Extended X- ray emission from PSR B /LS 2883 and other gamma- ray binaries

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1 Extended X- ray emission from PSR B /LS 2883 and other gamma- ray binaries George Pavlov (Pennsylvania State University) Oleg Kargaltsev (George Washington University) Martin Durant (University of Toronto)

2 Introduc)on: Pulsar tails We know from X-ray and radio observations of solitary pulsars that the PWNe of supersonically moving pulsars consist of a bow shock and a long tail Red: X-rays Blue: radio (Kargaltsev et al 2008) Lengths of the tails reach a few parsecs, ~ 10 3 R head (Gaensler et al 2004)

3 If the pulsar is in a high-mass binary with a strong companion wind, the bow shock will be within the orbit, but the tail can be seen well outside the binary (e.g., Bogovalov et al. 2008, Bosch-Ramon & Barkov 2011) As the pulsar is rotating around the high-mass companion, the tail should be bent in the opposite direction (e.g., Dubus 2006). Therefore, we can expect extended emission around HMBs; its size and morphology depend on the properties of the pulsar s and companion s winds and geometry of the system. To detect such a tail, high spatial resolution is required.

4 Chandra X- ray Observatory (NASA) Launched in July arcsec angular resolution similar to best ground-based optical telescopes allows to obtain sharp images Advanced CCD Imaging Spectrometer (ACIS): kev energy range, high quantum efficiency, large effective area, low background of CCD detectors à excellent sensitivity

5 PSR B : PSR B /LS 2883 Binary: P orb = d, e = 0.87, d = 2.3 kpc, a = 7 AU (~3 mas), i = 22 o P = 47.8 ms, τ = P/2Pdot = 330 kyr, Edot = erg/s B = G LS 2883: O9.5Ve, M ~ 30 M sol, T eff = ( ) 10 3 K, R = R sol, L = erg/s (Negueruela et al 2011, Moldon et al. 2011)

6 First Chandra observajon 2009 May 14, 667 days after periastron, 39 days after apastron, true anomaly θ = 182 o ACIS-I3 chip, 124 pix subarray (8 1 FOV), frame time 0.44 s, exposure 28.3 ks, livetime 25.7 ks (Pavlov et al 2011) Extended emission seen up to 4 ( cm) south of the PSR, 6σ significance Absorbed power-law (PL) spectrum: N H = (2.9±0.2) cm -2 Γ = 1.6 ± 0.3, flux ~ erg/cm 2 /s L X ~ erg/s ~ 0.1 of compact source luminosity

7 Conclusions from first Chandra observajon Asymmetric extended emission detected up to ~ 10 4 AU from the binary Interpreted as synchrotron radiation from a shocked pulsar wind blown out of the binary by the wind of the high-mass companion Its luminosity is a small fraction, ~ 10-4 Edot, of the PSR spindown luminosity, at a lower end for PWN efficiencies (but there is the brighter unresolved PWN component). The structure of the putative PWN is unclear; we interpreted it as a bent tail blown out of the binary, with an average flow velocity ~0.1c, magnetic field B ~ 1mG, Lorentz factor for electrons in the flow γ ~ This interpretation implies that the shape and luminosity of the extended emission should depend on the binary phase.

8 Second Chandra observajon 2011 December 17, 370 days after periastron, 248 days before apastron, true anomaly θ = 169 o The same observational setup, exposure 62 ks, livetime 56.3 ks 1 2 Disk of the equatorial wind

9 (Durant et al. 2013) In the second observation extended emission shows a spiral structure (the Snail) extended source is a factor of 3 brighter compact source brightened by only 80%

10 Image deconvolution, which helps to improve resolution, confirms the spiral-like structure

11 Spectra of extended and compact emission fit a power-law compact à synchrotron emission extended Normalization vs. photon index Γ second observation compact extended

12 Simple modeling of the bent tail Ballistic approximation: Colors correspond to time t elapsed since tail launch. Insets show the tail shapes near the binary. V flow = c. The yellow-red parts (solid) correspond to t < 6 yrs. Nearly circular (red-colored) part of the tail for 2 nd observation corresponds to launching points from around periastron (disk passage); it is beyond the 6 yr time for 1 st observation (dashed blue lines).

13 If 6 years is synchrotron cooling time, then magnetic field B ~ 0.4 (τ syn /6 yr) -2/3 (E/1 kev) -1/3 mg Lorentz factor γ ~ (E/1 kev) 1/2 à implies re-acceleration The increase of luminosity in 2nd observation might be explained by the assumption that we see that part of the Snail which was launched when the pulsar was passing through the dense equatorial disk of the high-mass companion, while in 1st observation that part was too old and invisible because of synchrotron cooling Puzzle: lack of extended radio emission on a similar scale (talk by Moldon) To understand the nature of the variable Snail structure, a series of Chandra observations throughout the orbit is needed. Next observation should be taken in May Proposal submitted to Chandra Cycle 15 (5 observations, 380 ks total).

14 LS 5039 Binary: P orb = d, e = 0.35, a = 0.2 AU (0.08 mas), i = 20 o, d = 2.5 kpc Compact source: M = M sol ; neutron star or black hole? High-mass companion: O7V(f)

15 (Durant et al. 2011) Chandra and XMM-Newton imaging observations Chandra on-axis observation shows extended emission up to 2 from the point source

16 The extended emission is better seen when the point source image is subtracted

17 Radial distribution of counts Excess over PSF is seen at r > 10 However, at least part of it can be due to a dust scattering halo The halo spectrum is expected to be softer than the point source spectrum

18 Power-law fits point inner outer Inner nebula (20 60 ): about the same Γ as for point source à not a halo? Outer nebula ( ): softer spectrum à a halo?

19 Based on the spectrum, the inner nebula might be due to synchrotron radiation from relativistic electrons blown out of the binary, likely mixed with the companion s wind. This assumption is supported the asymmetric brightness distribution. Azimuthal distribution in the inner nebula χ 2 = 15.9 (5 d.o.f) for a constant value hypothesis

20 Thus, we may indeed see an extended PWN around the binary, with a flux of ~ erg cm -2 s -1 (4% of the point source flux), although it is not a firm conclusion If true, the compact object in LS 5039 is a pulsar, and the extended PWN might be a tail wound up in a tight spiral in the orbital plane It might be a mixture of synchrotron emission from relativistic pulsar wind electrons and thermal emission from companion s wind heated in the shock Further observations are needed to investigate the spectrum and morphology

21 LS I P orb = 26.5 d, e = 0.72, a = 0.7 AU (0.4 mas), i ~ 30 o, d = 2 kpc High-mass companion: B0Ve Nature of compact object unknown (M = 1 4 M sol ) Paredes et al (2007) saw a hint of extended X-ray emission in a 50 ks Chandra observation:

22 Radial distribution Excess over PSF at 5 12, 3.2σ significance Additionally supported by the asymmetry of 2D distribution Flux ext ~ erg cm -2 s -1 Rea (2010) found no extended emission in a 95 ks Chandra observation. That observation, however, was taken in CC mode (1D image only), not sensitive to such faint emission. The question remains open. Deeper imaging Chandra observations needed

23 Summary Extended X-ray emission around high-mass gamma-ray binaries has been detected: B certainly LS 5039 likely LS I may be L ext ~ 0.1% ~ 10% of compact source luminosity Size ~ binary orbit size Synchrotron radiation from pulsar s wind blown out of the binary by the companion s wind (γ ~ , B ~ mg) or (and) thermal radiation from companion s wind heated in the shock (T ~ a few kev) B : Extended emission is variable; spiral-like morphology (Snail); V flow ~ c

24 Open issues Is there indeed extended emission around LS 5039 and LS I ? Deeper Chandra imaging needed. Synchrotron from PW or thermal from companion s wind? Could be determined from deep Chandra observations If it is synchrotron, why there is no extended radio emission on the same scale? Lack of electrons with lower energies? Modeling needed. The cause for variability in B : brightened part of the spiral because brighter tail segments are launched from some part of orbit (e.g., around periastron)? Multiple observations throughout the orbit needed.