Download Understanding Pulsars: White Dwarfs, Neutron Stars, and Black Holes - Prof. Gregory B. Tay and more Study notes Astronomy in PDF only on Docsity!
- White Dwarf If initial star mass < 8 MSun or so. (and remember: Maximum WD mass is 1.4 MSun , radius is about that of the Earth)
- Neutron Star If initial mass > 8 MSun and < 25 MSun.
- Black Hole If initial mass > 25 MSun.
Final States of a Star
Pulsars Discovery of LGM1 by Jocelyn Bell and Tony Hewish (Cambridge) in 1967. Nobel Prize to Hewish in 1974. Pulse periods observed from 0.001 sec to 10 seconds - DEMO Explanation: "beamed" radiation from rapidly spinning neutron star. Usually neutron stars are pulsars for 10^7 years after supernova.
The Crab Pulsar Neutron Stars
Leftover core from Type II supernova
- a tightly packed ball of neutrons. Diameter: 20 km only! Mass: 1.4 - 3(?) MSun Density: 10^14 g / cm^3! Surface gravity: 10^12 higher Escape velocity: 0.6c Rotation rate: few to many times per second!!! Magnetic field: 10^12 x Earth's! A neutron star over the^ Sandias?
An Isolated Neutron Star
T ~ 2 million K Size ~ 30 km
The Lighthouse model of a pulsar
Pulsars are incredibly accurate clocks!
Example: period of the first discovered "millisecond pulsar" is: P = 0.00155780644887275 sec It is slowing down at a rate of 1.051054 x 10 -19^ sec/sec The slowing-down rate is slowing down at a rate of: 0.98 x 10 -31^ /sec
Multi-wavelength observations of Pulsars
Pulsar Exotica Binary pulsars: two pulsars in orbit around each other. Einstein predicted that binary orbits should "decay", i.e. the masses would spiral in towards each other, losing energy through "gravitational radiation". Confirmed by binary pulsar. year Curve: prediction of decaying orbit. Points: measurements. Planets around pulsars: A pulsar was found in 1992 to have three planets! Masses about 3 MEarth, 1 MEarth, and 1 MMoon! Millisecond pulsars: periods of 1 to a few msec. Probably accreted matter from a binary companion that made it spin faster. Gamma-ray Bursts: some pulsars produce bursts of gamma-rays, called Soft Gamma-Ray Repeaters or SGRs
Time history of the 4 confirmed SGRs:
Woods & Thompson 2004
Soft Gamma-Ray Repeaters
X-ray image "Eiso ~ a few 1044 erg in gamma-rays Where does this energy come from?
- Accretion? No sign of a disk
- Rotation? Not enough energy available
- Magnetic fields? Yes Clicker Question:
What is our basic model for a pulsar?
A: a rotating white dwarf B: a rotating neutron star C: a rotating black hole D: an oscillating star
The Fossil Record is Marked by Mass Extinction
Events
Extinction Genus loss End Ordovician 60% End Devonian 57% End Permian 82% End Triassic 53% End Cretaceous 47% From Solé & Newman 2002 Effects of a nearby GRB on Earth Melott et al. 2004 Raphaeli 2001 B ~ 0.3 μG Gaensler et al 2005 Growth of the Radio Afterglow VLA 8.5 GHz Size at t+7 days 1016 cm (1000 AU) Velocity to t + 30 days ~ 0.8 c Decrease in vexp Image Evolution VLA 8.5 GHz E ~ 10^45 ergs One-sided (anisotropic) outflow Taylor et al 2005 Radio Light Curves bump (Gaensler et al. 2005; Gelfand et al. 2005)
From Cameron et al. 2005 Radio Afterglow has a Steep Spectrum ~ ν-0.6^ at t+7 days down to 220 MHz Flux > 1 Jy at early times and low frequencies. Adapted from Duncan and Thompson 1992 Clicker Question:
The energy source for the repeated
gamma-ray bursts (SGRs) from some
neutron stars is what?
A: fusion of hydrogen on the surface B: energy released by material accreting onto the surface. C: the result of reconfigurations of the strong magnetic fields D: changes in the rotation rate of the neutron star. Clicker Question:
What happens to a neutron star that acquires a
mass of more than 3 Msun?
A: It will split into two or more neutron stars B: It will explode and blow itself to bits C: It will collapse to form a black hole D: It will produce a type II supernova, leaving a single neutron star. NS Merger Model for short GRBs
Mean redshift ~ 0.25 for short hard bursts (SHB)
No supernova association expected
SHBs often found at outskirts of galaxy (implies large
peculiar velocities)
SHBs found in
- Elliptical galaxies
- galaxies with low star
formation rates
Neutron Star merger
