The Diversity of Young Neutron Stars and Unification Prospects Joanna Bridge


НазваThe Diversity of Young Neutron Stars and Unification Prospects Joanna Bridge
Дата конвертації20.05.2013
Розмір500 b.
ТипПрезентации


The Diversity of Young Neutron Stars and Unification Prospects

  • Joanna Bridge

  • Astro 550

  • October 5, 2012


Roadmap

  • Diversity of Neutron Stars

    • Young Radio Pulsars (PSRs)
    • Millisecond Pulsars (MSPs)
    • Pulsar Wind Nebulae (PWN)
    • Rotating Radio Transients (RRATs)
    • Isolated Neutron Stars (INSs)
    • Magnetars
    • X-Ray Bright Compact Central Objects (CCOs)
  • Unification Prospects



The “classic” pulsars are called young radio pulsars



The discovery of pulsars was serendipitous (as much science is…)

  • 1967 - Jocelyn Bell and Anthony Hewish

  • discovered PSR B1919+21 (P = 1.337 s)



Neutron stars have strong magnetic fields

  • Neutron stars have strong magnetic fields

  • - Likely originated with progenitor, then compressed and concentrated (Seward and Charles 2010)

  • The strong magnetic field of a neutron star can be approximated as a dipole



General Characteristics:

  • General Characteristics:

  • Period - 1 ms to 8 s

  • B Field - 108 to 9 x 1013 G

  • A subset of pulsars rotate extremely rapidly

  • Millisecond pulsars (MSPs) have P < 20 ms and

  • B < 1010 G

  • MSPs are though to exist in binaries where the neutron star has been spun-up by accretion



MSPs are powered by accretion



The Crab pulsar was the first undisputed discovery of a neutron star formed from a supernova





The Crab pulsar wind nebula (PWN) is prototypical

  • Pulsar wind nebula are sometimes called “plerion” from the Greek “pleres”, meaning “full”



PWNe come in all shapes

  • Outstanding questions:

  • Does the wind consist of only e+/e- pairs or ions as well?

  • What is the fraction of energy in the B fields versus the particles?



Radio pulsars are encompassed in a bigger category of rotation-powered pulsars (RPPs)

  • The term “radio pulsar” is really a misnomer since some are radio-quiet

  • Powered due to the loss of rotational energy from magnetic field braking

  • Example of radio-quiet

  • pulsar: Geminga



Geminga was an unknown gamma-ray source for many years

  • 1991 - ROSAT observed an x-ray periodicity of P = 0.237 s

  • Likely it is an NS with unfortunate geometry for observation



RPPs span much of the P-P phase space



Rotating radio transient are…transient

  • 11 RRATs were discovered in the Parkes radio survey (McLaughlin et al. 2006)

  • - denoted by flashes every 2 to 30 ms

  • - then silent for 4 min to 3 hours

  • Follow-up observations show underlying period of 0.4 to 7 s

  • Large spin-down rate and high B fields (5 x 1013 G)



Rotating radio transients have no obvious periodicities

  • Since pulsars are largely discovered by their dependable periodicity, it is difficult to find a “malfunctioning lighthouse”

  • It was not until single pulses were searched for that RRATs were found

  • Since RRATs are more difficult to find than PSRs, it is likely they are much more numerous - studies put the ratio at 1 PSR for every 4 RRATs (McLaughlin 2006)



Rotating radio transients are…transient



Rotating radio transients are…transient



The signal arrival bursts must be de-dispersed



Why are RRATs so transient?



Isolated neutron stars have no nebula or SN remnant

  • The “magnificent seven” INSs were discovered in 1996



INSs are used to help to constrain the equation of state of super dense matter

  • INS soft x-ray spectra (due to cooling) can be fit as a blackbody!

  • -they are unmarred by SNR or magnetospheric activity

  • From there, can determine temperature, and therefore constrain radius and mass



Neutron star-like objects that don’t fall into other categories are referred to as central compact objects

  • The prototypical CCO is Cas A

  • -no x-ray periodicity

  • -no associated nebulosity

  • -unusual x-ray spectrum

  • Other interesting CCOs show ages older than their associated SNR

  • 1E 161348-5055 has no counterpart, no periodicity, just randomly decides to show variability every few years - who knows what it is…



Then there are the magnetars, the “drama queens”

  • At their brightest, magnetars “can outshine all other cosmic soft-gamma-ray sources combined”

  • (Kaspi 2010, Hurley 2005)



Can such a diverse set of objects be unified?

  • A theory of magnetothermal evolution has been proposed to tie the disparate neutron stars together (Pons et al. 2009)

  • There is a correlation between B field strength and surface temperature

  • -thermal evolution and B field decay are linked

  • 1.) Temperature affects crustal electrical resistivity

  • 2.) Resistivity affects B field evolution

  • 3.) Decay of B field produces heat, affecting temperature evolution



Coupling of temperature and B field may explain many objects’ field strengths

  • An object with a larger birth B field strength has significant field decay, thus stays hotter longer - this is observed with magnetars

  • The high-B field INSs can be explained by the fact the “highest B sources remain hottest, hence most easily detected, longest” (Kaspi 2010)

  • RPPs, INSs and magnetars are explained if the mean birth B field is 1013.25 G - higher than previously found



Tying them all together…

  • Therefore, the different characteristics of INSs, RPPs and magnetars are a result of different birth B fields and present ages

  • Further, RRATs are an extreme form of RPPs with low intrinsic brightness, and MSPs are recycled RPPs, as previously thought



Summary

  • Neutron stars come in many different shapes and sizes

  • Ideas have been put forward to unify the theory behind the formation and emission mechanisms of these objects

  • There are still many unknowns - many of these objects have only recently been found!



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