Wind accretion in supergiant X-ray binaries A coherent picture within the porous wind framework Ignacio Negueruela Universidad de Alicante Granada May 2008

José Miguel Torrejón Universidad de Alicante & M.I.T. Silvia Martínez-Núñez Universidad de Alicante Pablo Reig University of Crete David M. Smith UCSC Pere Blay Universidad de Valencia Marc Ribó Universitat de Barcelona Granada May 2008

Accretion from the wind of a supergiant Accretion from the wind of a supergiant Roche-lobe overflow Roche-lobe overflow High Mass X-ray binaries Be/X-ray binaries

New “classes” of HMXBs found by INTEGRAL IGR J and a few other very absorbed sources. Most sources likely to be similar to old classes but more obscured. A group of flaring sources with very short outbursts and supergiant companions ( Smith et al. 2006, ApJ 638, 974; Negueruela et al. 2006, ESA-SP 604 (1), 165 )

Supergiant Fast X-ray Transients Very short (only a few hours) outbursts with complex structure ( Sguera et al. 2005, A&A 444, 221; 2006, ApJ 646, 452 ) X-ray spectra are hard and look typical of neutron stars in HMXBs ( González-Riestra et al. 2004, A&A 420, 589; Smith et al ) Several examples of sudden rises from L X < erg s -1 to L X  erg s -1 in minutes ( in’t Zand 2005, A&A 441, L1; Bamba et al. 2001, PASJ 52, 1179; Sakano et al. 2002, ApJS 138, 19 ) Lightcurve from XTE J during an outburst observed by INTEGRAL on 2003 March 22nd (Sguera et al. 2005)

Wind accretors High Mass X-ray binaries

Supergiant X-ray binaries ObjectPulseCounterpartPeriod Typical L X (erg s -1 ) 2S sB1 Iab11.6 d~ Vela X sB0.5 Iab8.9 d~ E sB2 Iae14.4 d~ GX sB1 Ia d~ U sB0 I3.7 d~ OAO sB IB I10.4 d~ U NOO6.5 Iaf+3.4 d~ U sO8 I8.4 d~ Cyg X-1 BHO9.7 Iab5.6 d~ 10 37

Vela X-1: Short term flaring Long term variability by a factor of 4 Supergiant X-ray binaries Flare from 4U Fritz et al (A&A 458, 885) Ribó et al (A&A, 449, 687)

Walter & Zurita Heras (2007, A&A 476, 335) attempt to define SFXTs with quantitative criteria: Count rate contrast > 100 in INTEGRAL passbands Outbursts last for hours. Typical (average) duration is 3ks for the strong flares and  4h for the whole outburst. A working definition of SFXTs What do they do when not detected by INTEGRAL? Sidoli et al. (2008, arXiv: ) carry out monitoring with Swift. Occasionally, they are at L X < erg s -1 Most of the time, they seem to emit at L X  erg s -1 (perhaps depending on source)

INTEGRAL long-term lightcurve of XTE J From Blay et al. (2008, A&A, soon) See poster by S. Martínez-Núñez

Activity from XTE J during GC monitoring September 2006 March 2007 August 2007 From Blay et al. (2008, A&A, soon)

Activity from XTE J during GC monitoring September 2006 March 2007 Detection limit L X > erg s -1 See poster by S. Martínez-Núñez

IGR J ksec Suzaku exposure on IGR J (PI Smith) Quiescence 1x10 33 ergs -1 Outburst 1.2x10 36 ergs -1

Wind accretors as seen by INTEGRAL Persistent SGXBs Irregularly flaring SFXTs (defined as variability factor >100 by Walter & Zurita Heras (2007, A&A 476, 335 ) XTE J , IGR SAX J IGR J Intermediate systems (smaller variability) AX XTE J Regular outbursters IGR J , IGR J

Parameters of SFXTs Optical counterpart to AX (VLT+FORS1) IGR J B0.5 Ib

Radiative winds as accretion fodder Heavy ions have large Thompson cross sections The  law   0.8 – 1.2 Review: Kudritzki & Puls 2000, ARA&A, 38, 613

Where are the low luminosity SGXBs?

The source of the instability Images stolen from Stan Owocki

Development of instability Velocity Density smooth wind Images stolen from Stan Owocki Owocki & Rybicki 1984, ApJ, 284, 337 cf. Feldmeier et al. 1997, A&A, 322, 878

Wind clumping Clumping factor Size and geometry of clumps Shells or blobs Optically thin? 1D simulations Runacres & Owocki 2002, A&A, 381, D simulations Dessart & Owocki 2003, A&A, 406, L1 Porous winds Owocki et al. 2004, ApJ, 616, 525 Oskinova et al. 2006, MNRAS, 372, 313 Constraints from spectra Prinja et al. 2005, A&A 430, L41 Bouret et al. 2005, A&A, 438, 301 Puls et al. 2006, A&A, 454, 625

Wind clumping If winds are clumped, Is the smooth wind approximation completely invalid? Why does it sort of work for SGXBs?

Porous winds We have used the “porous wind” model by Oskinova et al. (2007, A&A 476, 1331) Results do not depend strongly on model used Clumpiness parameterised by a single factor L 0, which must take values L 0  to fit optical and UV observations Taking L 0  0.2, we have a few 10 3 clumps out to 10 R *.

The porous wind as “seen” by the neutron star Number of clumps that will be inside the accretion radius of the neutron star in one orbit

Classical supergiant systems The neutron star is always inside the region where it sees most of the wind Circularised orbits help it not to get outside Note that SGXBs with an O-type supergiant do not evolve into SGXBs with B1-2 companions. They go TZO??

Supergiant fast X-ray transients The neutron star is in a region where for relatively frequent outbursts. Such systems may eventually evolve into SGXBs. But we still probably require

Eccentric SFXT Eccentricity results in systems that may show (quasi-)periodic changes in their behaviour

Regular outburster Neutron stars in systems with wide eccentric orbits spend most of the time in regions where they cannot accrete. P orb =15.7 d, BN0.5 II-III P orb =165 d, B0.7 Ia IGR J IGR J

Alternatives: the disk “model” Proposed by Sidoli et al. (2007, A&A 476, 1307) based on properties of IGR J Based on an object which is not an SFXT Has no physical motivation Requires huge disks around OB supergiants that should have observational signatures Requires SFXT outbursts to happen at regular outbursts against observations Is incompatible with observed lightcurves

IGR J ESO 2.2m + FEROS Dec 2006 to Feb 2007

Alternatives: centrifugal inhibition First proposed by Grebenev and Sunyaev (2007, AstL 33, 149) requires the neutron stars to be spinning close to their equilibrium period. There is no reason to expect normal neutron stars with B  G to be rotating at their equilibrium period. May make sense if B can have a wide range of values In this case, SFXTs should host magnetars ( Bozzo et al arXiv: )

Wind accretors: a coherent picture Warning: wind clumping is a working hypothesis. Physical parameters of clumps are unconstrained. However, the scenario presented is independent of clumping details. Values favoured are compatible with those derived from optical and UV observations of wind lines (e.g., Oskinova et al ). Calculations in good agreement with independent estimates by Walter & Zurita- Heras (2007).

Wind accretors: a coherent picture The scenario presented provides a coherent framework where all wind accretors fit. Peculiarities can be explained as due to particularities within the framework. It provides an explanation for both the outbursts and the quiescence of SFXTs. In addition, it explains at once some puzzling properties of SGXBs. However, it does not exclude that other mechanisms are also at work.