The Diversity of Young Neutron Stars and Unification Prospects

1. The Diversity of Young Neutron Stars and Unification Prospects Joanna Bridge Astro 550 October 5, 2012

2. 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

3. The “classic” pulsars are called young radio pulsars (Credit: NASA)

4. The discovery of pulsars was serendipitous (as much science is…) 1967 - Jocelyn Bell and Anthony Hewish discovered PSR B1919+21 (P = 1.337 s) Although originally found in the radio, pulsars emit at all wavelengths Several thousand have been discovered Jocelyn Bell with the telescope used during the discovery of pulsars (Credit: Jocelyn Bell Burnell)

5. Radio pulsars are powered by their 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 (Credit: NASA)

6. Radio pulsars are powered by their magnetic fields 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

7. MSPs are powered by accretion

8. The Crab pulsar was the first undisputed discovery of a neutron star formed from a supernova (Credit: CXC/NASA/ESA) (Credit: HST/NASA)

9. (Abdo et al. 2010)

10. The Crab pulsar wind nebula (PWN) is prototypical Pulsar wind nebula are sometimes called “plerion” from the Greek “pleres”, meaning “full” PWN result from the interaction of the relativistic charged particles with the interstellar medium When particles cross the B field lines, synchrotron radiation is produced (Credit: Chandra/HST/NASA; NRAO/NSF)

11. 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? (Credit: Chandra/HST/NASA; NRAO/NSF)

12. 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 (Credit: XMM-Newton/NASA)

13. 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 Geminga stands for “Gemini gamma-ray source” as well as meaning “it’s not there” in a dialect of Milan (Credit: Fermi/NASA)

14. ˙ RPPs span much of the P-P phase space (Kaspi 2010)

15. 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)

16. 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)

17. Rotating radio transients are…transient (McLaughlin 2006)

18. Rotating radio transients are…transient (McLaughlin 2009)

19. The signal arrival bursts must be de-dispersed Using the value of the dispersion measure, the output from each channel can be summed to obtain the pulse (McLaughlin 2006)

20. Why are RRATs so transient? Other ideas: Asteroids Switching between magnetospheric states If the hard-to-find RRATs are so numerous, what does that mean for the neutron-star birthrate? Perhaps they are dying pulsars, slowly shutting off as they approach the death line?

21. Isolated neutron stars have no nebula or SN remnant The “magnificent seven” INSs were discovered in 1996 Characteristics: Quasi-thermal x-ray spectra Relatively close (<500 pc) Lack of radio counterpart Long spin periods (3-11 s) Preferentially higher B fields

22. 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 (Credit: LOFT/ESA)

23. 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… (Credit: CXC/NASA)

24. 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) Soft gamma repeaters (SGRs) Anomalous X-Ray sources (AXTs) (Credit: NASA)

25. 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

26. 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

27. 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

28. 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!