A Debris Disk Around an Isolated Young Neutron Star

1. A Debris Disk Around an Isolated Young Neutron Star Deepto Chakrabarty Zhongxiang Wang David L. Kaplan Kavli Institute for Astrophysics and Space Research Massachusetts Institute of Technology (MIT) References: Wang, Chakrabarty, & Kaplan (2006), Nature, 440, 772 Wang, Kaplan, & Chakrabarty (2006), ApJ, submitted (astro-ph/0606686)

2. Pulsars and Neutron Stars • Pulsars: rotating, highly magnetized neutron stars • Formed in core-collapse supernova explosions from massive stellar progenitors (8-20 solar masses). Neutron star birth mass ~1.4 solar masses. • “Typical” birth spin period of ~15-30 ms, surface magnetic field B~1012 G (BUT: magnetars) • Spin down due to magnetic dipole radiation over 107-108 years • Estimate magnetic field strength of rotation-powered pulsars using magnetic dipole emission model, B • Estimate spin-down age from P and dP/dt. • Nearly 2000 radio (rotation-powered) pulsars known • If member of binary: classical accretion-powered X-ray pulsars (0.1-1000 s), over 50 known

3. Life History of Pulsars: Spin and Magnetic Evolution Pulsars born with B~1012 G, P~20 ms. Spin-down due to radiative loss of rotational KE If in binary, then companion may eventually fill Roche lobe. Accretion spins up pulsar to equilibrium spin period Sustained accretion (~109 yr) attenuates pulsar magnetic field to B~108 G, leading to equilibrium spin P~few ms (Not directly observed yet!) 4. At end of accretion phase (companion exhausted or binary disrupted), millisecond radio pulsar remains (magnetars) 1 2 3 4

4. Overview of Young Neutron Stars • “Crab-like” pulsars (~25 known) • Strong non-thermal emission from gamma-ray to infrared • Synchrotron (pulsar wind) nebula (“plerion”) • Supernova remnants • Usually radio pulsars • Classical “prototype” for young neutron stars • Radio-quiet central compact objects in supernova remnants (7 known) • Young compact objects in supernova remnants. Some are X-ray pulsars. • Soft thermal X-ray spectrum • No detection of radio pulsations, wind nebula, or any non-thermal emission • “Magnetars” (~12 known) • Anomalous X-ray pulsars and soft gamma repeaters • Slow spin periods, evidence for extremely strong magnetic fields.

5. Young neutron stars: “Crab-like” pulsars • ~25 known • Spin periods 10-100 ms • Surface magnetic field of order 1012 G. • Generally associated with supernova remnants, confirming young age • Strong non-thermal emission from pulsar in gamma-ray to infrared bands • Synchrotron (pulsar wind) nebula • Usually radio pulsars • Classical “prototype” for young neutron stars

6. Young Neutron Stars: Radio-Quiet Neutron Stars in Supernova Remnants • 7 known • Young compact objects in supernova remnants. Some are X-ray pulsars. • Soft thermal X-ray spectrum • No detection of radio pulsations, wind nebula, or any non-thermal emission • Some have known spins of order 100 ms. • Magnetic field strengths unknown

7. Anomalous X-Ray Pulsars (Magnetars?) • Narrow range of spin periods (5-12 s). Spinning down. Glitches. Young timing age. • At least one associated with a supernova remnant. • Soft X-ray spectra, very distinct from accretion-powered pulsars. Low luminosity. • X-ray luminosity greatly exceeds the spin-down energy loss. • No evidence for binary companions (X-ray timing, counterparts). Extremely low mass limits. • Optical/IR counterparts. Bursts (like SGRs). • High inferred magnetic field from spin parameters (not a unique indicator of AXP properties) • Magnetar versus isolated disk accretion? Standard accretion disk evidently ruled out.

8. Fallback in Supernovae • Fallback(Colgate 1971). Some ejecta from supernova explosion may remain bound and fall back onto the compact stellar remnant. This could affect SN explosion process, or could even lead to the formation of a black hole (Chevalier 1989). • Fallback disk(Michel & Dessler 1981; Chevalier 1989). If the fallback material has sufficient specific angular momentum, • l > (GMNSRNS)1/2 ~ 1016 cm2 s-1 • then it can form a disk. Modeling of rotating pre-supernova massive stars suggest that l ~ 1017 cm2 s-1 is possible (Heger et al. 2000), indicating that fallback disks are possible. • Initial survival of fallback disks (considered in context of accretion models for gamma-ray bursts by Narayan et al. 2001): Initially high mass accretion rate (1 Msun/s) as neutrino-cooling dominates. Rate will eventually drop below a critical value where photon cooling dominates and radiation pressure halts steady accretion. Mass remaining in disk highly uncertain. • Later survival of fallback disks(Eksi, Hernquist, & Narayan 2005): propeller regime (even if super-Eddington)

9. Allowed Regions for an Accretion Disk Around a 1012 G Neutron Star Eksi, Hernquist, & Narayan 2005

10. Possible “Applications” of Fallback Disks • Anomalous X-ray pulsars(Chatterjee et al. 2000; Alpar 2001; Marsden et al. 2001; Eksi & Alpar 2003) Suggestions that X-ray emission from these objects powered by fallback accretion disk • Soft gamma repeaters (Ertan & Alpar 2003), compact central objects in SNRs(Xu et al. 2003; Shi & Xu 2003) Suggestions that X-ray emission from these objects powered by fallback accretion disk • Radio pulsars (Marsden et al. 2001; Menou et al. 2001; Alpar et al. 2001; Qiao et al. 2003) Fallback disks would have consequences for radio pulsars, including: discrepancy betweeen timing and supernova ages (Marsden et al. 2001), braking indices (Menou et al. 2001), distribution of spins and spin derivatives (Alpar et al. 2001), timing variability (Qiao et al. 2003). • Jets from young pulsars in Crab and Vela supernova remnants might be collimated by fallback disks (Blackman & Perna 2004) • SN 1987A lightcurve deviations from pure radioactive decay might be explained by fallback disk accretion (Meyer-Hofmeister 1992; Mineshinge et al. 1993). • Planets around pulsars may be formed from fallback disks (Lin et al. 1991; Podsiadlowski 1993) Observational Constraints on Fallback Disks • Interestingly, there are no significant observational constraints on the existence of fallback disks. Searches for protoplanetary disks around pulsars have not concentrated on young systems. • Prediction(e.g., Perna, Hernquist & Narayan 2000): compare to NS/LMXB disk without binary companion to truncate outer edge. Due to central irradiation, expect strong thermal IR/millimeter excess from outer disk.

11. Our Program: Search for Fallback Disks Around Young Neutron Stars • Begun in 2002. • Select young NSs without optical/IR contamination from non-thermal emission • Exclude Crab-like pulsars with strong non-thermal optical/IR emission • 5 compact central objects in supernova remnants: Cas A, PKS 1209-52, Pup A, RCW 103, RX J0852-46 • 1 anomalous X-ray pulsar: 4U 0142+61 (the brightest AXP; known opt/IR cpts.) • Deep optical and near-IR imaging from Magellan 6.5-m telescopes at Las Campanas. • Deep near-IR imaging from CTIO 4-m • Deep mid-IR imaging from Spitzer Space Telescope Results of Survey • Non-detection (upper limits) in 5 compact central objects. Spitzer images for 1 of these saturated by strong diffuse background. • 1 detection of a mid-infrared counterpart, in an anomalous X-ray pulsar!

12. Properties of the Anomalous X-Ray Pulsar 4U 0142+61 • Brightest known AXP. • Spin period P=8.7 s. • X-ray pulsed fraction ~few percent. • Spin-down age ~105 yr. • X-ray luminosity ~1035 erg/s. X-ray spectrum: blackbody + power law • Spin-down luminosity ~1033 erg/s • Stringent timing limits essentially rule out a stellar binary companion. • Faint optical counterpart. Optical emission pulsed (27% pulsed fraction) in phase with X-rays. • Faint near-IR counterpart. FK/FX ~ 10-4 • Interstellar reddening AV=3.5 ± 0.4 (Durant & van Kerkwijk 2006a) • Distance 3.6 (± 0.4) kpc (Durant & van Kerkwijk 2006b)

13. Spitzer and Keck IR Imaging of 4U 0142+61 F8.0 = 52(5) Jy F4.5 = 36(4) Jy K=20 • Based on density of sources in 4.5 m image, chance coincidence probability is 0.2%. No other objects detected within 4 arcsec of source in deep K image, down to K=22 (1.1 Jy). • For IR color-color diagram, we used 33 field stars from 2MASS point source catalog. Also shown main sequence and giant star models from synthetic spectra, appropriately reddened. • Object has extremely unusual colors. No amount of reddening can explain this. K counterparts of all other detected IRAC objects are brighter than our target. • We conclude that this is the mid-IR counterpart of 4U 0142+61.

14. new data just in! Optical/IR Spectral Energy Distribution of 4U 0142+61 • Optical and infrared emission evidently come from different components (less obvious but still likely for lower reddening case) • Optical component consistent with a power-law spectrum, pulsed at spin period (large pulsed fraction), probably arises in magnetosphere (F. Özel, in prep.) • A single temperature blackbody gives a mediocre fit, with T=920 K and R=5 R. Too large given companion limits. • Multi-temperature (700K-1200K) thermal model fits the data better, and naturally suggests a disk or shell origin. • We reject a shell geometry since it would produce ~1000 the observed X-ray and optical absorption. • We note that an accretion disk powering the X-ray source has already been ruled out, since it would have high optical flux due to X-ray heating of the inner disk (Hulleman, van Kerkwijk, & Kulkarni 2000).

15. X-Ray Heated Accretion Disk Model Overpredicts Spectrum

16. Interpreting the Spectrum as a Passive Illuminated Disk of Dust and Gas • Assume a passive disk illuminated by magnetar emission from X-ray pulsar. • At these low temperatures, continuum emission entirely from dust. (Similar to dusty protostellar disks.) • We presume that there is also gas present, and that gas pressure support establishes hydrostatic equilibrium, yielding a slightly flared disk irradiated by the central point source. (Note difference from protostellar case: NS is a point source.) • Note that 4 of the 8 AXPs have near-IR counterparts, all with FK/FX ~ 10-4. Also, in 2 cases, there is correlated X-ray/IR variability. Supports X-ray heating origin. (Optical variability poorly constrained, but pulse fraction excludes X-ray heating.)

17. Modeling and Fit Parameters for the Disk • We used the X-ray heated disk model of Vrtilek et al. (1990) developed for low-mass X-ray binaries. Disk thickness H ~ R9/7, and temperature at radius R is • Integrate temperature profile over disk surface. We fixed cos i=0.5 and d=3.9 kpc, and considered high X-ray albedo values ( >0.9), as indicated by studies of X-ray reprocessing in LMXBs (de Jong et al. 1996). • Best fit: rin= 2.9 R, rout= 9.7 R,  =0.97. For comparison, note that rco= 0.01 R, rLC= 0.61 R. Inner and outer disk radii sensitive to choice of  • Inner disk temperature well constrained to 1200 K by shape of near-IR spectrum, comparable to dust sublimation temperature (1000-2000 K). Inner boundary of dust disk may be set by sublimation. Disk gas possible inside this radius, cooling by line emission. • Outer disk radius inferred from frequency below which spectrum turns over to Rayleigh-Jeans spectrum. Poorly constrained by our data. Observations in far-IR or millimeter necessary.

18. Estimating the Mass and Lifetime of the Disk • Dust emission in far-IR and millimeter bands is typically optically thin, allowing for direct measure of the disk mass (Beckwith et al. 1990). If we assume that the 1 mm flux is no higher than the 8 micron flux, then we can set an upper limit on the disk mass assuming a dust-to-gas ratio of 1 percent. Note that the appropriate opacity is highly uncertain (ranges between 0.002 and 0.1 in ordinary protostellar disks). Opacity, dust-to-gas ratio might be very different for high metallicity in supernova-processed material. • Extrapolating spectral fit to 1 mm reduces flux limit by 2 orders of magnitude lower, giving a disk mass limit of ~10 Earth masses. (This is similar to flux ratio in protostellar disks.) Comparable to mass of Earth-mass planets found around a millisecond pulsar by Wolszczan & Frail (1992). • Disk lifetime estimate from upper limit on disk mass loss rate (propeller spin-down): dM/dt < 10-11 M/yr. Resulting disk survival lifetime >106 yr, longer than spin-down age. (SGR flares?)

19. Length Scales for Pulsars and Disks magnetic axis spin axis dipole magnetic field accretion disk corotating v ~ rm Important length scales: rm = magnetospheric radius, where Keplerian rco = corotation radius, where rLC = light cylinder radius, where rco r If disk penetrates within rLC, then spin evolution of pulsar is affected. Then inner disk radius set by rm. Will have either propeller spin-down (rm>rco) or accretion spin-up (rm<rco).

20. Arguments for Debris Disk Interpretation • Thermal shape of IR spectrum. Coincidence of Tin and dust sublimation temperature. Uniform FIR/FX ratio and correlated X-ray/IR variability in AXPs, consistent with reprocessing. Alternative Interpretations? • Accretion disk emission. Ertan et al. (2006) argue that the optical and infrared emission can be fit by a single accretion-disk model. They challenge the Hulleman et al. (2000) rejection of an accetion model, arguing that it is still allowed for appropriate choice of assumptions, e.g. reprocessing efficiency. • Magnetospheric emission. Chris Thompson (CITA) has suggested that a purely magnetospheric model may be able to explain both the optical and infrared data, without appealing to a disk at all. Observational Tests? • Correlated variability: Do X-ray and IR have correlated variability consistent with reprocessing? (Recent work by Durant?) Are IR and optical emission correlated or uncorrelated? • Phase-lagged IR pulsations: Look for IR pulsations with small pulsed fraction, delayed by ~1 s with respect to X-ray pulsations. (Compared to <0.3 s delay for optical pulsation.) • Rayleigh-Jeans emission from disk: Look for long-wavelength continuum emission • Dust emission line features: Use low-resolution spectroscopy to look for discrete features.

21. Some Implications of Our Pulsar Debris Disk Interpretation • Fallback in supernovae.Bolsters general notion of SN fallback. (SN 1987A?) • Fallback and young neutron stars. • If fallback disks are common around all young NSs, then this may affect estimates of pulsar ages and magnetic moments from spin-down. • If fallback disks are peculiar to AXPs, then this may point to a distinct formation mechanism for magnetars (e.g., rapidly rotating progenitors). This may also explain the narrow clustering of AXP spins. Is accretion power in AXPs really ruled out? • Difficult to discriminate because of lack of observations. Heating of disk key to detectability. Could radio pulsar wind be as effective as X-rays? • Pulsars and planet formation. • Resemblance of our pulsar debris disks to ordinary protoplanetary disks raises the possibility of planet formation around young pulsars. • Recall planetary system in old (~109 yr) 6.2 ms pulsar PSR B1257+12 (Wolszczan & Frail 1992). Different evolutionary path for this system. • Unclear whether environment around AXP is conducive to formation/survival of planets. However, formation time scale (~106 yr) probably exceeds magnetar lifetime. • Planet detection around AXPs would be very difficult due to slow pulses. Searches around young radio pulsars might be impeded by timing noise.

22. Possible “Applications” of Fallback Disks • Anomalous X-ray pulsars(Chatterjee et al. 2000; Alpar 2001; Marsden et al. 2001; Eksi & Alpar 2003) Suggestions that X-ray emission from these objects powered by fallback accretion disk • Soft gamma repeaters (Ertan & Alpar 2003), compact central objects in SNRs(Xu et al. 2003; Shi & Xu 2003) Suggestions that X-ray emission from these objects powered by fallback accretion disk • Radio pulsars (Marsden et al. 2001; Menou et al. 2001; Alpar et al. 2001; Qiao et al. 2003) Fallback disks would have consequences for radio pulsars, including: discrepancy betweeen timing and supernova ages (Marsden et al. 2001), braking indices (Menou et al. 2001), distribution of spins and spin derivatives (Alpar et al. 2001), timing variability (Qiao et al. 2003). • Jets from young pulsars in Crab and Vela supernova remnants might be collimated by fallback disks (Blackman & Perna 2004) • SN 1987A lightcurve deviations from pure radioactive decay might be explained by fallback disk accretion (Meyer-Hofmeister 1992; Mineshinge et al. 1993). • Planets around pulsars may be formed from fallback disks (Lin et al. 1991; Podsiadlowski 1993)

23. Comparison: Pulsar Planets • The first extrasolar planets were discovered by Wolszczan & Frail (1992) around the 6.2 millisecond radio pulsar PSR 1257+12. • 3 planets: • Planet A: M=0.02 Earth masses, P=25.3 d • Planet B: M=4.3 Earth masses, P=66.5 d • Planet C: M=3.9 Earth masses: P=98.2 d • These planets lie in the same orbital plane and the B and C are in a 3:2 resonance, supporting a disk origin for the planets (Konacki & Wolszczan 2003). • Millisecond pulsar evolution is significantly different than isolated pulsar evolution.

24. Search for Debris Disks Around Other Young Neutron Stars Limits on IR/X-Ray Ratio in Other Targets Predicted IR Flux for Other Pulsars

25. Previous Searches for Debris Disks Around Nearby Pulsars • Infrared surveys: • van Buren & Terebey (1993), Foster & Fischer (1996), Koch-Miramond et al. (2002), Lazio & Fischer (2004) • Submillimeter surveys: • Phillips & Chandler (1994), Greaves & Holland (2000), Lohmer et al. (2004) • Specific for PSR B1257+12: • Bryden et al. (2006) and references therein:

26. Related work: Debris disks around massive white dwarfs • Infrared excess interpreted as debris disk emission has been detected from two massive DAZ white dwarfs: • G29-38 (Zuckerman & Becklin 1987) • GD 362 (Kilic et al. 2005; Becklin et al. 2005) • Accretion from this disk may explain the high metallicity at the WD photosphere, short settling time for heavy elements. • Dusty disks have been predicted to exist around massive white dwarfs by Livio, Pringle, & Wood (2005), from dissipation of a disrupted WD in a WD-WD merger event.

27. Summary • Infrared emission in 4U 0142+61 (and probably all AXPs) likely arises from a dust disk. This is supported by the similar IR/X-ray ratio seen in the AXPs, and the correlated variability. The X-ray heating of the disk is crucial to our being able to detect it. • How widespread is this phenomenon? We are not yet able to reach the same IR/X-ray flux ratio in either compact central objects (e.g., Cas A) or ordinary X-ray pulsars. • Presence of disk is an AXP demonstrates that such disks are possible. Origin as a supernova fallback remnant plausible. But AXP evolution presumably not typical of evolution of other pulsars. • Only known planetary system is in a millisecond pulsar, presumably requiring binary evolution. However, limits on planetary systems are still being measured. What does this mean for planets around ordinary pulsars? • Follow-up observations: • More deep searches for disk emission in AXPs. Searches for disk emission from other X-ray pulsars. • Correlated X-ray/optical/IR variability studies of AXPs. • Disk emission due to particle heating by radio pulsar winds? • Mid-IR spectroscopy of 4U 0142+61 to look for dust line features. References: Wang, Chakrabarty, & Kaplan (2006), Nature, 440, 772 Wang, Kaplan, & Chakrabarty (2006), ApJ, submitted (astro-ph/0606686)

28. Latest Spectral Energy Distribution for 4U 0142+61