Colloquium, IIA, Bangalore, May 23, 2006
R. T. GangadharaIndian Institute of Astrophysics
Bangalore
PLAN• DISCOVERY OF PULSARS• PULSAR RADIO PROFILES• EMISSION MECHANISM
-Curvature Radiation• EMISSION HEIGHT:
-Aberration-Retardation-Polar cap current
DISCOVERY OF PULSARSThe first Pulsating Source of Radio (PSR) was discovered in August 1967 by Jocelyn Bell and Tony Hewish in Cambridge, England.
-Project set out to investigate interplanetary scintillation.
4Extremely regular pulses:Clock.
4Variable intensity:Interference or Scintillation.
4Appeared 4 minutes earliereach day:Therefore, celestial.
4Sinusoidal change of pulseperiod:Doppler shift due to EarthSun motion.
4Pulse duration ~ 0.02 sec:Source no larger than Earth.
PSR B1919+21Freq. = 81.5 MHzPeriod = 1.337 sec
PROPERTIES:-
NATURE OF PULSARSWhat are the options?
A pulsating compact object ? White Dwarf or Neutron Star.- resonant periodicity > 1 sec for white dwarfs. - resonant periodicity > 1 ms for neutron stars.- doubtful to account for observed pulse stability but plausible.
Orbiting object ? Binary Stars or Small Satellites.
- gravitational radiation losses would decay the orbit on time scale ~ 2 days !- tidal disruption of satellite severe.
Intelligent extra-terrestrial sources ? Little Green Men !- transmitter power at stellar distances.- more than one source found !!
~ 1024 W
Rotating compact object ? Neutron Star.- simple analysis gives Pmin» 3p Gr 1�2, similar numbers to oscillation.
So, White Dwarfs and Neutron Stars are the prime candidates …Notable discoveries were the Crab (33 ms) and Vela (89 ms) pulsars.White Dwarf models were quickly ruled out on break-up grounds.
Neutron star phenomenon looks to be the clear winner !Hewish et al. 1968, Nature, 217, 709
PULSAR SPIN PERIOD
Conservation of angular momentum: Ms Rs2 Ws = M R2 WEx: Sun
Period ~ 27 days, Angular velocity Ws ~ 2.7x10- 6radsec- 1
If R = 10 km, then W= Ws Rs R 2~ 104 radsec- 1
Periodofrotation P = 2 p W~ 0.6millisec
Observed range of Pulsar periods: 1.57 millisec to 5.1 sec
Radius Rs = 6.96x108 m
Animation of Crab pulsar spinning and emitting two beams of radiation
P
Period:P= 2 p�W
LIGHT HOUSE MODEL OF PULSAR
SOUNDS OF RADIO PULSARS
Light Curve
Inte
nsit
y
Single and Average pulses of PSR B0329+54Spin period = 0.71 sec
PULSE PHASE (bins]
Radio Freq. = 606 MHz
Pulse intensity fluctuations
PSR B0950+08Pulse-to-pulse fluctuations
Hankins & Cordes (1981)
Rankin (1983)PULSAR EMISSION BEAM IS CONAL
PULSAR EMISSION BEAM IS PATCHY Lyne & Manchester (1988)
Pulse profiles from the radio to gamma--rays
NUV profile is similar to optical (Kern et al. 2003).
-May be same emission mechanism.
Soft x-ray:-Shallow pulsations with a sine-like light curve.-Single broad maximum per pulse.
Thermal emission from the surface of NSWith non-uniform temp. distr.
Optical and NUV pulse str. Indicates non-thermalemission.
Supernova Explosion of a massive star leads to the formation of neutron star.
The Crab Nebula Pulsar•only 1000 years old!
•still inside its supernova remnant
•emits a pulsar wind and jets
•produces visible pulses –only young pulsars have enough energy to do this
•20-km in diameter and is spinning at 30 rps
Magnetic dipole radiation of rapidly rotating neutron star
Crab nebula is powered by magnetic dipole radiation emitted by a rapidly rotating and highly magnetized neutron star (Pacini 1967).
Rate of rotational energy loss : E = -d
dt 1
2 I W2 = - I WW
Power radiated by magnetic dipole : W =2
3 M¦2 W4
c3
Rate of change of angular velocity: W = - ik2 M2 sin2 a3 c3 I
y{ W3Braking index n is defined by W= k Wn
Pulsar age :
Pulsarmagneticfield: Bs = 3.2 ´ 1019 P P×
G
t = 1n - 1
PP
= P
2 P , for n = 3.
Pulsar Population
DNS binaries live here
Internal structure of Neutron star
Gangadhara, R. T. & Gupta, Y., 2001, ApJ, 555, 31
P = 0.71 s
How do they work?
(Sturrock 1971; Ruderman & Sutherland 1975)
V / c
Equation for a field line in the Cartesian coordinate system (x,y,z):
Equation for field line : r = re sin2 q
Magnetic field line
Field line constant : re
r® = x, y, z
= re sin3 q cosf , sin3 q sinf , sin2 q cosqc
(Gangadhara, R. T., 2005, ApJ, 628, 923)
g = 340, 390
PSR B0450-18
n = 318 MHz , p = 0.5489 s , a = 24° , b = 4°_____________________________________________________________Cone f ' Df ' q f rem r /rLC g P/P1No. (°) (°) (°) (°) (km)_____________________________________________________________
1 7.78±0.07 1.01±0.07 3.50±0.01 -43.98±0.30 236±17 0.20±0.00 284±0.37 1.00±0.01 _______________________________________________________________________________________
P1 = 1.13E-16 erg/s
rem r�rLC
Gupta, Y., & Gangadhara, R. T. 2003, ApJ, 584, 418
P1 = 1.13x10- 16erg�s
PSR B1237+25
n = 318 MHz , p = 1.3824 s , a = 53° , b = 0°____________________________________________________________Cone f ' Df ' q f rem r /rLC g P/P1No. (°) (°) (°) (°) (km)____________________________________________________________
1 1.84±0.07 0.28±0.07 0.98±0.05 89.45±0.02 161±40 0.19±0.01 382±5 1.00±0.092 3.70±0.05 0.72±0.05 1.97±0.03 88.89±0.02 414±29 0.24±0.00 415±2 0.85±0.033 6.04±0.04 0.93±0.04 3.21±0.02 88.18±0.01 535±23 0.19±0.00 384±1 0.99±0.02____________________________________________________________
P1 = 6.23 x10- 17erg�s
PSR B1237+25
n = 318 MHz , p = 1.3824 s , a = 53° , b = 0°____________________________________________________________Cone f ' Df ' q f rem r /rLC g P/P1No. (°) (°) (°) (°) (km)____________________________________________________________
1 1.84±0.07 0.28±0.07 0.98±0.05 89.45±0.02 161±40 0.19±0.01 382±5 1.00±0.092 3.70±0.05 0.72±0.05 1.97±0.03 88.89±0.02 414±29 0.24±0.00 415±2 0.85±0.033 6.04±0.04 0.93±0.04 3.21±0.02 88.18±0.01 535±23 0.19±0.00 384±1 0.99±0.02____________________________________________________________
P1 = 6.23 x10- 17erg�s
MILLISECOND PULSAR:PSR J0437-4715
Discovered in Parks by Johnston et al. (1993).
- Nearest and bright millisecond pulsar (~180 pc)
- Spin period 5.75 ms- Has a low mass white dwarf in binary with
orbital period 5.74 days
Data by Johnston and Manchester (2005) from Parks.
(Gil & Krawczyk 1997)
Phase
Sing
le p
ulse
s
CONCLUSION
® Duetogeometricrestrictions, observer receivesthecore inner emissionfrom lowerheightswhile theconalemissionfrom higherheights.
® Pulsar profiles become asymmetricdue toaberration,retardationandpolarcapcurrents.
® Radius - to - frequency mapping RFMcan be explained by considering curvatureradiation.
CONCLUSION
• Developed a method for estimating the absolute emission heights of core as well as conal components.
In millisecond pulsar: J0437-4715 (i) A/R phase shift is quite high (~20 deg).(ii) Core component is emitted from an altitude close
to NS surface (~20 Km).(iii) Emission altitude of cones increases from core
to outer most cone (~90 Km).
(Gil & Krawczyk 1997)PSR J0437-47151440 MHz
Km 48 r4
20
≈−=
=
β
α
lc tcoc
LCr/r2−≈′φδ
LCr/r2≈′φδ
GMRT RESULTS
• Millisecond Pulsar PSR J0514-4002 in globular cluster NGC 1851 - Rotational period of 4.99 ms- Orbital period of 18.8 days
• PSR J1833-1034 in supernova remnant G21.5-0.9 - Rotational period of 61.86 ms
Paulo et al., 2004, ApJ, 606L, 53
Gupta et al., 2005, Current Science, Submitted
Neutron Binary Stars
– Intense X-rays from neutron stars in binary systems There are several types of X-ray binaries
• X-ray bursters from gas falling on the neutron star
• X-ray pulsars from hot-spots on the neutron star• infalling gas can “spin up” an old neutron star
BINARY PULSARS
A Double Pulsar Binary System
Jodrell Bank Observatory, UKMultibeam detector Parks