1. Neutron Stars and Debris Disks
Andy Shearer and Vitaly Neustroev
Centre for Astronomy
NUI, Galway
Ireland
22/02/2019

2. Debris Disks and Pulsar Theory
Underlying pulsar theory unchanged for 40 years
rapidly rotating magnetised neutron star
radio energy emission - coherent synchrotron
high energy emission - incoherent synchrotron or curvature radiation
No agreement on how.
Probably the only point of agreement between all these theories is the association of pulsars with magnetized, rotating neutron stars - Lyutikov, M., Blandford, R., & Machabeli, G., 1999, MNRAS, 305, 338
Understanding now helped by
Rotating Radio Transients - RRATs
AXPs
Transient pulsars and transient phenomena
.....
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3. Some Background
Pulsar-disk systems have been proposed for over 25 years
Debris Disks - either
fossil disk created during the supernova event
Michel & Dessler, 1981, ApJ, 251, 654, Pulsar Disk System
Michel, Nature, 1988, 333, 644, Neutron star disk formation from supernova fall-back and possible observational consequences
continually fed from the ISM
e.g. Popov, Colpi, Treves, Turolla, Lipunov, & Prokhorov, 2000, ApJ, 530, 896
pulsar planetary system first discovered in and to date the lowest mass planet
Wolszczan, A.; Frail, D. A., 1992, Nature, 355, 145
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4. Some more background - Spin History
But
n=2.4 - PSR B
Vela
Crab
2.91 PSR J
2.837 PSR B
Additional torque from a fall-back disk
Menou, Perna & Hernquist, ApJ, L63, 554 (2001)
Mass in flow ~ 10-6 M☉ / year for the Crab
But requires limitations on the radial extent of the
disk to avoid optical detection
An additional source of torque for Vela?
Possibly a fall-back/debris disk?
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5. UltraCam observations - Dhillon et al, MNRAS, 363, 609, 2005
First observation of a neutron star disk - AXP- 4U
unpulsed
Wang et al, 2006, Nature, 440, 772
Pulsed
Figure 3. Energy distribution for 4U At low frequencies (1014–1015 Hz), the points marked
V, R, and I indicate the observed V, R, and I-band fluxes. The vertical error bars reflect the uncer-
tainties, while the horizontal ones indicate the filter bandwidths. The set of points above the mea-
surements indicate dereddened fluxes for AV = 5.4, as inferred from the X-ray column density.10,11
The errors include a 3% uncertainty in the reddening correction.9 At high frequencies (1017–1018 Hz),
the crosses show the incident X-ray spectrum as inferred from ASCA measurements.10 The diamonds
show the spectrum after correction for interstellar absorption, and the two thick dashed curves show
the two components used in the fit10: a power law of the form F = 103(h /1 keV)−2.67 μJy and a
black body with T = 4.4 Å~ 106 K and R = 12d5 km. The latter component is extrapolated to lower
frequencies to show it cannot reproduce the optical fluxes. Drawn in thin solid lines are models for
the optical emission. The curve marked disk is for an accretion disk that has inner radius equal
to the corotation radius Rco = (GMP2/4 2)1/3 = 0.010R⊙ (for M = 1.4M⊙, P = 8.7 s), outer
radius of 1014 cm, and inclination of 60 degrees. Both irradiation and viscous heating are taken into
account13; the former dominates in the optical and the latter causes the bump in the ultra-violet.
The calculation is for d = 5 kpc, but since the disk luminosity scales almost linearly with the X-ray
luminosity, the result is not sensitive to distance. The optical fluxes expected in the accretion model
are greatly in excess of those observed. Only for a truncated disk with a small outer disk radius,
rout ∼ < 0.1d10/11
5 (f/0.25)−2/11 R⊙, can the observed fluxes be reproduced, as is shown by the dashed
curve. The thin solid curves marked BB, H, and He are black body, pure hydrogen and pure helium
white dwarf model atmospheres for T = 4 Å~ 105 K. The normalization to the optical data implies
R = 0.011d5, 0.017d5 and 0.015d5 R⊙, respectively. None of the spectra can reproduce the X-ray
emission.
Pulsars are rotating, magnetized neutron stars that are born in supernova explosions following the collapse of the cores of massive stars. If some of the explosion ejecta fails to escape, it may fall back onto the neutron star or it may possess sufficient angular momentum to form a disk. Such `fallback' is both a general prediction of current supernova models and, if the material pushes the neutron star over its stability limit, a possible mode of black hole formation. Fallback disks could dramatically affect the early evolution of pulsars, yet there are few observational constraints on whether significant fallback occurs or even the actual existence of such disks. Here we report the discovery of mid-infrared emission from a cool disk around an isolated young X-ray pulsar. The disk does not power the pulsar's X-ray emission but is passively illuminated by these X-rays. The estimated mass of the disk is of the order of 10 Earth masses, and its lifetime (>= 106years) significantly exceeds the spin-down age of the pulsar, supporting a supernova fallback origin. The disk resembles protoplanetary disks seen around ordinary young stars, suggesting the possibility of planet formation around young neutron stars.
UltraCam observations - Dhillon et al, MNRAS, 363, 609, 2005
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6. Other evidence for debris disks?
Shibanov, Y. A, et al, 2006, A&A, 448, 313

8. Perna, Hernquist & Narayan, 2000, ApJ, 54, 344
Not a new idea ...
Perna, Hernquist & Narayan, 2000, ApJ, 54, 344

9. Spitzer - NICMOS - Subaru Image PSR B0656+15
Spitzer 3.6μm Spitzer 5.8μm
Subaru B NICMOS 160W
Subaru -Shibanov et al., A&A, 448, 313 (2006)
Nicmos - Koptsevich et al, A&A., 370, 1004 (2001)
Spitzer - Spitzer archive.
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10. Predicted Debris Disk Fluxes for Normal Pulsars
Predicted Debris Disk Fluxes for Normal Pulsars rinner= light cylinder radius router=1R
Geminga parallax 2007Ap&SS
Faherty, Jacqueline; Walter, Frederick M.; Anderson, Jay
1055 distance 2002ApJ H
Gamma-Ray Emission from an Outer Gap Accelerator: Constraints on Magnetospheric Current, Magnetic Inclination, and Distance for PSR B Hirotani, Kouichi; Shibata, Shinpei

11. Rotating RAdio Transients - RRATs
Transient phenomena - Debris Disk hints?
Rotating RAdio Transients - RRATs
To date ~10 pulsars show transient behaviour
McLaughlin et al, Nature, 2006, 439, 817
At least one with an X-ray counterpart
Reynolds et al, 2006, ApJ, 639, L71
Accreting material triggering magneto-spheric activity?
Li, 2006, ApJ, 646, L139, also PSR B
Authors:
Li, Xiang-Dong
Affiliation:
AA(Department of Astronomy, Nanjing University, Nanjing , China;
Publication:
The Astrophysical Journal, Volume 646, Issue 2, pp. L139-L142. (ApJ Homepage)
Publication Date:
08/2006
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12. Other stochastic emitters - Giant Radio Pulse[GRPs] Pulsars
A few pulsars emit GRPs where the randomly occurring pulses have a flux many thousand times greater than the normal
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13. Ground based AO Observations
arXiv: v2
Possible interpretations
X-ray heated fossil debris disk
Viscously heated disk
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14. Summary - why near IR observations are important
At least one young neutron star, 4U , has been observed to have a debris-disk, but
how many others do?
how long does a disk last?
are such disks suitable for planet formation?
disks or rings?
How many young ‘normal’ pulsars have associated debris disks?
t < 100,000 years Vela, Crab
middle aged pulsars - PSR B , Geminga,
Can debris disks explain
Stochastic pulsar behaviour?
Spin evolution and birth spin period?
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15. Summary - Why SM4 will be important
ACS
Visible polarisation
WFC3
NIR imaging for 0.1μJy fluxes
NICMOS
NIR imaging and polarimetry
compare LONG and SHORT polarisation fluxes
Observations at 2 microns
Need multi-wavelength studies
X-Ray - IR variability for AXP disks
WFC3 and ground based adaptive optic observations [ELTs]
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