# Licensed under a 3-clause BSD style license - see LICENSE.rst
"""Pulsar source models"""
from __future__ import absolute_import, division, print_function, unicode_literals
import numpy as np
from astropy.units import Quantity
from ...extern.validator import validate_physical_type
__all__ = [
'Pulsar',
'SimplePulsar',
]
DEFAULT_I = Quantity(1e45, 'g cm2')
"""Pulsar default moment of inertia"""
DEFAULT_R = Quantity(1e6, 'cm')
"""Pulsar default radius of the neutron star"""
B_CONST = Quantity(3.2e19, 'gauss s^(-1/2)')
"""Pulsar default magnetic field constant"""
[docs]class SimplePulsar(object):
"""Magnetic dipole spin-down model for a pulsar.
Reference: http://www.cv.nrao.edu/course/astr534/Pulsars.html
Parameters
----------
P : `~astropy.units.Quantity`
Rotation period (sec)
P_dot : `~astropy.units.Quantity`
Rotation period derivative (sec sec^-1)
I : `~astropy.units.Quantity`
Moment of inertia (g cm^2)
R : `~astropy.units.Quantity`
Radius of the pulsar (cm)
"""
def __init__(self, P, P_dot, I=DEFAULT_I, R=DEFAULT_R):
validate_physical_type('P', P, 'time')
validate_physical_type('P_dot', P_dot, 'dimensionless')
self.P = P
self.P_dot = P_dot
self.I = I
self.R = R
@property
def luminosity_spindown(self):
"""Spin-down luminosity (`~astropy.units.Quantity`).
Notes
-----
The spin-down luminosity is given by:
.. math::
\\dot{L} = 4\\pi^2 I \\frac{\\dot{P}}{P^{3}}
"""
return 4 * np.pi ** 2 * self.I * self.P_dot / self.P ** 3
@property
def tau(self):
"""Characteristic age (`~astropy.units.Quantity`).
Notes
-----
The characteristic age is given by:
.. math::
\\tau = \\frac{P}{2\\dot{P}}
"""
return (self.P / (2 * self.P_dot)).to('yr')
@property
def magnetic_field(self):
"""Magnetic field strength at the polar cap (`~astropy.units.Quantity`).
Notes
-----
The magnetic field is given by:
.. math::
B = 3.2\\cdot 10^{19} (P\\dot{P})^{1/2} [\\textnormal(Gauss)]
"""
return B_CONST * np.sqrt(self.P * self.P_dot)
[docs]class Pulsar(SimplePulsar):
"""Magnetic dipole spin-down pulsar model.
Reference: http://www.cv.nrao.edu/course/astr534/Pulsars.html
Parameters
----------
P_0 : float
Period at birth
logB : float
Logarithm of the magnetic field, which is constant
n : float
Spin-down braking index
I : float
Moment of inertia
R : float
Radius
"""
def __init__(self, P_0=Quantity(0.1, 's'), logB=10, n=3, I=DEFAULT_I,
R=DEFAULT_R, age=None, L_0=None, morphology='Delta2D'):
self.I = I
self.R = R
self.P_0 = P_0
self.logB = logB
self.P_dot_0 = (Quantity(10 ** logB, 'gauss') / B_CONST) ** 2 / P_0
self.tau_0 = P_0 / (2 * self.P_dot_0)
self.n = float(n)
self.beta = (n + 1.) / (n - 1.)
self.morphology = morphology
if age is not None:
validate_physical_type('age', age, 'time')
self.age = age
if L_0 is None:
self.L_0 = 4 * np.pi ** 2 * self.I * self.P_dot_0 / self.P_0 ** 3
[docs] def luminosity_tev(self, t=None, fraction=0.1):
"""Gamma-ray luminosity assumed to be a certain fraction of the spin-down luminosity.
Parameters
----------
t : `~astropy.units.Quantity`
Time after birth of the pulsar.
"""
if t is not None:
validate_physical_type('t', t, 'time')
elif hasattr(self, 'age'):
t = self.age
else:
raise ValueError('Need time variable or age attribute.')
return self.luminosity_spindown(t) * fraction
[docs] def luminosity_spindown(self, t=None):
"""Spin down luminosity at age t.
Parameters
----------
t : `~astropy.units.Quantity`
Time after birth of the pulsar.
Notes
-----
The spin-down luminosity is given by:
.. math::
\\dot{L}(t) = \\dot{L}_0 \\left(1 + \\frac{t}{\\tau_0}\\right)^{\\frac{n + 1}{n - 1}}
"""
if t is not None:
validate_physical_type('t', t, 'time')
elif hasattr(self, 'age'):
t = self.age
else:
raise ValueError('Need time variable or age attribute.')
return self.L_0 * (1 + (t / self.tau_0)) ** self.beta
[docs] def period(self, t=None):
"""Period at age t.
Parameters
----------
t : `~astropy.units.Quantity`
Time after birth of the pulsar.
Notes
-----
The period is given by:
.. math::
P(t) = P_0\\left(1 + \\frac{t}{\\tau_0}\\right)^{\\frac{1}{n - 1}}
"""
if t is not None:
validate_physical_type('t', t, 'time')
elif hasattr(self, 'age'):
t = self.age
else:
raise ValueError('Need time variable or age attribute.')
return self.P_0 * (1 + (t / self.tau_0)) ** self.beta
[docs] def energy_integrated(self, t=None):
"""Total released energy at age t.
Time-integrated spin-down luminosity since birth.
Parameters
----------
t : `~astropy.units.Quantity`
Time after birth of the pulsar.
Notes
-----
The time integrated energy is given by:
.. math::
E(t) = \\dot{L}_0 \\tau_0 \\frac{t}{t + \\tau_0}
"""
if t is not None:
validate_physical_type('t', t, 'time')
elif hasattr(self, 'age'):
t = self.age
else:
raise ValueError('Need time variable or age attribute.')
return self.L_0 * self.tau_0 * (t / (t + self.tau_0))
[docs] def period_dot(self, t=None):
"""Period derivative at age t.
P_dot for a given period and magnetic field B, assuming a dipole
spin-down.
Parameters
----------
t : `~astropy.units.Quantity`
Time after birth of the pulsar.
Notes
-----
The period derivative is given by:
.. math::
\\dot{P}(t) = \\frac{B^2}{3.2 \\cdot 10^{19} P(t)}
"""
if t is not None:
validate_physical_type('t', t, 'time')
elif hasattr(self, 'age'):
t = self.age
else:
raise ValueError('Need time variable or age attribute.')
return Quantity(10 ** self.logB, 'gauss') ** 2 / (self.period(t) * B_CONST ** 2)
[docs] def tau(self, t=None):
"""Characteristic age at real age t.
Parameters
----------
t : `~astropy.units.Quantity`
Time after birth of the pulsar.
Notes
-----
The characteristic age is given by:
.. math::
\\tau = \\frac{P}{2\\dot{P}}
"""
if t is not None:
validate_physical_type('t', t, 'time')
elif hasattr(self, 'age'):
t = self.age
else:
raise ValueError('Need time variable or age attribute.')
return self.period(t) / 2 * self.period_dot(t)
[docs] def magnetic_field(self, t=None):
"""Magnetic field strength at the polar cap. Assumed to be constant.
Notes
-----
The magnetic field is given by:
.. math::
B = 3.2\\cdot 10^{19} (P\\dot{P})^{1/2} [\\textnormal(Gauss)]
"""
if t is not None:
validate_physical_type('t', t, 'time')
elif hasattr(self, 'age'):
t = self.age
else:
raise ValueError('Need time variable or age attribute.')
return B_CONST * np.sqrt(self.period(t) * self.period_dot(t))