Long-term radio observations of orbital phase wander in six eclipsing pulsar binaries. Brian Prager Department of Astronomy University of Virginia

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1 Long-term radio observations of orbital phase wander in six eclipsing pulsar binaries. Brian Prager Department of Astronomy University of Virginia Collaborators: Scott Ransom (NRAO), Phil Arras (UVa), Paulo Freire (MaxPlanck-Institut für Radioastronomie), Jason Hessels (ASTRON), Ingrid Stairs (University British Columbia) 1

2 Background Neutron Stars Evolution towards becoming a neutron star. Image Credit: Journal of the International Association of Physics Students 2

3 Background Pulsars Pulsars are an observational subset of neutron star where emission from the magnetic poles pass the line of sight to the Earth. Example pulse profile of a pulsar with a rotational period of approximately 1 second. Image Credit: Journal of the International Association of Physics Students 3

4 Pulsar Timing Timing Models Pulsar timing is designed to accurately predict the arrival time of every pulse. This process produces very precise measurements of various physical properties such as: Properties of PSR J as obtained through pulsar timing. (Demorest et al. 2010) Location & Proper Motion Spin-rate and its derivatives Binary parameters Free electron column density in the ISM 4

5 Pulsar Timing Millisecond Pulsars The most accurately timed pulsars are the millisecond pulsars (MSPs). They rotate approximately once every few milliseconds. Their rotational periods are stable to a part in ~

6 MSPs Formation Mechanism MSPs are formed when a companion star donates its angular momentum to a regular pulsar, spinning it up to rapid rotation rates. This evolutionary stage is short lived and therefore quite rarely discovered. In addition to this, the unbound gas in the system can cause eclipses that make discovering these systems even more difficult. 6

7 MSPs - Spider Pulsars Pulsars found in or around this evolutionary phase are dubbed 'Spiders' due to their 'eating' the companion star. Spider pulsars with longer orbital periods (P 4 hrs) and larger companion masses (M 0.08Mʘ) are called 'Redbacks'. 7

8 Redbacks Research Opportunities We are just beginning to discover larger numbers of spider pulsars, which is a boon to a wide variety of different fields of research. Studies of MSP formation. Pulsar eclipse mechanisms. Outflow from low-mass stars. Convection in low-mass stars. Magnetic activity in low-mass stars. 8

9 Redbacks Research Opportunities We are just beginning to discover larger numbers of spider pulsars, which is a boon to a wide variety of different fields of research. Studies of MSP formation. Pulsar eclipse mechanisms. Outflow from low-mass stars. Convection in low-mass stars. Magnetic activity in low-mass stars. 9

10 Magnetic Activity Our sample M28 NGC6440 Terzan 5 We have data going back 15 years for six redbacks within three different globular clusters. 10

11 Magnetic Activity Phase Wander Every redback studied to date shows stochastic changes in its reference orbital phase. Changes in the interior of the companion star are thought to be the cause of this effect. We can therefore relate changes in the orbital properties of a system to the internal properties of the low-mass companion. 11

12 Magnetic Activity Applegate Model The favored mechanism for explaining this effect is the Applegate model. The Applegate model couples the magnetic field in the lowmass star to its differentially rotating layers, producing torques on the orbit that are modulated over a magnetic cycle. We can calculate not only the strength of the magnetic field, but the length of the magnetic cycle. This is particularly useful as companion stars to Redbacks should be rapidly rotating. Spectroscopic studies of magnetic activity cannot probe rapid rotation rates due to rotational broadening dominating the spectra. 12

13 Magnetic Activity Applegate Model We tested the Applegate model by looking for two pieces of observational evidence: 1) The presence of a magnetic cycle. 2) The energy requirements for deforming the companion star are reasonable. 13

14 Applegate Model - Evidence We tested the Applegate model by looking for two pieces of observational evidence: 1) The presence of a magnetic cycle. 2) The energy requirements for deforming the companion star are reasonable. 14

15 Applegate Model - Evidence We used a Lomb-scargle periodogram to account for uneven sampling and to incorporate measurement errors to search for periodicity. We find that the orbital phase wander is closer to a red noise process than periodic. Any periodic signal must be Pcyc 30 years. 15

16 Applegate Model - Evidence At this point, it is possible that the magnetic cycle in these stars is simply much longer than our observed data span. The Sun is thought to undergo long magnetic cycles in addition to the well known 11 year cycle (eq. The Gleissberg Cycle) Studies of variable stars with active chromospheres have found some systems have decades long magnetic cycles. 16

17 Applegate Model - Evidence We tested the Applegate model by looking for two pieces of observational evidence: 1) The presence of a magnetic cycle. 2) The energy requirements for deforming the companion star are reasonable. 17

18 Applegate Model - Evidence 18

19 Applegate Model Future Work The Applegate model may be present in our data as a higher order effect. Correct identification of the Applegate model in pulsar timing can be used to expand the stellar activity-rotation relationship for rapidly rotating stars. Vr 19

20 Conclusions The recent discovery of numerous Redback pulsars will allow for new constraints to be placed on the process of MSP formation and the behavior of low-mass stars. Stochastic changes in the orbital properties of a Redback are related to the interior physics of low-mass stars. The Applegate model is not applicable to Redbacks at present. Future studies of pulsar timing will investigate additional constraints on the interior of low-mass stars. Questions? 20

21 Extra Slides Spider Table 21

22 Extra Slides Orbital Period Distribution 22

23 Extra Slides Magnetic Distribution 23

24 Extra Slides J2 24