Gamma-ray binaries as non-accreting pulsar systems

Gamma-rays from binaries: GeV and TeV *See Cortina’s and Dubois’s talks in this conferencefor observational results in the TeV and GeV domain. *See Stella’s for a MW overview. LS I +61 303 Average spectrum:Index: 2.21, Cutoff: 6.3 GeVLS 5039 Average spectrum:Index: 1.9, Cutoff: 2.1 GeV2

Gamma-rays from binaries and gamma-ray binaries • gamma-ray binaries:when gamma-ray emission above 10 MeV dominates the SED output • This is the case for LS 5039, PSR B1259-63, LSI+61 303 • This is not the case for Cyg X-1 (for which L vhe ~ 10 -4 L x )• Other clear distinctive phenomenology: orbital variability vs. flaresPlot from Chernyakova et al. 2006 LS I +61 303 3

Dichotomy in composition Plot from Mirabel 2006 4

A few ideas on gamma-ray generation from pulsar binaries Models do share generic ingredients • Binary system with massive star(giving target material, photons: essentially anisotropicIC models)• Binary orbit constrain geometry in non-trivial ways • Other radiative processes:Synchrotron / absorption & pair creation / cascading Different dominance: -pulsar wind zone processes (e - accelerated by pulsar or inter-windshocks or particle energization in theinner wind already produced gammaemission therein) -shock processes (e - accelerated by inter-wind shock,interact in the MSWZ) Ideas go back to Maraschi and Treves 1981; Protheroe and Stanev 1987, Arons and Tavani 1993, 1994; Tavani & Arons 1997; Bednarek 1997; Kirk et al. 1999; … 5

A few ideas on gamma-ray generation from pulsar binaries Models do share generic ingredients • In a variable geometry along the orbit• Pulsar (or close-to-psr shock) inject electrons • Which interact via (anisotropic, KN) IC • Photons are absorbed and initiate cascades • Which are computed by Monte Carlo or othermethods Things to note from geometry e.g. for LS 5039 • Detailed account of processes in a varyinggeometry, plus full analysis of cascading Plots from Sierpowska-Bartosik & DFT 2008 6

A few ideas on gamma-ray generation from pulsar binaries Opacities are very dependent on phase, and their combined role generate LC and spectra Different energies, same inclinationDifferent inclinations, same energiese.g. for LS 5039 Left plots from Dubus 2006, Right plots from Sierpowska-Bartosik & DFT 2008 7

A few ideas on gamma-ray generation from pulsar binaries Anti-correlation of GeV and TeV fluxes is a sustained prediction of these models(as a result of physical process and the geometry, less of the possible pulsar in the system) Left: Bednarek 2006 Right: Aharonian et al. 2006 [Probably all in place since Protheroe &Stanev 1987. It was also found by Dubus et al. 2008 Sierpowska-Bartosik & DFT 2007, 2008b] and others working in black hole models. Prediction of orbital spectral variation in the GeV range is also overall ok (e.g., at the hardness ratio level) 8

A few ideas on gamma-ray generation from pulsar binaries Other good and bads: -No model predicted the Fermi GeV cutoffs (nor the ones withBHs) Comparison between LS and LSI: LS I opacities in the PWZ a factor offew-to-few-10 lower than in LS -Models predicted less flux than observed (not dramatic, andespecially, not true at the tens of GeV range) -PWZ and Shock models are not exclusive (ultimatelydepending on opacities, hybrid models not in place, manyparameters) -Gamma-ray astronomy as a window for the study of pulsarwindsThe emission of the unshocked wind isincompatible with observations whenthe former is formed by monoenergeticparticles. Plots from Cerutti et al 2009, Sierpowska-Bartosik & DFT 2008, 2009 9

Why is a non-accreting pulsar a tenable alternative? 1) The mass function is compatible with a pulsar binary • The mass function values are low. See J. Casares’s talk • If ignorance of the orbital inclination can be represented by a random distribution, thelikelihood for the system to be formed by a neutron star is greater than to be formed bya black hole compact object. Casares et al. (2005a,b); Grundstrom et al. (2007); Aragona et al (2009) 10

Why is a non-accreting pulsar a tenable alternative? (II) 2) No obvious sign of accretion See next slides for the search of lines • LSs are in the lower-end of the HMXB distribution of X-ray luminosities, albeit stillcompatible with some wind-fed accreting binaries. • If Bondi accretion: The accretion rate is dM=dM w /(2r a /d s ) 2 ~ 5×10 14 g s −1 [wheredM w is the stellar wind mass loss rate, d s is the orbital separation, and r a = 2GM c /v w2is the Bondi capture radius, with M c the mass of the compact object and v w thewind speed], that when transformed into power implies unrealistic efficiencies.• No variability on s-scales, as low/hard state LMXRB or disc accreting HMXB.• No sign of a corona in X-rays, instead, there is an apparent lack of spectralcutoffs at 30 keV to 100 keV, distinguishing these systems from both accretingHMXBs and low state LMXBs. • To avoid disc accretion, if all of the pulsar spin-down power L sd is transferred tothe pulsar wind, pressure equality implies L sd > 4(dM)v w c. Writing L sd as functionof P and B, it results P < 230 B 1/2 (dM/10 15 g s -1 ) −1/4 ms,• If a fast (young) millisecond pulsar is present, accretion onto it is not expected. e.g., Chernyakova, Neronov & Walter 2006; Takahashi et al. 2009, Rea et al. 2010 11

Why is a non-accreting pulsar a tenable alternative? (III) 3) It is ok not to see radio-pulses (opacities too large) • The opacity to free-free absorption in the equatorial disc of the star in LS I 61 303(using fiducial values) is • This absence of radio pulses also happens in the periastron of PSR B 1259-63 • In the polar flow of the star, the opacity is also high (f gives a measure of clumpiness) e.g., Zdziarski, Neronov & Chernyakova (2008) 12

Why is a non-accreting pulsar a tenable alternative?4) The radio morphology is stable and no jets are seen in any scale Simultaneous radio-morphology at different angular resolution • The size of the radio emitting region of LS I +61 303 is less than 6 mas (12projected AU), and the presence of persistent jets above this scale is excluded. • The comparison of VLBA images at similar orbital phases, but 10 orbital cycles apartshows a high level of similarity • Interaction of steady flows (from a relativistic pulsar wind, jet and/or stellar wind) ratherthan by the interaction of such an outflow with wind clumps. • If radio emission is produced by a milli-arcsecond scale jet, its stability and periodicity isdifficult to reconcile with the non-persistent nature of a large scale feature LS I +61 303 on 25-26/10/2006, phase 0.62e.g., Dhawan et al. 2006, Albert et al. 2009; ; there is a similar radio morphology in LS 5039, see Ribó 2008 13

Why is a non-accreting pulsar a tenable alternative?4) The radio morphology is stable and no jets are seen in any scale Non-trivial radio morphology is expected in pulsar binaries (cometary tails are evenproduced in interactions with the ISM for isolated pulsars) • Whereas on a small scale, the shocked material flows away from the binary on astraight path, following the direction given by the vector difference of the stellar windand orbital speeds v w − v orb ; on a larger scale, the material shears with the orbitalmotion Top: simulations by Dubus 2006 Bottom observations by Paredes et al 2000 e.g., Dubus 2006, observation by Paredes et al. 2000 14

Why is a non-accreting pulsar a tenable alternative?4) Young ages are consistent • Tracing back of the proper motion places upper limit to the age (e.g. Ribó 2002) 5) Pulsar systems would be consistent with population predictions of Be XRBs • 64 Be XRBs are known, and in 42 the compact object was confirmed to be a neutronstar by the presence of the X-ray pulsations; in not a single one of them, a black holewas confirmed (these are older pulsars, only a few are expected to host a young psr) • An evolutionary effect? The rotation of Be stars could be achieved during a period ofRoche-lobe overflow from the more massive compact object progenitor, and if it loosesmost of the mass in the early pre-SN phase process, becoming a He star with mass ofonly a few M ⊙ , when exploding as a supernova it can only leave a neutron star behind e.g., Belczynski & Ziolkowski 2009; Tauris & van den Heuvel 2006 15

X-ray variability in gamma-ray binariesLS I +61 303 95 ks Chandraobservation near apastron 1ks binning lightcurves in fivetime intervals Bkg. Subtracted (0.3-10 keV)lightcurve binned at 100s • On top of a slow variability of a factor of 2, there were 2 flares lasting 3 and 1.5 ks• Not unseen in this source: Sidoli et al. 2006; Paredes et al. 2007; Esposito et al. 2007; Smith et al. 2009 have found some similar flares From Rea, DFT, van der Klis, Jonker, Mendez, Sierpowska-Bartosik 2010 16

Do we understand these flares in pulsar models? LS I +61 303 • Variability not observed in isolated pulsars • In an accretion scenario, flares like these can be explained by variability in theaccretion rate• In the rotational-powered pulsar scenario, as the interaction between the pulsarwind and clumps in the Be wind. If the typical density of a clump in the wind is n 10 10 cm −3 and its size is R 10 11cm, then δNH n R 10 21 cm −2 . The strong dependence between NH, Γ and thepower-law normalization, intrinsinc to the power-law modeling, might hide thisvariability in an increase of the hardness of the emission Neronov & Chernyakova. et al. 2007 17

Shorter timescales variability resembling a magnetar?• On 2008 September 10th, Swift-BAT triggered on a short SGR-like burst from the direction of LS I +61 303. • The other telescope onboard, Swift-XRT did not detect the burst because started observing 921s after the BAT trigger. • The burst location, lightcurve, duration, fluence, and spectra were fully consistent with a SGR/AXP • Archival analysis of Chandra ACIS-I observation showed many faint sources in the field of view, some with VLAdetections (16 compact radio sources within a 10 arcmin field of view with a peak flux density above four times the rmsnoise). • Magnetars are not associated with compact radio sources.• Thus, any X-ray source without a radio counterpart within the field of view could be the origin of the burst.GCN 8209, 8211, 8215; ATELs 1715,1731, 174018

Accretion signatures in gamma-ray binaries? • The presence of spectral lines is highly dependent on the continuum spectrum and on theorbital phase of the system. • The best data available for these kind of studies come e.g., from the XMM-Newtonsatellite (large collecting area, spectral resolution, grating spectrometer).• However, at a given orbital phase, only very short observations have been taken (aiming atmonitoring the continuum spectral variability over the orbit).• In the past XMM-Newton spectra of, e.g, LS 5039, an EW 60 eV is the current 1σlimit on the presence of Fe Kα (and only in a small part of the orbit). In the availableSuzaku observations, the limit on the detection of lines is 40 eV (see Takahashi et al.2009).• With the current short pointings (at a given orbital phase) there are not enough counts to usethe high-resolution spectral capabilities. 19

Accretion signatures: a word of caution • Consider the binary 4U 1700-37,• A very similar HMXB to LS 5039 (i.e., similar companion star and 4-days orbitalperiod, but no TeV emission, and clearly accreting) located at 1.5 kpc • Left: XMM-Newton spectrum of 4U 1700-37 (first published by van der Meer et al. 2005). • Right: how would these accretion lines look, assuming all are present, in the available datafor LS 5039 LS 5039 15 ks XMM simulationof the response of anspectrum full withaccretion lines 20

Pulsation search • Deep searches for pulsations have been performed in the radio band at several frequencies.However, no radio pulsations have been detected so far from candidates. • At PSR B1259–63’s periastron there is no radio pulsation. Its periastron (given thelarge orbit, 3.4 year period) is approximately the same size than the major axis of theorbit of LS I +61 303 (26 days period) and it is way larger than LS 5039’s obit (4 daysperiod). • The strong massive companion winds might have prevented radio pulsations to be detectedbecause of the strong free-free absorption all over the orbits.• PSR B1259-63 does not show X-ray pulsation either in the periastron (~15% pulsedfraction).• X-ray emission not-magnetospheric dominated? • beam not well-pointed? • A clearly difficult game, hopefully to be aided by GeV observations. 21

Pulsation X-ray search: the current status LS I +61 303 Pulsed fraction 3σ UL on the semiamplitudeof a sinusoidal signal95 ks Chandraobservation near apastron 0.3 – 2 keV Overall similar situation: 2 – 4 keV • Search for periods with frequencies 0.005 – 175 Hz (timingresolution of the CC-mode: 0.00285 s) • Pulse fraction upper limits between 7 – 15 % for the wholeenergy range (larger in smaller energy bands) 4 – 10 keV • No significant short bursts in bins of 0.1 s from Rea, DFT, van der Klis, Jonker, Mendez, Sierpowska-Bartosik 2010 22

Pulsation X-ray search: the current status (II)LS 5039 ~100 ks Chandra But given the size of the orbit of LS 5039 as compared with LSI +61 303, we do not expect much difference in the results. ? from Rea, DFT, van der Klis, Jonker, Mendez, Sierpowska-Bartosik 2010b; in preparation23

The GeV Fermi data in perspective LS I +61 303 Average spectrum:Index: 2.21, Cutoff: 6.3 GeVLS 5039 Average spectrum:Index: 1.9, Cutoff: 2.1 GeV• The Fermi PSR catalog has 46 high confidence pulsar detections. The pulsar spectra is fitted withan exponentially cutoff power-law model, with the large majority of the cutoff values being < 3 GeV. • Exponential cut-offs are reminiscent of the Fermi pulsar spectra; is this a sign of magnetosphericemission in these systems? But the orbital variability of spectrum? From the spectral viewpoint, it makes sense to ask What if the gamma-ray binaries are part of the same populationand the GeV emission is coming from the pulsar magnetospheres? See Abdo et al. 2009a,b for the Fermi results on binaries 24

The problem and a hypothesis towards its solution PSR • GeV-emissionpulsed • Cutoff is abalance btwacceleration/curvature loss Unpulsedradiation.Why? • Not enoughstatistics • Washed out inuncertainorbitalparameters Orbitalvariation? • Curvatureemissionproduced nearpulsar • No obviousreason formodulation The spectral distribution points to a natural pulsar interpretation of the GeV emission, but the orbitalvariability argues against it, and overall, the GeV-TeV anti-correlation remains unexplained.The 2-components GeV flux hypothesis GeV emission is formed by both a magnetospheric component, pulsed, steady along the orbit, plusan inter- or intra-wind shock component, unpulsed, modulated with the orbital motion. TeV emission is only formed by the modulated signal, having the pulsed component died out atsuch energies. 25

Could it work? Total SUPC emission Total INFC emission PWZ/Shock SUPC contribution PWZ/Shock INFC contribution Pulsed emission Lines do not represent a model, just the idea 26








Could it work? In this proposal it is possible to expect• the anti-correlation of GeV-TeV fluxes,• a possible increase of the energy (or disappearance!) of the GeV measured exponential cutoff(this would explain why there is no cutoff in LS 5039 INFC; high cutoff in LS I +61 303) and provides an interpretation for • why Fermi sees spectral pulsar-like spectra which varies with the orbital phase: it is just an effectof when, which contribution significantly contribute to the total flux.• makes it even harder to see the pulsation in GeV data (the pulsed fraction is only a fraction of theflux observed) In addition: • makes the INFC (maximum of TeV, minimum of GeV) the best place to look for pulsations (thepulsed fraction is the highest)34

Are we far? Color lines are PWZ models, as in Sierpowska-Bartosik & DFT 2008 The pulsar model is a typical Fermi spectrum, added ad-hoc. 35

Concluding remarks -Ground-based Cherenkov arrays and Fermi have confirmed that binaries are sources of HE/VHEgamma-rays. -High mass companions influence lead to orbital VHE modulation (absorption & cascadinginvolved) -Binary plerions are a new window for pulsar wind physics.-All in all, the possibility of pulsar-composed gamma-ray binaries remain as viable theoretical andphenomenological alternative, if not the preferred one based on MW observations -But it is not a proven one yet.-Future experiments in the 10-100 GeV range are crucial to disentangle model features -We need more binaries to play. Thank you! 36

Accretion signatures: the current statusLS I +61 303 95 ks Chandraobservation near apastron For the total spectrum, the 3σ upper limit on theequivalent width of the presence of lines range between50 − 550 eV depending on the energy (between 0.3–10keV) and assumed line width (σ E = 0.05 − 0.5 keV).Spectra of the 5 time intervals fitted with an absorbed power-law function and 1σ sensitivity for thepresence of spectral lines (defined as the 1σ limit for the detection of flux variability on top of thecontinuum model as a function of energy, lines as broad as the energy-dependent instrument resolution). from Rea, DFT, van der Klis, Jonker, Mendez, Sierpowska-Bartosik 2010 38

Gamma-ray pulsation limits in context • X-ray pulsations from rotational powered pulsars have been detected in tens of objects, ofwhich only fourteen are to date also TeV emitters (see e.g. Mattana et al. 2009), due to thepresence of an energetic Pulsar Wind Nebula (PWN).Features of isolated rotational-powered X-ray emitting pulsars Rotational powerPulsation period Characteristic ages Magnetic fieldsX-ray pulsed fractionsRange for physical value 3 × 10 35 < L sd < 5 × 10 38 erg s −1 33 18 % • A PF < 15% for spin periods as fast as 5.6 ms (much faster than the fastest rotationalpowered pulsar known to be a TeV emitter), indicates that if the compact object hosted is arotational-powered pulsar, its X-ray emission cannot be dominated by synchrotron emissionfrom the pulsar magnetosphere alone… or the beam is really mis-pointed.See Mattana et al. 2009 for a compilation of physical values 39

Why is a non-accreting pulsar a tenable alternative?4) The radio morphology is stable and no jets are seen in any scale 4a) A pulsar-wind like feature? • No large features or high-velocity flows were noted in any of the observing days,which implies at least its non-permanent nature.• The changes within 3 hours were found to be insignificant, so the velocity of theoutflow can not be much over 0.05c. • The position and direction of the cometary tail can be understood if the pulsar movesin a dense, slow equatorial wind LS I +61 303 e.g., Dhawan et al. 2006, Albert et al. 2009, montage on the right by Dubus 2008; there is a similarly changing radio morphology in LS 5039, see Ribó 2008 40

Gamma-ray binaries as non-accreting pulsar systems

Create successful ePaper yourself

Delete template?

Save as template?