Pulsars and Radio Transients Scott Ransom National Radio Astronomy Observatory / University of Virginia TIARA Summer School on Radio Astronomy 2016
Radio Transients Non-thermal emission Emission types split at Tb ~ 1012 K Inverse Compton causes relativistic electrons in incoherent synchrotron to lose energy Field still in its infancy Slow, incoherent Timescales of days, weeks, months AGN variability, GRB afterglows, stellar flares Fast, coherent Timescales of milliseconds or less Pulsars, giant pulses, Fast Radio Bursts
Macquart et al. 2015, SKA Book arxiv:1501.07535
Fast Radio Bursts (FRBs) ~20 seen so far Milliseconds in duration Highly dispersed Unknown origins Most all from Parkes GBT, Arecibo, Molonglo Lorimer et al. 2007, Science Extremely high brightness temps Until recently, mostly found in archival data Definitely real, and likely extra-galactic
Dispersion Lower frequency radio waves are delayed with respect to higher frequency radio waves by the ionized interstellar medium t DM -2 High Freq Low Freq (DM = Dispersion Measure) Coherent Dedispersion exactly removes this effect, but is very computationally difficult
Spitler et al. 2016, Nature 10 additional bursts seen over 2 days and 3 scans with Arecibo Same DM in all bursts Highly variable spectral indices More since (GBT & Arecibo) Event is not cataclysmic Young neutron star origins?
The Discovery of Pulsars PhD student Jocelyn Bell and Prof. Antony Hewish Initially Little Green Men Hewish won Nobel Prize in 1974
What are their radio properties? Continuum sources Typically somewhat to highly linearly polarized Steep radio spectra (index of -1 to -3, typical obs freqs 0.3-3 GHz) Point sources Special ISM effects (freq dependent) Highly time variable Wide variety of timescales Very faint average flux density ~mjy
Confusion? None for pulsars! Pulsars separated via time (or spin frequency!) rather than spatially. Gain variations? Who cares?! Observations are continually on and off source. Large beam? Doesn't matter! Sub-arcsec positions come from pulsar timing. Timing solns for 33 Ter5 MSPs (VLA contours in green)
Fundamental Physics with Pulsars Gravitational wave detection (e.g. high precision timing) Physics at nuclear density (e.g. neutron star interiors) Strong-field gravity tests (e.g. binary pulsar dynamics) Also many others: Plasma physics (e.g. magnetospheres, pulsar eclipses) Astrophysics (e.g. stellar masses and evolution) Fluid dynamics (e.g. supernovae collapse) Magnetohydrodynamics (e.g. pulsar winds) Relativistic electrodynamics (e.g. pulsar magnetospheres) Atomic physics (e.g. NS atmospheres) Solid state physics (e.g. NS crust properties)
Basic Physical Information from Pulsars Rotating dipole magnet in a vacuum (I = 1045 g cm2): radiates energy and therefore spins-down 2 observables: period (P) and period-derivative (p-dot) Surface magnetic field strength (B) Spin-down luminosity (E-dot) Age (T) and Characteristic Age ( c) (braking index: n ~ 3)
P-Pdot Diagram Pulsar HertzsprungRussell Diagram HR Diagram: Temp (color) vs Luminosity P-Pdot Diagram Period vs Spindown rate
Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Old Low B
Crab Nebula SN1054AD Pulsar rotates 30 times per second! Anasazi Indian cave pictogram, Chaco Canyon, NM
The Crab is visible at all energies! Red = Radio Green = Optical Blue = X-ray
Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Pulsars move down and right across the diagram as they lose energy (assuming that the magnetic field doesn't change...) Old Low B
Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Normal (average B, slow spin) Old Low B
Science with normal pulsars Used to: study the unknown pulsar emission mechanism probe the interstellar medium (scattering, scintillation, rotation measures, electron distribution) Scintillation Walker et al 2008 Measure PSR distances (HI absorption) Drifting Sub-pulses Bhattacharyya et al 2007
Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Normal (average B, slow spin) Eventually they slow down so much that there is not enough spin to generate the electric fields which produce emission. Low B Their lifetimes are 10-100 Myrs. Old
Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Normal (average B, slow spin) Old Millisecond (low B, very fast, very old, very stable spin, best for basic physics tests) Low B
Millisecond Pulsars: via Recycling Supernova produces a neutron star Red Giant transfers matter to neutron star Alpar et al 1982 Radhakrishnan & Srinivasan 1984 Millisecond Pulsar emerges with a white dwarf companion Picture credits: Bill Saxton, NRAO/AUI/NSF
Pulsar Flavors Young High B Young (high B, fast spin, very energetic) Normal (average B, slow spin) ng! Old (low B, very fast, very old, very stable spin, best for basic physics tests) Re cy cli Millisecond Low B
The Primary Pulsar Telescopes Arecibo GBT Parkes Jodrell Bank
Millisecond Pulsars are Very Precise Clocks PSR J1737+0747 At 12:40PM PST February 17 2015: P = 4.570136528819804 ms +/- 0.000000000000001 ms The last digit changes by 1 every 2 minutes! This digit changes by 1 every ~4000 years! This extreme precision is what allows us to use pulsars as tools to do unique physics!
Science comes from pulsar timing
The Binary Pulsar: B1913+16 First binary pulsar discovered at Arecibo Observatory by Hulse and Taylor in 1974 NS-NS Binary Ppsr = 59.03 ms Porb = 7.752 hrs a sin(i)/c = 2.342 lt-s e = 0.6171 ω = 4.2 deg/yr Mc = 1.3874(7) M Mp = 1.4411(7) M
The Binary Pulsar: B1913+16 Three Relativistic Observables: ω, γ, Porb Indirect detection of Gravitational Radiation In 1993, Russell Hulse and Joseph Taylor were awarded the Nobel Prize for their work on PSR B1913+16! From Weisberg & Taylor, 2003
J1614-2230: Incredible Shapiro Delay Signal Full Shapiro Signal No General Relativity Mwd = 0.500(6) M Mpsr = 1.97(4) M! Inclination = 89.17(2) deg! Full Relativistic Solution Demorest et al. 2010, Nature, 467, 1081D see Ozel et al. 2010, ApJL, 724, 1990
Direct Gravitational Wave Detection (Pulsar Timing Array) Looking for nhz freq gravitational waves from super massive black hole binaries Need good MSPs: Significance scales with the number of MSPs being timed Must time 20+ pulsars for 10+ years at precision of ~100 nanosec! Bill Saxton (NRAO/AUI) For more information, see nanograv.org Australia Europe North America
Summary Radio transients and pulsars are producing very exciting science FRBs, constraining masses of neutron stars, GWs Computing and rise of excellent digital systems has allowed this new science Only know of a few percent of the pulsars in the Galaxy, and fast transients are largely unexplored the future will be great Upcoming telescopes will be spectacular: MeerKAT, FAST, SKA, ngvla?
Essential Radio Astronomy http://science.nrao.edu/opportunities/courses/era/