May 2008 - Ljubljana

1. Magnetic Field Upper Limits for Jet Formation in X-ray binaries & AGNs M. Kaufman Bernadó1,* & M. Massi1 1Max Planck Institut für Radioastronomie, Bonn, Germany *Humboldt Research Fellow May 2008 - Ljubljana

2. INDEX 1. Introduction 2. Jet formation cycle A and B – Basic condition 3. Neutron Star X-ray Binaries 4. Black Hole X-ray Binaries and Supermassive Black Holes 5. Implications 6. Summary

3. INDEX 1. Introduction 2. Jet formation cycle A and B – Basic condition 3. Neutron Star X-ray Binaries 4. Black Hole X-ray Binaries and Supermassive Black Holes 5. Implicactions 6. Summary 1. Introduction

4. X-ray Binary System X-ray Binary System Microquasar Accretion Disk: X-ray emission Compact Object: ACCRETOR Neutron Star or Black Hole Companion Star: MASS DONOR Low- or High-Mass Microquasars are defined as the XRB systems where either high-resolution radio interferometric techniques have shown the presence of collimated jets or a flat/inverted radio spectrum has been observed (indirect evidence of an expanding continuous jet). The nature of the compact object, NS or BH, is still uncertain for several microquasars.

5. Magnetic Field Upper Limits for Jet Formation Necessary initial condition: alow magnetic fieldat the NS surface or at the last stable orbit of the accretion disk of a BH. Aim: to quantify this important parameter and therefore give an upper limit for the magnetic field strength for which an ejection could happen in a NS or BH XRB system, as well as to predict the corresponding behaviour for Active Galactic Nuclei using standard scaling. When will an accreting NS become a microquasar and when, on the other hand, an X-ray pulsar? When will a BH XRB system be able to evolve into a microquasar phase?

6. INDEX 1. Introduction 2. Jet formation cycle A and B – Basic condition 3. Neutron Star X-ray Binaries 4. Black Hole X-ray Binaries and Supermassive Black Holes 5. Implications 6. Summary 2. Jet formation cycle A and B Basic condition

7. Initial Condition for Jet Formation: twisted B Magnetic Lines are compressed Cycle A PB < Pp PB AMPLIFIED PB vs Pp START PB > Pp increase The formation of a jet is based on a competition process between the magnetic field pressure, PB, and the plasma pressure, Pp. Summarised in a flowchart. Because of the increasing compression of the magnetic field lines, the magnetic pressure will grow and may become larger than the gas pressure on the surface of the accretion disk, where the density is lower. Then, the magnetic field becomes “active”, i.e. dynamically dominant, PB > Pp,and the plasma has to follow the twisted magnetic field lines, creating two spinning-plasma flows. The strength of the large-scale poloidal field must be low enough that the Pp dominates PB (Blandford 1976). Only under that condition, PB < Pp, the differentially rotating disk is able to bend the magnetic field lines in a magnetic spiral (Meier et al. 2001). Numerical simulations show that the launch of a jet involves a weak large-scale poloidal magnetic field anchored in rapidly rotating disks or compact objects (Meier et al. 2001).

8. START PB > Pp B Twisted? increase increase NO YES two spinning plasma flows QUIESCENT no JET is formed a JET is formed BH: LOW/HARD ------------ NS: IS / HB Neutron Star: X-ray Pulsar Cycle B new compression of the magnetic lines reconnection BH: VERY HIGH ----------- NS: NB stored magnetic energy released untwisted B BH: HIGH/SOFT ------------- NS: BS / FB The generation of jets and their presence in XRBs is coupled to the evolution of a cycle that can be observed in the X-ray states of this kind of systems. We therefore complement the jet formation flowchart showing the parallelism between the presence of a jet and the different X-ray states.

9. The strong magnetic field cannot be twisted and is dynamically dominant: the plasma is forced to move along the magnetic field lines and converges onto the magnetic poles of the neutron star. There, it releases its energy, creating two X-ray emitting caps (Psaltis 2004).

10. The distance at which the magnetic and plasma pressure balance each other. Alfvén Radius RA / R* = 1 NS Surface Radius RA / RLSO = 1 The Basic Condition BH Last Stable Orbit Magnetic Field Upper Limit PB < Pp JET FORMATION

11. INDEX 1. Introduction 2. Jet formation cycle A and B – Basic condition 3. Neutron Star X-ray Binaries 4. Black Hole X-ray Binary and Supermassive Black Holes 5. Implications 6. Summary 3. Neutron Star X-ray Binaries

13. for NS XRBs, Using observed values of B and Classical X-ray Pulsars ms X-ray Pulsars Atoll-sources Z-sources Millisecond (ms) X-ray Pulsars: - rapidly spinning - few detected / all LMXB Classical X-ray Pulsars: - called “slow” accretion-powered pulsars, period ~ 1s or more. - HMXB (five LMXB) Atoll and Z - Sources: - LMXBs divided in these two types depending on their timing and spectral properties - Z-type have larger mass accretion rates than Atoll-type

14. The intersection between the function, RA/R*, and the basic condition plane, RA/R*=1, indicates the combination of the magnetic field and the mass accretion rate values for which plasma pressure and magnetic field pressure balance each other at the surface of the star. This ensures that the initial condition for jet formation, PB < Pp is fulfilled over the whole accretion disk.

15. Z sources Atoll Sources ms X-ray Pulsars 108.2 G 107.7 G 107.5G Theses theoretical values are in complete agreement with the up to now existing observational data: For their average B~108 G, the basic condition would only be fulfilled for a mass accretion rate , , whereas the maximum observed accretion rate is nearly one order of magnitude lower, . In fact, in the millisecond source SAX J1808.4-3658, which shows hints for a radio jet, during bright states, peak values of were measured and the upper limit of the magnetic field strength was found to be a few times 107 G (Gilfanov et al. 1998). Instead, if Upper Limit for B The association of a classical X-ray pulsar (B ~ 1012 G) with jets is excluded even if they accrete at the Eddington critical rate. Millisecond X-ray pulsarcould switch to a microquasar phase during maximum accretion rate. We see that Atoll-sources are indeed potential sources for generating jets and in fact they have not only been detected in radio (Fender & Hendry 2000; Rupen et al. 2005) but more recently evidence of jets in these sources has been found (Migliari et al. 2006 and Russell et al. 2007). The magnetic field strength has been determined in a Z-source, with jets, Scorpius X-1, from magnetoacoustic oscillations in kHz QPO reaching values of 107-8 G (Titarchuk et al. 2001). Classical X-ray pulsar: in agreement with the systematic search of radio emission in this kind of sources with so far negative result (Fender et al. 1997; Fender & Hendry 2000; Migliari & Fender 2006).

16. INDEX 1. Introduction 2. Jet formation cycle A and B – Basic condition 3. Neutron Star X-ray Binaries 4. Black Hole X-ray Binaries and Supermassive Black Holes 5. Implications 6. Summary 4. Black Hole X-ray Binaries & Supermassive Black Holes

20. Schwarzschild Stellar Mass BH Kerr Stellar Mass BH Upper Limit for B with Eddington mass accretion rate Stellar-Mass BHSchw Stellar-Mass BHKerr 1.35 x 108 G BHSchw with BHKerr 5 x 108 G

21. Upper Limit for B with Eddington mass accretion rate Schwarzschild and Kerr Supermassive BHs SupermassiveBHSchw Supermassive BHKerr 105.4 G 105.9 G SBHSchw SBHKerr Straightforward dependency of the magnetic field strength with the mass of the BH allowing us to establish its upper limit for the jet formation in the case of supermassive BHs as well:

22. Note: in the specific case of a supermassive Schwarzschild BH of 108 we get B < 104.3 G. For a BH of the same mass Blandford & Payne (1982) established B < 104 G at 10rg. Scaling our value, which is relative to RLSO=6rg, to 10rg, we get B < 104 G in complete agreement with the results of Blandford & Payne (1982).

23. INDEX 1. Introduction 2. Jet formation cycle A and B – Basic condition 3. Neutron Star X-ray Binaries 4. Black Hole X-ray Binaries and Supermassive Black Holes 5. Implications 6. Summary 5. Implications

24. Nature of the compact object in XRB systems ms X-ray pulsars spin distribution The analysis of the basic condition for jet formation has two interesting possible implications:

25. The nature of the compact object in XRB systems & the magnetic field decay Measurements of surface magnetic field strengths by cyclotron resonance effects were carried out in a dozen of X-ray classical pulsars show that B is tightly concentrated over: (1 - 4) x 1012 G (Makishima et al. 1999) To achieve the basic conditions for forming a jet the field must decay to B=107-8 G

26. Therefore this kind of magnetic field decay process excludes the possibility of a NS-HMXB evolving into a microquasar phase since this decay is longer than the lifetime of the high-mass companion star, 107 yr for . The nature of the compact object in XRB systems & the magnetic field decay Analysis of pulsars data have indicated that B decays 4 orders of magnitude by Ohmic dissipation in a timescale longer than 109 yr (Konar & Bhattacharya 2001). In this case then, the only possible accretor would be a Black Hole.

27. The nature of the compact object in XRB systems & the magnetic field decay Faster decays of the magnetic field can occur with the high-accretion-induced crust screening process. (Zhang 1998) Circinus X-1 is an XRB with a type I X-ray bursts NS. It has a confirmed jet, so it is a microquasar. • It is so young that its orbit has not yet had time to become circular circularization time ~ 105 yr. • (Ransom et al. 2005) • It has already reached B to fulfill the basic condition for jet formation. In fact, Romani (1995) has deduced a characteristic timescale for the initial field decay by screeing in the range of 104 yr < t < 106 yr.

28. The nature of the compact object in XRB systems & the magnetic field decay A decay in the B due only to Ohmic dissipation implies the presence of a BH as the compact object in a microquasar-HMXB because of the long timescales of this process. Only in the case of high-accretion-induced crust screening process the timescales can be as short as t ~ 105 yr and the issue of the nature of the compact object remains open.

29. ms X-ray pulsars spin distribution One of the major open issues concerning millisecond X-ray pulsars is the absence of sub-millisecond X-ray pulsars. The spin distribution sharply cuts off well before the strict upper limit on the NSs spin rate that is given by the centrifugal breakup limit (0.3 ms depending on the NS equation of state). The physics setting that limit is unclear. (Chakrabarty 2005) Due to the possibility of jets in millisecond X-ray pulsars, then the jet might be the suitable agent of angular momentum sink, as in the bipolar outflows from young stellar objects. The transport rate of angular momentum by the jet can be two thirds or more of the estimated rate transported through the relevant portion of the disk. (Woitas et al. 2005)

30. INDEX 1. Introduction 2. Jet formation cycle A and B – Basic condition 3. Neutron Star X-ray Binaries 4. Black Hole X-ray Binaries and Supermassive Black Holes 5. Implications 6. Summary 6. Summary

31. Z sources Atoll Sources 108.2 G Supermassive BHSchw Supermassive BHKerr 107.7 G 105.4 G Stellar-Mass BHSchw Stellar-Mass BHKerr Implications 105.9 G 1.35 x 108 G 107.5G,and max. ms X-ray Pulsars • Compact object nature vs. B decay • ms X-ray pulsars spin distribution vs. presence of jets 5 x 108 G Magnetic Field Upper Limits for Jet Formation • The association of a classical X-ray pulsar - B ~ 1012 G - with jets is excluded.