The puzzling diversity of pulsar wind nebulae
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The puzzling diversity of
pulsar wind nebulae
George Pavlov (Pennsylvania State University)
Oleg Kargaltsev (George Washington University)
Bettina Posselt (Pennsylvania State University)
Noel Klingler (George Washington University)
Jeremy Hare (George Washington University)
Chandra (and XMM) detected over 60 PWNe
Two major classes
(I) Young PWNe within SNRs:
subsonic motion, torus-jet
morphologies in very young ones,
interaction with SNR reverse
shock in older one (Pat’s talk).
(II) Older PWNe, left their
SNRs, supersonic motion,
bow-shock/tail morphologies
Both classes show great variety
of shapes and other properties
-- particularly surprising for
Class II PWNe
RMHD simulations of isotropic outflow for supersonically moving pulsar
(Bucciantini, Amato, Del Zanna 2005)
Density, velocity, magnetization
Synchrotron radiation for different magnetizations
Forward shock
Contact Discontinuity
Termination shock
Some PWNe resemble the simulations.
Example: The Mouse
PSR J1747-2958
Edot =2.5×1036 erg/s
τsd = 25 kyr (true age ~100 kyr)
D = 5 kpc
Vt ~ 400 km/s
120 ks Chandra ACIS (Klingler et al 2015)
X-ray (blue) superimposed on Radio (red,VLA, beam = 3.5”)
At 5 kpc:
30” à 0.36 pc, 3’ à 4.4 pc
Comparison with models yields Mach ~ 60, V ~ 600 km/s,
n ~ 0.3 cm-3 (Gaensler et al 2004)
X-ray and radio – synchrotron radiation from shocked PW
Rapid synchrotron cooling à
strong magnetic field ~ 100 μG
Quite different picture:
Geminga PWN
Chandra ACIS 600 ks (Posselt, XVP-PWN
Collaboration 2015)
Edot = 3×1034 erg/s, τsd = 340 kyr, d ~0.2 kpc ,Vt ~340 km/s
Two ~4’-long lateral tails and 50’’-long axial tail
No radio or H-alpha PWN
The axial tail might be a jet, and the lateral tails
resemble a limb-brightened shell (bow shock),
but no emission is seen between them
(and such bow shocks are usually seen in Balmer lines,
not in X-rays)
The lateral tails could be bent jets?
But then what is the axial tail?
Equatorial torus seen edge-on?
Spectra are very hard, Γ ≈ 0.7 – 1.0 for the lateral tails,
Γ ≈ 1.4 – 2.0 for the axial tail; no statistically significant
softening. Additional acceleration in lateral tails?
The 3D PWN geometry remains unclear, it is not explained by simple theoretical models
(Ask Bettina Posselt for details.)
B0355+54 (Mushroom PWN)
Another kind:
Edot = 5×1034 erg/s, τsd = 650 kyr, d ~ 1 kpc, Vt ~ 60 km/s
Chandra ACIS, 390 ks, Klingler, XVP-PWN
Collaboration, 2015
Filled-in “head” (mushroom cap), brightened
along the axis, with a sharp lower boundary
and strange “whiskers”, a dimmer “stem, ” and
an even dimmer 7’(~2 pc) - long broadening
trail.
Only a hint of spectral softening, from Γ = 1.5 in the compact nebula to Γ = 1.7 in the trail
Yet another type:
J1509-5850
Edot = 5.1×1035 erg/s, τsd = 154 kyr, d ~ 3.8 kpc, Vt not measured
Chandra ACIS, 370 ks, Klingler et al 2015
Bowshock-like structure around the pulsar, similar to Geminga.
Long (~6 pc) tail with high outflow speed, >> Vpsr. Very unusual
“headlight”. Radio tail up to 11 pc, quite different from the Mouse.
Magnetic field parallel to the tail in the Mouse, perpendicular in
J1509. Faint Halpha bow shock (Brownsberger & Romani 2014).
Photon index Γ ~ 1.9 in the tail,
Even longer radio tail
hint of softening in the “headlight”.
(Hui & Becker 2007, Ng et al 2010)
Even more exotic:
Lighthouse nebula
IGR J11014-6103
D ~ 7 kpc, Vt > 1000 km/s (based on SNR MSH 11-61A association
Pavan at al. 2013
and SNR age estimate)
In addition to the “usual” tail-like nebula behind the pulsar, there is
a very long (~11 pc ) jet-like structure almost perpendicular to the
direction of motion. Interpreted as a precessing jet (~66 yr period).
Photon indices:: Γ = 1.6 +/- 0.2 for the “jet”, Γ = 1.9 +/- 0.1 for the “tail”
Another misaligned outflow: B2224+65 (Guitar nebula)
2’ (~1 pc)-long jet-like feature in X-rays
d ~ 1.5 kpc (Hui & Becker 2007, Johnson & Wang 2010)
Ė = 1.2 x 1033 erg/s
Age ~ 1 Myr
Vt ≈ 900 km/s
A guitar-like nebula in H-alpha (Cordes et al 1993,
Chatterjee & Cordes 2004).
Photon index in the one-sided jet”: Γ = 1.60 +/- 0.15
The true nature of the “jet” remains unclear. Might be a leak of
high-energy electrons from the bow-shock apex and further drift
in the ambient magnetic field (Bandiera 2008).
J0357+3205: Turtle’s tail Chandra ACIS 105 ks
d ~ 0.5 kpc Ė = 5 1033 erg/s
Age ~ 540 kyrs
Vt ≈ 390 km/s (De Luca et al 2013)
Compact PWN (“bullet”) is
essentially absent!
9’(~1.3 pc) trail, brightness increases with distance
from pulsar, no traces of spectral softening.
Might be thermal emission from ISM heated by
the shock driven by the fast-moving pulsar
(Marelli et al 2013) ?
No H-alpha
De Luca et al. 2013Some structures show fast variability.
J1809-1917 – wagging tail PWN
D ~ 3.5 kpc, Edot = 1.8*1036 erg/s, age ~ 50 kyr, Vt = ?
Chandra ACIS,
20 + 47 + 65 ks
(Pavlov et al 2015)
Tail (or jet?) changed its shape and length in just 8 months! A kink instability?
(Ask Bettina Posselt for a movie on variability of the Geminga PWN)Not only the detected X-ray PWNe of supersonically moving pulsars
have very different morphologies and spectral properties, but some
apparently similar pulsars do not show clear PWNe at all (or perhaps
show very small and faint PWNe).
ACIS-S, 5 ks
PSR B0656+14
Birzan et al 2015
Nearby, middle-aged pulsar (d = 290 pc, age = 110 kyr,
Edot = 3.8 1034 erg/s), Vt = 60 km/s.
No spectacular nebula, only a slight excess over the PSF profile in
a 4’’ – 15’’ annulus, with a very soft spectrum possibly due to
contribution of dust scattering halo.
HRC-I, 40 ksVery similar situation with another middle-aged pulsar
B1055-52
Posselt et al 2015
D ~350 pc, Edot = 3*1034 erg/s, age = 535 kyr, Vt ~ 70 km/s
50 ks Chandra ACIS-S image shows only a slight excess over
the PSF profile in a 5’’ – 20’’ annulus, mostly in the soft band.
What is the reason? Projection effect? Pulsar velocity almost
along the line of sight (Vt unusually small)? Likely, but B0355
also has a low Vt but shows a spectacular PWN. Some
intrinsic peculiarities of the pulsars (e.g., close alignment of
magnetic and spin axes)? Not clear yet.Thus,
X-ray PWNe of supersonically moving pulsars show great diversity:
Generally, 3 elements: Tail (trail?), Head, Collimated misaligned outflow
Not all of them are seen in a given PWN…
The tails/trails are typically a few pc long, with high flow speed in at least some of
them. The visible flow is likely decelerated by entrainment of ambient matter
(tail à trail). We see synchrotron cooling in some cases but not always.
The heads can be “filled-in” (Mouse, Mushroom) or “empty” (shell-like), sometimes
with a short axial tail (Geminga, J1509-5850).
The misaligned outflows could be jets along the pulsar spin axis or outflows of
very high energy electrons leaked from bow-shock apex region.
Some tails show variability with a few months timescale.
Some pulsars show very faint and/or very small X-ray PWN (or no PWN at all?).
Might be a projection effect?
More advanced (3D) models are needed.Even more diversity if we include binary pulsars. Here is an example A dynamic nebula of the B1259-63/LS 2883 high-mass binary The animation shows 3 images taken on Dec 2011, May 2013 and Feb 2014 with about equal exposures ~60 ks. We see an extended object moving from the binary with high apparent velocity, perhaps with acceleration. What is this?
High-mass binary LS 2883 with PSR B1259-63
Fast-spinning, massive
(M~30 M¤, L=6×104L¤)
star with a strong wind.
The wind is dense and slow
in the decretion disk,
tenuous and fast outside the
disk.
Pulsar B1259-63:
Spin period = 48 ms
Edot = 8×1035 erg/s
Spin-down age =330 kyr
Should emit pulsar wind
X-ray flux and spectrum vary with orbital period.
Orbit:
Gamma-ray flashes near periastron,
3.4 yr orbital period
apparently when the pulsar interacts with
7 AU (3 mas) max. separation
the decretion disk during 2nd passage.
0.87 eccentricityOrbital positions, dates, and days from periastron for 3 observations
16563(83)"
2014 Feb 8
1151 d
2011 Dec 17
370 d
2013 May 19
886 dDistance of the extended source from the binary versus time
8
-
-
6
Distance, arcseconds
-
4
-
-
2 -
0
0 200 400 600 800 1000 1200 1400
Days since periastron passage
Linear fit: V = (0.07+/-0.01)c
If there is no acceleration, the cloud was ejected from the binary
around periastron of 2010 Dec 14
Apparent acceleration (?) 90+/- 40 cm/sec2Luminosity decreased from 1 to 0.2*1031 erg/s (~ 3% à 0.7% of binary luminosity) The spectra are consistent with power laws, photon indices Γ = 1.2+/-0.1, 1.3+/-0.2, and 0.8+/-0.4 (no softening!) Synchrotron radiation? B ~ 10-4 G, Ee ~ 10 – 100 TeV Confidence contours in Photon Index – Normalization plane
Possible interpretations: (1) Variable termination shock in the circumbinary medium, similar to PWNe around isolated pulsars (Kargaltsev et al 2014) But, it requires unrealistically high ambient pressure, pamb ~ 10-10 dyn cm-2, to explain the shock size. (11) Ejected clump of stellar (disk) matter moving in the unshocked pulsar wind (Pavlov et al 2015) -- looks more plausible. The ram pressure of the unshocked pulsar wind can also accelerate the clump: vdot ~ ppw A mcl-1. mcl ~ 1021 g for the apparent (low-significance) estimated acceleration. The clump can be ejected due to interaction of the pulsar (PW) with the decretion disk. The X-ray emission is due to synchrotron radiation of the pulsar wind shocked by the collision with the clump. X-ray luminosity LX,cl = ξX Edot (rcl/2r)2, ξX ~ 1.5×10-3 Other interpretations are possible, new observations are planned.
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