PSR 11131-04.
428
WU Xin-ji et al.
magnetic inclinations, while the intermediate pulse may have been formed when the inclination in near to 90”, making it possible to observe emission from both polar regions of the neutron star. Another possibility of intermediate pulse is a very small magnetic inclination coupled with an extremely non-uniform intensity distribution over the radiating cone. 1. The Single
Peak Pulsar
PRR 1356-60
This is a pulsar with a very large dispersion (LBl= 295 pc/cm3). Thanks to the use of a powerful de-dispersing system, a good signal-to-noise was obtained. The mean pulse has a very strong linear polarization (84%). Roth the total intensity and the linear polarization are symmetrical single peaks and there is rather strong depolarization in the wings. The position angle of the linear polarization basically does not vary with the pulse longitude. 2. The Double Peak Pulsar
PRR 0148-06
The total intensity of the mean pulse and the linear polarization both show good double peaks and the position angle varies smoothly with the pulse longitude. The maximumgradient of the position angle occurs at the middle of the mean pulse, that is, between the two peaks. All these are in complete agreement with the predictions of the coronal cap model. The linear polarization is rather strong, but the circular polarization is weak. 3. The Complex Multiple
Peak Pulsar
P3R 1831-04
Its apparent beamwidth is very large, the half-power width is already lOgo. The profile of the mean pulse is rather complicated, showing many components. The linear polarization also shows a complex multi-peak structure. The mean linear polarization is rather low. 4. Pulsar
with an Intermediate
Pulse
P6R 1055-52
The observed results at high frequency (1560Mtlz) agree in many respects with those at low frequency. The main pulse and the intermediate pulse are separated from each other by 155”, but the radiation is still regarded as coming separately from two magnetic polar regions. One of the reasons is that the separation between the two pulse is independent of the frequency. If the radiation comes from the same region, then the separation at high frequency will be narrower than at low frequency. Another reason is that the intermediate pulse comes from a partial cone with a vanishing tail so that the separation is only 155”. This pulsar is a weak object, the mean flux density of the main pulse is only 2.5 mJy, and the intermediate pulse is only 2.4 qJy. We used data over 11675 cycles and a good signal-to-noise ratio was obtained after period folding and accumulation. The main pulse has a strong linear polarization, as large as 82X, particularly its first component which is almost 100% linearly polarized. Of the intermediate pulse, the first component is again strongly polarized, but the following components , so the mean polarization for this have very weak polarization
FourSouthern Pulsars
pulse is only 35%. Roth the intermediate circular polarization.
429
and main pulses
have clear
TABLE 1 lists the parameters of these four pulsars. The nean linear polarization (RLP) is defined as the ratio of the areas under the effective linear polarisation curve (Z,) and the total intensity curve (I), The mean circular polarisation (MCP) is similarly defined with the effective circular polarication curve (V) replacing L. A& is the equivalent apparent be-width, and A&o and A*lo are the be-widths at half power and l/l0 peak value, respectively. At is the time lag caused by dispersion by interstellar medium on a 5MRe wide pulse, and Z+,urse is the accumulated number of pulses.
TABLE 1 PRR
P fWf Z$ul*e l s p&ems
At 118 9l560 BJY MLP
0148-06 1465 25.1 1597 0.4 1055-52% 197 30.1 11675 0.45 1356-60’ 1831-04
126 290
295. 78.8
7813 8017
4.5 1.2
11.0 2.5 2.4 7.0 11.0
0.29 0.82 0.35 0.84 0.15
MCP 0.06 0.13 0.21 0.15 0.11
AI& 0
A%o
A@jo
15.0 30.2 37.4 16.2 23.5 30.4 21.6 18.9 45.9 17.2 15.5 28.1 63.3 109.2 136.4
’ First line-the main pulse; second line-the intermediate ’ Interstellar medium dispersion widened Acp
pulse.
4. DISCUSSION Here we give only part of the results of our polarization observation in 1988 of southern pulsars at Packes, Australia. For the four pulsars reported here, only low frequency observations were available before. Becauseof the power law spectrum, the high frequency flux is very small and is difficult to measure. Apart fro= P8R 1356-60 for which we cannot examine the variation of the beamwidth with frequency becauee of the large broadening by interstellar dispersion, all other three pulsars showed a decrease in the apparent beamwidth with increasing frequency. While of the power law form, the spectra are rather flat, with an index of only about 0.1. The variation with frequency of the linear polarization did not show any regularity. Roth peaks of PSI2 0148-06 show the sane linear polarization of 58X at 611MRs, much higher than the 35% and 32% shown at 156OkRIs. For PRR 1831-04, again the polarieation is stronger at the lower than at the higher frequency. On the other hand, for PRR1356-60, the polarization at 156OMIis (84%) is higher than at the lower frequency of 95OMRs. In the case of PSR 1055-52, the situation is that the linear polarieation is about the same at 156ORRs and 611HRe; for the main pulse, it is respectively 82X and 90%; for intermediate pulse, 35% and 32%. Local variations are present in the mean pulse profile in high and low frequencies, showing thet the different components have rather different spectral indices. For the double peak pulsar P8IZOl48-06, the intensity ratio between the two peaks is 1.6 at 1560MII~ and 1.3 at 611MRs, showing that the leading component has a flatter spectrum
430
WU Xin-ji et al.
than the following component. For PSR1055-52, the main pulse has a three-peak structure. The intensity ratios of the three peaks are 2.3:1.0:1.2 at 1560MHe and 1.3:l.O:l.l at 63lMHz. This shows that the leading peak has a much flatter spectrum the follwoing two peaks. PSR 1831-04 has a complex multi-peak structure. At 611 MHz, it shows at least 5 components, with intensities (relative to the middle on) of 0.90 : 0.74 : 1.00 : 0.40 : 0.60. At 1560MHe, the mean pulse also shows a multi-peak structure, with peaks nos. 1, 3, 4, 5 the clearest. The intensity ratios can be estimated to be 1.75 : 1.13 : 1.00 : 0.90 : 1.13. It is very clear that the central component has the steepest spectrum while those at the wings have the flattest.
ACKNOWLEDGEMENTOne of the authors (WU Xin-ji) thanks the Institute of Radiophysics, CSIRO, Australia, the Parkes National Radio Astronomy Observatory and Professor QU Qin-yue for valuable support and help.
REFERENCES Jones, P.B., Astrophys. J., 236 (1980), 661. [ll r21 Narayan, R. and Vivwkanand, M., Astron. Astrophys.
122 (1983), 45. Wu X.J. and Shen, Z.Q., Chinese Science Bulletin, 34 (1989), 131 1116. Narayan, R. and Vivekanand, M., Astron. Astrophys. 113 (1982), 141 L3. Lyne, A.G. and Manchester, R.N., Mon. Not. R. Astron. Sot. 234 [51 (1988), 477. Gusejnov, O.K. and Yusifov, I.M., Sov. Astron. Sot., 29 (1985), [61 136. Wu, X.J. and Xu Wen, in The Magnetospheric Structure and [71 Emission Mechanisms of Radio Pulsar, 1990, IAU Cello. No. 128 Wu X.J., Qiao, G.J., Xia, X.Y. and Li F., Astrophys. Space Sci. [81 119 (1986), 101. Candy, B.N. and Blair, D.G., Astrophys. J. 307 (1986), 535. 191 [lOI Wu X.J., Deng, G.X. and Chen Hao, in The Origin and Evolution of Neutron Star, 1987, IAU Symp. No. 125, ~60. Ashworth, M., Mon. Not. R. Astron. Sot., 230 (1988), 87. Ill3 r121 Wu X.J. and Manchester, R.N., in The Magnetospheric Structure and Emission Mechanisms of Radio Pulsar, 1990, IAU Collo. No. 128. [I31 Taylor, H.J. and Stineherg, D.R., Annu. Rev. Astron. Astrophys. 24 (1986), 285. [I41 Backes, D.C., in Pulsars, 1980, IAU Symp. No. 95, Ed. W. Sieker and R. Wielebinski, D. Reidel, p.163. r151 Rankin, M.R., Astrophys. J., 274 (1983). 333. [I61 Hamilton, P.A., McCulloch, P.M., Ables, J.G. and Komesaroff, M.M., Mon. Not. R. Astron. Sot., 180 (1977). 1. 1171 McCulloch, P.M., Hamilton, P.A., Manchester, R.N. and Ables, J.G., Mon. Not. R. Astron. Sot., 183 (1978), 645. [ISI Manchester, R.N., Hamilton, P.A. and McCulloch, P.M., Mon. Not. R. Astron. Sot., 192 (1980), 153.
0 Pergaaon Press plc Printed in Great Britain 02751062/91$10.00+.00
~~I~TI~ O~~VATI~S OF FWR SOUTHERN PULSARSAT 2560 MHz* Xin_ji'vz~3 R. N. Manchester3 A. G. Lyne’ ‘Centre of Astronomy and Astrophysics, CCAST (World Laboratory] ‘Department of Geophysics, Peking University 3Division of Radiophysics, CSIRO, Australia ‘NRAL. University of Manchester, Jodreli Bank, UK
w
ABSTRACT We present some interesting results from the mean pulse polarization observations of four southern pulsars made at the Australian National Radio Astronozty Observatory, Parkes, using the 64-s telescope in June and July, 1988. The 2 x 16 x 5 MHz filter system fros Jodrell Bank has proved excellent in de-dispersing the pulse signals and measuring their polarization properties. We give the data for the four pulsars in some detail and discuss their spectral behaviour.
Key words:
Pulsars-polarization
1. INTRODUCTION Polarization of mean pulse of pulsars is one of the basic topics in pulsar observational study. The Bean pulse often has a strong linear polarization with stable characteristics. This means that the sagnetic field of a pulsar is also very stable. Measurement of the polarization properties is particularly important in the study of polar cap models. Such studies as the question whether the radiating cone has an elliptical section [l-31, the sagnetic inclination (4-71, the evaluation of the position paraseters Q and /I~ where the sightline sweeps past the radiating cone 15, 81, the evolution of the pulsars [7, 91, the widths of drifting pulses and subpulses [lo, Ill, and the derivation of accurate formula for radio lusinoaity [8, 121 become sore than superficial only after adopting some polarization parameters. For some pulsars, the position angle of linear polarization shows eudden 90° jumps as the pulsar longitude changes, or the intensity of circular polarization show sudden changes in the direction of polarization at certain longitudes. For some pulsars, strong de-polarization is present in certain parts of their mean pulse profile. Observations of two orthogonal polarization modes and of the phenomenon of de-polarization and the study of the mechanisms have anounted to 1 Progras supported by National Educational Commission Astronomical Section Plan for Observations Abroad and the National Natural Science Foundation. Original version received 1990 May 15; passed by Referee November 14; revised version received 1991 March 5.
424
WU Xin-ji et al.
topics of great concern 113, 141. Multi-frequency observation not only provides spectral characteristics of the pulsar emission, it also provides information on the three-dimensional structure of the radiating cone. This is because according to the available theoretical models different frequencies are emitted in different locations, the higher frequency regions are closer to the surface of the neutron star than the lower frequency regions. Therefore, the separations between the components in the mean pulse profile, the polarization properties, the frequency dependence of the spectrur etc. have always been important topics for the statistical and theoretical studies. Rankin (1983, [15]) analysed the multi-frequency properties of intensity and polarization of mean pulse for a set of pulsars and arrived at the conclusion that the central portion (core) of the radiating cone has strong emission and so changed the traditional view of hollow cone; this is one of the important results in the observational and statistical studies of pulsars over the last ten years. In view of the importance of the polarization properties of pulsars in our knowledge of their emission properties and theoretical models, recent years have seen great strides being made in polarization observation in the northern hemisphere [5J. In the southern hemisphere, however, such observation has been at a halt and there was no such observation between 1977 and the present one reported here. Of the 155 pulsars discovered in the second Molonglo survey, only a few have polarization aeasured. Because of its geographical advantage and of the excellent work of its astronomers, Australia occupies a special position in the observational study of pulsars. Of the pulsars discovered, one half are contributed by Australia. In polarization observation, results at 400, 630 and 1612 MHz were successively published in 1977, 1978 and 1980 [IS-181. There were 18 pulsars at 400 MHz, 43 at 630 MHs and 38 at 1612 MHz. Of these, 9, 23 and 22, repectively are south of latitude -30°. They belong to 25 different pulsars. The resson why the southern polarization observation lagged behind is that the 64-B Parkes radio telescope had only a single channel receiver system, had no de-dispersing capability and could observe only the stronger pulsars. For our present observation we used a newly and successfully designed de-dispersing filter system, which raised greatly the sensitivity. We obtained polarization data on 47 sourthern pulsars, which is the largest s-pie obtained so far for the southern sky. Of the 47 pulsars, 17 were measured for the first time for polarization, and 21, for polarization at high frequency. In this paper we present the results of four representatives with first-time high frequency polarization data,
2. ANALYSIS AND TREATMENTOF OBSERVATIONS AND DATA Our observation was carried out on the Parkes 64-metre radio telescope of the National Radio Astronomy Observatory, Australia. The dates were 1988 June 1 and 2, July 14, 15 and 18. The Parkes polarimeter was as shown in the block diagram by Hamilton [16]. But the present observation differed from the previous ones in two respects. The pre-amplifier was cooled, so the systen noise temperature was lower, the equivalent flux when pointing at cool sky
FourSouthern Pulsars
425
was 66Jy. The second difference was that we used the new 2 x 16 x 5HRz muiti-channel fiiter system, recently successfully designed at Jodrell Rank. Each channel has a bandwidth of 5 M?Ie. The polsriueter h8s four outlets giving the Stokes parauetera U and V, the two components of the intensity IX, I,. The total bandwidth of each route is 80 MHz, evenly slloted to 16 filters. After detection, a multi-channel l-bit digital system samples the signal from esch filter over 1.07 l s intervals and writes the record on magnetic tape. Because we selected a high observing frequency (156O~s), the effect of dispersion by interstellar medium on pulses with a bandwidth of 5I4Hs is very small and allows the observation of pulsars with larger dispersions. The radio source 1934-63 (with known flux of 16Jy) was used for C8libr8ting the Because of differing periods, the comparison source of the receiver. number of recorded d8t8 points within a period is different for different pulsars, so 8 standardized treatsent is needed. For the longer-period pulsars, the data within each period are grouped into 256 groups; for the shorter-period pulsars, we take the actual number of sample points, when that number resulting fros the 1.07 ss scupling is less than 256. A key step in the data analysis is the period folding of the output of each of the 64 channels to yield high S/N ratio mean einglechannel results. Fig. la gives the actual exaaple of PSR 1356-60. Channel numbers l-16 refer to the parameter IX, 17-32, 1,. 33-48 to the parameter U, and 49-64, the paraueter V. Because of the effect of dispersion by interstellar sedium, the pulse arrives at different times at different frequencies. Take IX 8s example, Channel 16 has a central frequency of 1597.5MRs while Channel 1, one of 1522.5MHa. The high frequency pulse arrives first, the low frequency pulse arrives later , so we find a regular time lag. From the data of Pig. 1 we can ueaeure the dispersion measure DM. The time lag At over different frequencies caused dispersion in the interstellar space is AC -
DM
1.2 x
IO_‘v’
Av
(1)
At is in ss, o and Au are in MHz and DM is in pc/cu3. The difference in frequency between two neighbouring channels is Av = 5MRz and it is easy to calculate the lag of the first 15 of the 16 channels relative to the last. The process of de-dispersing ia very simple: we only have to add to the data after period folding for each channel the appropriate time lag and then add together the results for the 16 channels. Fig. lb shows the results for the four Stokes pammeters I (=Ix t Iy), 8 = (Ix - Iy), U, V for PPSR 156-60, labelled I, 2, 3, 4, respectively. The final results are the total intensity I, the effective linear polarization L = (eZ t ti)“‘, the effective circular polarization V (positive for left, negative for right circular polarization) and the position angle of the Linear polarization IP= (1/2)arctan (U/O). Figs.2~5 give the variation with longitude of the four pammeters for the four pulsars, the solid line is I, the dashed line is L, the dotted line is V and the curve in the upper part of the diagrau is u. The error box given in the left hand bottom
426
WU Xin-ji et al.
0’ pdse
Fig.
phase
g 0.2
I 0.4
0.6 Pulse phase
’
0.8
-
1 a) Results after period folding of the four parameters IX (Channels l-161, I, (17-321, II (33-46), Y (47-64) of PSR 1356-60. b) Mean results of the four Stokes parameters I, Q, (I, V of PSR 1356-60.
corner represent, in the vertical direction, the uncertainty (20) in the total intensity caused by random noise and, in the horizontal direction, the effect of time lag caused by interstellar dispersion. The error in the position angle of the linear polarization is caused by random noise and the error in the position of the antenna illuminator and is represented by the error bar. The effective linear polarization is defined to be always positive and it includes the effect of noise. To solve this problem we should subtract a value representing the noise level of L which can be equated with the mean L value outside the mean pulse. The value of L shown in the figure is the value after subtracting this noise level.
3. THE RESULTS Figs. 2-5 give the results of the first time high frequency polarization measurement of four pulsars. These include typical objects with single peak, double peak, and broad and complex multiple peak pulses as well as the relatively rare intermediate pulse. It is now broadly accepted that the pulsar radiating cone consists of three parts: a central part (core), a weak inner annulus and a strong outer annulus and the intensity distribution is not continuous over the cone annuli [16]. Because the sight line sweeps different parts of the cone, so we observe different profiles of the mean pulse. Very broad profiles are caused by small
Four Southern Pulsars
42-l
*‘..
0
50 Lcqitude(deg.)
Fig.
1
toirl
inrcnsity-
snd
Mean
pulse
poiiiion
*ngfe per)
profiler
100 LonKitude(deg.)
(lower
Fig.
curvesc
linear----.
circuirr
..-a..)
of linear
component
(up-
.f
*-.
3
angle
Mean
pulse
of Iincar
profiler
compoocor (Key,
and
posician
of PSR 1055-52
rce Fig.
2)
PSK 0148-06.
180
-
120 60 [ 150
100
M
L.
01 0
Fig.
angle
50
4
Mean
pulse
of linear
100
profiler
componcnr
(Key,
se< Fig.
-I .?
and
poririon
aI PSR
13S6-60.
2.)
0
Fig. angle
100
300
200 ~ngitud~deg.)
5
Mean of linear (Key,
pulrr
profiler
somponenc see Fig.
and of
2.)
poritian
PSR 11131-04.
428
WU Xin-ji et al.
magnetic inclinations, while the intermediate pulse may have been formed when the inclination in near to 90”, making it possible to observe emission from both polar regions of the neutron star. Another possibility of intermediate pulse is a very small magnetic inclination coupled with an extremely non-uniform intensity distribution over the radiating cone. 1. The Single
Peak Pulsar
PRR 1356-60
This is a pulsar with a very large dispersion (LBl= 295 pc/cm3). Thanks to the use of a powerful de-dispersing system, a good signal-to-noise was obtained. The mean pulse has a very strong linear polarization (84%). Roth the total intensity and the linear polarization are symmetrical single peaks and there is rather strong depolarization in the wings. The position angle of the linear polarization basically does not vary with the pulse longitude. 2. The Double Peak Pulsar
PRR 0148-06
The total intensity of the mean pulse and the linear polarization both show good double peaks and the position angle varies smoothly with the pulse longitude. The maximumgradient of the position angle occurs at the middle of the mean pulse, that is, between the two peaks. All these are in complete agreement with the predictions of the coronal cap model. The linear polarization is rather strong, but the circular polarization is weak. 3. The Complex Multiple
Peak Pulsar
P3R 1831-04
Its apparent beamwidth is very large, the half-power width is already lOgo. The profile of the mean pulse is rather complicated, showing many components. The linear polarization also shows a complex multi-peak structure. The mean linear polarization is rather low. 4. Pulsar
with an Intermediate
Pulse
P6R 1055-52
The observed results at high frequency (1560Mtlz) agree in many respects with those at low frequency. The main pulse and the intermediate pulse are separated from each other by 155”, but the radiation is still regarded as coming separately from two magnetic polar regions. One of the reasons is that the separation between the two pulse is independent of the frequency. If the radiation comes from the same region, then the separation at high frequency will be narrower than at low frequency. Another reason is that the intermediate pulse comes from a partial cone with a vanishing tail so that the separation is only 155”. This pulsar is a weak object, the mean flux density of the main pulse is only 2.5 mJy, and the intermediate pulse is only 2.4 qJy. We used data over 11675 cycles and a good signal-to-noise ratio was obtained after period folding and accumulation. The main pulse has a strong linear polarization, as large as 82X, particularly its first component which is almost 100% linearly polarized. Of the intermediate pulse, the first component is again strongly polarized, but the following components , so the mean polarization for this have very weak polarization
FourSouthern Pulsars
pulse is only 35%. Roth the intermediate circular polarization.
429
and main pulses
have clear
TABLE 1 lists the parameters of these four pulsars. The nean linear polarization (RLP) is defined as the ratio of the areas under the effective linear polarisation curve (Z,) and the total intensity curve (I), The mean circular polarisation (MCP) is similarly defined with the effective circular polarication curve (V) replacing L. A& is the equivalent apparent be-width, and A&o and A*lo are the be-widths at half power and l/l0 peak value, respectively. At is the time lag caused by dispersion by interstellar medium on a 5MRe wide pulse, and Z+,urse is the accumulated number of pulses.
TABLE 1 PRR
P fWf Z$ul*e l s p&ems
At 118 9l560 BJY MLP
0148-06 1465 25.1 1597 0.4 1055-52% 197 30.1 11675 0.45 1356-60’ 1831-04
126 290
295. 78.8
7813 8017
4.5 1.2
11.0 2.5 2.4 7.0 11.0
0.29 0.82 0.35 0.84 0.15
MCP 0.06 0.13 0.21 0.15 0.11
AI& 0
A%o
A@jo
15.0 30.2 37.4 16.2 23.5 30.4 21.6 18.9 45.9 17.2 15.5 28.1 63.3 109.2 136.4
’ First line-the main pulse; second line-the intermediate ’ Interstellar medium dispersion widened Acp
pulse.
4. DISCUSSION Here we give only part of the results of our polarization observation in 1988 of southern pulsars at Packes, Australia. For the four pulsars reported here, only low frequency observations were available before. Becauseof the power law spectrum, the high frequency flux is very small and is difficult to measure. Apart fro= P8R 1356-60 for which we cannot examine the variation of the beamwidth with frequency becauee of the large broadening by interstellar dispersion, all other three pulsars showed a decrease in the apparent beamwidth with increasing frequency. While of the power law form, the spectra are rather flat, with an index of only about 0.1. The variation with frequency of the linear polarization did not show any regularity. Roth peaks of PSI2 0148-06 show the sane linear polarization of 58X at 611MRs, much higher than the 35% and 32% shown at 156OkRIs. For PRR 1831-04, again the polarieation is stronger at the lower than at the higher frequency. On the other hand, for PRR1356-60, the polarization at 156OMIis (84%) is higher than at the lower frequency of 95OMRs. In the case of PSR 1055-52, the situation is that the linear polarieation is about the same at 156ORRs and 611HRe; for the main pulse, it is respectively 82X and 90%; for intermediate pulse, 35% and 32%. Local variations are present in the mean pulse profile in high and low frequencies, showing thet the different components have rather different spectral indices. For the double peak pulsar P8IZOl48-06, the intensity ratio between the two peaks is 1.6 at 1560MII~ and 1.3 at 611MRs, showing that the leading component has a flatter spectrum
430
WU Xin-ji et al.
than the following component. For PSR1055-52, the main pulse has a three-peak structure. The intensity ratios of the three peaks are 2.3:1.0:1.2 at 1560MHe and 1.3:l.O:l.l at 63lMHz. This shows that the leading peak has a much flatter spectrum the follwoing two peaks. PSR 1831-04 has a complex multi-peak structure. At 611 MHz, it shows at least 5 components, with intensities (relative to the middle on) of 0.90 : 0.74 : 1.00 : 0.40 : 0.60. At 1560MHe, the mean pulse also shows a multi-peak structure, with peaks nos. 1, 3, 4, 5 the clearest. The intensity ratios can be estimated to be 1.75 : 1.13 : 1.00 : 0.90 : 1.13. It is very clear that the central component has the steepest spectrum while those at the wings have the flattest.
ACKNOWLEDGEMENTOne of the authors (WU Xin-ji) thanks the Institute of Radiophysics, CSIRO, Australia, the Parkes National Radio Astronomy Observatory and Professor QU Qin-yue for valuable support and help.
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