Millisecond Pulsars at

X-ray Energies

Slavko Bogdanov

In collaboration with:

Scott Ransom (NRAO)

Ingrid Stairs (UBC)

Paulo Freire (MPIfR)

Fernando Camilo (Columbia)

Maura McLaughlin(WVU)

Duncan Lorimer(WVU)

Jason Hessels(ASTRON)

Werner Becker (MPE)

Josh Grindlay (Harvard/CfA)

George Rybicki (Harvard/CfA)

Maureen van den Berg (UvA)

Craig Heinke (U of Alberta)

Vicky Kaspi (McGill)

Anne Archibald (McGill)

Haldan Cohn (Indiana U)

Phyllis Lugger (Indiana U)

Mathieu Servillat (Harvard/CfA)

� First detected in ROSAT All-Sky Survey- PSR J0437−4715(Becker & Trümper 1993)

� ~10 MSPs detected by ROSAT

� ~50 detected to date with Chandra and XMM-Newton

� Most (~30) detected in deep Chandra

observations of globular clusters

� Very faint X-ray sources (L

X ≤1033 erg s–1,

typical: LX

≈1030–31 erg s–1)

MSPs in X-rays (~0.1−−−−10 keV)

Becker & Trümper (1993)

47 TucChandra ACIS-S

0.3−6 keV281 ks

Heinke et al. ApJ, 625, 796 (2005)

Bogdanov et al. ApJ, 646, 1104 (2006)

M28 (NGC 6626)

ACIS–S

0.3–6 keV

237 ks

Bogdanov et al. ApJ, 730, 81 (2011)

NGC 6397

Chandra ACIS-S

0.3−2 keV

290 ks

D ≈≈≈≈ 2.5 kpcNH ≈≈≈≈ 1 ×××× 1021 cm-2

PSR J1740–5340

U18

Bogdanov et al. ApJ, 709, 241 (2010)

I - Energetic MSPs

� Ė ≥≥≥≥ 1036 erg s−−−−1 LX ≈≈≈≈ 1033 erg s−−−−1

� Hard, non-thermal radiation

� Narrow pulses

⇒ particle acceleration in magnetosphere

Zavlin AP&SS, 308, 297 (2007)Bogdanov et al. ApJ, 730, 81 (2011)

PSR J0024–7204W (47 Tuc)

PSR J1740–5340 (NGC 6397)

II - Eclipsing Binary MSPs

� LX ≈≈≈≈ 1031−−−−32 erg s−−−−1

� Hard, non-thermal emission

� Orbital variability

⇒ intra-binary shock due to interaction of pulsar wind with companion

Bogdanov et al. ApJ, 730, 81 (2011)

PSR J1824-2452H

(M28)

Bogdanov et al. ApJ, 709, 241 (2010)

Bogdanov et al. ApJ, 630, 88 (2005)

Eclipsing Binary MSPs: PSR J1023+0038 – “the missing link”

Bogdanov et al. ApJ, 762, 96 (2011)

Chandra ACIS-S

Eclipsing Binary MSPs: PSR J1723−−−−2837

Bogdanov et al. in prep.

Chandra ACIS-S

Eclipsing Binary MSPs

� Pronounced X-ray eclipses due to geometric occultation of intra-binary shock by secondary star

Arons & Tavani, ApJ, 403, 249 (1993)

Bogdanov et al. ApJ, 762, 96 (2011)

� Depth and duration of X-ray eclipses imply a shock localized at face of companion and/or L1.

� Shock luminosity & location constrain pulsar geometry & physics:

• Pulsar wind has to be anisotropic & concentrated in orbital plane

⇒ pulsar spin axis aligned with orbital angular momentum axis

• At shock, wind is magnetically dominated

PSR J1023+0038

Eclipsing Binary MSPs

III Typical MSPs

� Ė ≈≈≈≈ 1033−−−−34 erg s−−−−1 LX ≈≈≈≈ 1030−−−−31 erg s−−−−1

� Soft, thermal X-rays from Reff ≤≤≤≤ 2 km

� Broad pulses ⇒ surface PC emission

Bogdanov ApJ, 762, 96 (2013)

PSR J0437−4715

Bogdanov & Grindlay ApJ, 703, 1557 (2009)

PSR J0030+0451

XMM–Newton

130 ks

130 ks

e±

e±

X-rays

� Thermal X-ray emission due to polar cap heating by a return current of

relativistic particles from pulsar magnetosphere

X-rays

� Surface radiation can serve as a valuable probe of neutron star properties

(compactness, B-field geometry, surface composition,…)

Modeling thermal X-ray emission from MSPs

� Ingredients:

- rotating neutron star

- two X-ray emitting hot spots

- general & special relativity * Schwarzschild metric

(or approximation)

* Doppler boosting/aberration

* propagation time delays

- optically thick hydrogen atmosphere

Viironen & Poutanen (2004)

αααα = pulsar obliquity

ζζζζ = ∠∠∠∠ b/w line of sight & pulsar spin axis

φφφφ(t) = rotational phase

θθθθ = photon ∠∠∠∠ w.r.t surface normal

ψψψψ = photon ∠∠∠∠ at infinity

b = photon impact parameter at infinity

ζζζζαααα

θθθθ

ζζζζ

Viironen & Poutanen (2004)

Nollert et al. (1989)

Flat

Schwarzschild

Bending of photon trajectories

For M = 1.4 M�, R = 10 km~80% of the neutron

star surface is visible at a given instant.

Modeling thermal emission from MSPs

Courtesy of G.B. Rybicki

BB

H atm.

• Non-magnetic (B < 1010 G ~ 0 G) Hydrogen Atmosphere:

- harder than blackbody for same effective temperature

- anisotropic emission pattern ⇒ limb-darkening- 100% pure H due to gravitational sedimentation

McClintock, Narayan, & Rybicki (2004)

} Zavlin et al. (1996)

Romani (1987)

Bogdanov, Grindlay, & Rybicki, ApJ, 689, 407 (2008)

Synthetic MSP X-ray pulse profiles- R = 10 km, M = 1.4 M�- Teff = 2 × 106 K (H atmosphere)

- 2 antipodal, point-like polar caps

Thermal X-ray emission is observable for all (α, ζ)

- P = 4 ms, R = 10 km, M = 1.4 M�

- Teff = 2 × 106 K (H atmosphere)

- 2 antipodal, point-like polar caps

Blackbody

Blackbody + Doppler

H atmosphere

H atmospere + Doppler

- Can determine emission properties of NS surface

(H atm. vs blackbody)

- Can constrain magnetic field and viewing geometries ⇒ input for γ-ray modeling

α=10°, ζ=30°

α=30°, ζ=60°

α=60°, ζ=80°

α=20°, ζ=80°

Model MSP X-ray pulse profiles

Bogdanov, Rybicki, & Grindlay, ApJ, 670, 668 (2007)

9 km12 km16 km

for M = 1.4 M

�

* Fits to X-ray pulse profiles of MSPs can be used to infer NS compactness

1 + zg = (1 – 2GM/c2R)–1/2

⇒ constrain NS EOS(Pavlov & Zavlin 1997; Zavlin

& Pavlov 1998)

}

α=10°, ζ=30°

α=30°, ζ=60°

α=60°, ζ=80°

α=20°, ζ=80°

Model MSP X-ray pulse profiles

Bogdanov, Rybicki, & Grindlay, ApJ, 670, 668 (2007)

Lattimer & Prakash (2004)

Probing the EoS of Cold Ultra-dense Matter

Lattimer & Prakash (2004)

� NH ≈≈≈≈ 2 ×××× 1019 cm–2

� LX = 3 ×××× 1030 ergs s–1

� MPSR = 1.76 ± 0.20 M�

(radio timing; Verbiest et al. 2008)

� D = 156.3 ± 1.3 pc (VLBI; Deller et al. 2008)

PSR J0437−−−−4715

Bogdanov ApJ, 762, 96 (2013)

PSR J0437−−−−4715

R > 11.1 km (3σσσσ conf.) for M = 1.76 M�

130 ks

- Pulsations inconsistent with blackbody

- Anisotropic emission pattern required ⇒ atmosphere on surface

Bogdanov ApJ, 762, 96 (2013)

PSR J0437−−−−4715

- Magnetic dipole not centered on star!Bogdanov ApJ, 762, 96 (2013)

� Solitary MSP

� νννν = 205 Hz � D ≈≈≈≈ 300 pc� NH ≈≈≈≈ 1 ×××× 1020 cm–2

� LX = 4 ×××× 1030 ergs s–1

PSR J0030+0451

Bogdanov & Grindlay, ApJ, 703, 1557 (2009)

R > 10.7 km (95% conf.)

R > 10.4 km (99.9% conf.)

for M = 1.4 M�

X-ray Emission from MSPs

• Majority of radio MSPs have soft, thermal X-ray spectradue to heated magnetic polar caps (Teff ~ 106 K)

• Eclipsing binary MSPs can constrain pulsar wind physics

• Modeling thermal X-ray Emission: promising method for constraints on elusive NS EOS:

� Non-transient (always “on”) and non-variable

� “Weak” magnetic fields (Bsurf~108–9 G)⇒ B-field does not affect radiativeproperties of atmosphere

� Dominant thermal emission(≥95% of total counts @ 0.1–2 keV)

� Radiation from small fraction of NS surface(Reff ≤ 2 km) ⇒ emission region size and shape important at ≤1% level

� High precision distances (±0.8% for PSR J0437−4715; Deller et al. 2008)⇒ uncertainty in (Reff/D)2 greatly reduced

� Independent, accurate mass measurements possible from radio timing ⇒ unique constraint on RNS

MSPs: important targets for future X-ray missions

Section Title - 28GSFC and partner competition sensitive and proprietary – do not share or copy .

Explorer AO Site Visit

50 phase-resolved spectra

� Dramatic increase in photon statistics (>10-fold for PSR J0437−4715 compared to best available XMM-Newton data) enables use of novel 2-D modeling approach in the E − φ plane.

� ≤5% RNS

measurement in ~1 Ms with NICER

Section Title - 29GSFC and partner competition sensitive and proprietary – do not share or copy .

Explorer AO Site Visit

50 phase-resolved spectra

Pulsed thermal X-ray emission is observable for all

combinations of viewing angle (ζ) and pulsar magnetic inclination (α) due to light bending

Blind X-ray timing searches could discover all nearby MSPs in field and in GCs (with Gen-X?)

Bogdanov, Grindlay, & Rybicki, ApJ, 689, 407 (2008)

Proof of concept:

Blind discovery of PSR J0437-4715 in acceleration search

in 20 ks Chandra HRC-S observation