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Advances in Space Research 52 (2013) 732–739

The case for massive, evolving winds in black hole X-ray binaries

Joey Neilsen ⇑,1

Boston University Department of Astronomy, 725 Commonwealth Avenue, Room 416D, Boston, MA 02215, United States

Received 30 November 2012; received in revised form 18 April 2013; accepted 22 April 2013Available online 3 May 2013

Abstract

In the last decade, high-resolution X-ray spectroscopy has revolutionized our understanding of the role of accretion disk winds inblack hole X-ray binaries. Here I present a brief review of the state of wind studies in black hole X-ray binaries, focusing on recent argu-ments that disk winds are not only extremely massive, but also highly variable. I show how new and archival observations at high timingand spectral resolution continue to highlight the intricate links between the inner accretion flow, relativistic jets, and accretion disk winds.Finally, I discuss methods to infer the driving mechanisms of observed disk winds and their implications for connections between massaccretion and ejection processes.� 2013 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: accretion, Accretion disks; Black hole physics; Stars: winds, outflows; X-rays: binaries; X-rays: individual

1. Introduction

In the last 20 years, we have seen the discovery of a mul-titude of highly-ionized absorbers in moderate and high-resolution X-ray spectra of black hole and neutron starX-ray binaries (e.g. Ebisawa, 1997; Kotani et al., 1997;Brandt and Schulz, 2000; Kotani et al., 2000a; Kotaniet al., 2000b; Lee et al., 2002; Sidoli et al., 2001, 2002;Schulz and Brandt, 2002; Parmar et al., 2002; Boirin andParmar, 2003; Boirin et al., 2004; Boirin et al., 2005; Milleret al., 2004, 2006a,b, 2008; Miller et al., 2011; Neilsen andLee, 2009; Neilsen et al., 2011, 2012a; Neilsen and Homan,2012; Ueda et al., 2004, 2009; Martocchia et al., 2006;Kubota et al., 2007; Blum et al., 2010; Reynolds andMiller, 2010; King et al., 2012a; Dıaz Trigo et al., 2006,2007, 2009, 2012; Diaz Trigo et al., 2012). Often theseabsorbers are blueshifted, indicative of hot outflowinggas, i.e. accretion disk winds. The prevalence of disk windsin X-ray binaries suggests that these outflows may play acrucial role in the physics of accretion and ejection around

0273-1177/$36.00 � 2013 COSPAR. Published by Elsevier Ltd. All rights rese

http://dx.doi.org/10.1016/j.asr.2013.04.021

⇑ Tel.: +1 617 353 1533.E-mail address: neilsenj@bu.edu.

1 Einstein Fellow, Boston University.

compact objects. In this brief review, I discuss some recentdevelopments in the influence of ionized disk winds aroundblack holes.

2. Black hole accretion disk winds and the disk-jet connection

Much of the recent work on accretion and ejection pro-cesses in black hole outbursts has focused on radio/X-raycorrelations (e.g. Gallo et al., 2003; Corbel et al., 2003;Fender and Belloni, 2004; Fender et al., 2004, 2009,although see e.g. Gallo et al., 2012 and references thereinfor lingering questions about the precise nature of thesecorrelations). Briefly, we now know that typical black holetransients emerge from quiescence in X-ray hard states thatproduce steady, compact jets. They rise in luminosity inthis (probably radiatively inefficient; e.g. Esin et al., 1997)hard state, until at some point they undergo a transitiontowards a much softer state, possibly dominated by a rad-iatively-efficient disk. This transition has also been associ-ated with major relativistic plasma ejections and thedisappearance of steady jets. Eventually, the luminosityfalls and they return to quiescence via the hard state.

Over the last decade, this canonical picture of the“disk-jet connection” has proved to be a fruitful way to

rved.

1 2 5

0.1

1

Energy (keV)

Phot

ons

cm−

2 s−

1 keV

−1

ObsID 5460

ObsID 5461

MJD 53441

MJD 53461

10−

410

−3

0.01

0.1

1

Phot

ons

cm−

2 s−

1 keV

−1

MJD 53441

MJD 53461

105 20−5

05

χ

Energy (keV)

Fig. 1. Spectra of GRO J1655�40 from Neilsen and Homan (2012). (�2012. The American Astronomical Society. All rights reserved.) In both panels,black is the spectrum of the hard state and blue is the spectrum of the softer state. Left: Chandra HETGS spectra show only a single line during the firstobservation, but a rich series of lines from the accretion disk wind in the softer state. Right: RXTE PCA show significant differences in the correspondingbroadband X-ray spectra, but we argue (Section 2.1) that the changes in the ionizing flux cannot explain the differences in the lines.

J. Neilsen / Advances in Space Research 52 (2013) 732–739 733

characterize accretion and ejection processes around stel-lar-mass black holes, and has become the backbone ofour understanding of the spectral and timing behavior ofblack hole transients. But this story cannot be complete,for it fails to describe or account for the presence or theinfluence of another mode of mass ejection: highly-ionizedaccretion disk winds, whose behavior in outburst is onlynow becoming clear.

Just three years after the launch of Chandra, Lee et al.(2002) argued that winds could be associated with theaccretion disk, although they were not confined to disk-dominated states. Miller et al. (2008) confirmed that inboth GRO J1655�40 and GRS 1915+105, absorptionlines were stronger in spectrally soft states (see also Neil-sen and Lee, 2009). They suggested that higher ionizingflux might be responsible for the changes in the winds,but left open the possibility that other (e.g. geometric)changes might be required as well. Thus it remainedunclear how or why winds might change on outburst timescales: were they steady, passive bystanders that simplyresponded to variations in the ionizing flux, or did theyplay a role in outbursts, appearing and disappearing justlike jets?

2 For interpretation of color in Fig. 1, the reader is referred to the webversion of this article.

2.1. A case study in evolving winds

With two high-resolution Chandra HETGS observa-tions of accretion disk winds separated by less than threeweeks (Miller et al., 2008; Neilsen and Homan, 2012), the2005 outburst of the microquasar GRO 1655�40 presentsan ideal backdrop against which to test the hypothesis thatwinds do not evolve during outburst. The Chandra andRXTE spectra are shown in the left and right panels ofFig. 1, respectively. The first observation (shown in black)took place during a hard state, while the second observa-

tion (shown in blue)2 occurred during a much softer state.And while the first observation contained an FeXXVI

absorption line near 7 keV, the second provided an extre-mely rich absorption line spectrum that has been studiedin great detail (Miller et al., 2006a; Netzer, 2006; Milleret al., 2008; Kallman et al., 2009; Neilsen and Homan,2012; see Section 3 for a discussion of the origin of thiswind).

Here, let us consider the question: why are the twoChandra absorption line spectra so different? Are the differ-ences driven by changes in the photoionizing flux from thehard state to the soft state, or did the wind physicallyevolve over those 20 days? Our detailed analysis (Neilsenand Homan, 2012) indicates that the wind must haveevolved significantly between the two Chandra observa-tions. This argument can be understood both qualitiativelyand quantitatively:

1. A comparison of the hard state and soft state PCA spec-tra in Fig. 1 reveals a clear excess of photons withE > 10 keV, which we usually think of as ionizing pho-tons. Thus, at first glance it seems plausible that changesin the ionizing flux could explain the differences in thelines. In fact, however, the ionization of this wind is deter-

mined primarily by soft X-rays, since many of the visibleions during the softer state, like O, Ne, Na, Mg, Al, andSi, are effectively transparent to hard X-rays (due totheir small cross-sections above 10 keV). Since the softX-ray spectra of the two observations are quite similar,we conclude that the change in the relevant ionizing fluxis negligible and cannot, in and of itself, explain theobserved differences in the lines.

0.1

1Ph

oton

s cm

−2 s

−1 k

eV−

1

2 4 6 8

11.

5R

atio

Energy (keV)

Predicted but not observed

Fig. 2. Photoionization models of a steady wind in GRO J1655�40 fromNeilsen and Homan (2012). (�2012. The American Astronomical Society.All rights reserved.) Based on Kallman et al. (2009). If the same wind werepresent in both Chandra HETGS observations, we should have detected anumber of strong absorption lines during the hard state. The absence ofthese lines indicates wind variability over the course of the outburst.

734 J. Neilsen / Advances in Space Research 52 (2013) 732–739

2. The physical properties of the rich absorber during thesoft state are well known (Miller et al., 2006a, 2008;Kallman et al., 2009), so we can use photoionizationcodes like XSTAR (Bautista and Kallman, 2001) and the1 eV–1 MeV radiation field to generate predictionsabout ionized absorption during the hard state. Ourresults (Fig. 2; Neilsen and Homan, 2012) clearly indi-cate that a number of strong absorption lines would have

been visible during the hard state if the wind had beensteady; the non-detection of these lines confirms thatthe wind must have evolved during those 20 days. Forseveral simple but realistic scenarios for the geometricalevolution of the wind (Neilsen and Homan, 2012), weargue that the variations in its ionization and columndensity likely imply an increase in the density and massloss rate in the wind by a factor between 25 and 300.

To summarize briefly, after Lee et al. (2002): “ionizing fluxis only part of the solution.” Based on our careful treat-ment of photoionization, we find compelling qualitativeand quantitative evidence for significant physical changesin the accretion disk wind during the 2005 outburst ofGRO J1655�40. In the following section, we argue thatbroad parallels between this source and other black holessupport the conclusion that evolving winds may be anextremely common, if not universal, phenomenon.

2.2. Ubiquitous, massive evolving winds

As noted above, Lee et al. (2002) and Miller et al. (2008)pointed out that accretion disk winds seem to be associatedwith the accretion disk and/or spectrally soft states. Recentwork by Ponti et al. (2012) provides convincing evidencefor these earlier claims: their archival study of Chandra

HETGS, XMM-Newton, and Suzaku observations of stel-lar mass black holes in outburst demonstrates that accre-tion disk winds are preferentially detected in softerstates.3 In particular, winds are ubiquitous along thehigh-luminosity branch of the hardness-intensity diagramafter the spectrally hard state (see Lee et al., 2002; Milleret al., 2008; Neilsen and Homan, 2012 for rare cases ofweak winds at the high luminosity end of spectrally hardstates). In some cases, stringent upper limits have beenplaced on the existence of hard state winds (e.g. Blumet al., 2010; Miller et al., 2012).

Ponti et al. (2012) suggest that a static absorber with avariable ionization parameter may be unlikely to explaincompletely the observed behavior of winds in black hole

3 Note that disk winds are only detected in systems seen at high orbitalinclination, which is consistent with the interpretation that these absorbersare localized near the equatorial plane of the disk. Based on theinclinations of systems with detected disk winds, Ponti et al. (2012)estimate a typical wind opening angle hK 30�. This conclusion echoesestablished results from studies of neutron star low-mass X-ray binaries,where X-ray absorbers are known to be concentrated near the disk plane(e.g. Sidoli et al., 2001; Boirin and Parmar, 2003; Dıaz Trigo et al., 2006).

outbursts, although it is also noted that ionization effectsmay be important and that only detailed photoionizationstudies can confirm this suggestion. While this is certainlytrue, black hole wind variability studies on time scales fromseconds (Neilsen et al., 2011) to hundreds or thousands ofseconds (Lee et al., 2002; Miller et al., 2006b) to weeks andyears (Neilsen and Homan, 2012; Blum et al., 2010; Milleret al., 2012) have all required changes in the wind density.It therefore seems likely that the observed outburst behav-ior of winds will also require such changes, in which casewe can conclude that disk winds are preferentially butnot exclusively launched at high luminosity, around or afterthe time the black hole begins to exit the spectrally-hardstate.

To test this interpretation, we undertook a Chandra

HETGS campaign to catch this phase of a new outburst;the resulting spectra of 4U 1630�47 are shown in Fig. 3.The results will be published in detail in future work (Neil-sen et al., in preparation), but suffice it to say here that withthe robust detection of a strong outflow, this campaign wasremarkably successful. It should be noted that Kubotaet al. (2007) detected a wind in a similar phase of a prioroutburst of 4U 1630�47, so our new detection confirmsthat wind behavior is predictable. We conclude that windsare reliably launched during this outburst phase in blackhole X-ray binaries; confirming their absence or weaknessin harder and less luminous states will be the subject offuture work.

If winds were simply ionized gas along the line of sight,such a conclusion might be interesting but relatively insig-nificant. In reality, there is now a large and growing bodyof evidence indicating that disk winds in stellar mass blackholes may be extremely massive. In fact, as early as a dec-ade ago, it was discovered that wind mass loss rates _Mw

J. Neilsen / Advances in Space Research 52 (2013) 732–739 735

could be comparable to black hole accretion rates _Macc

(Lee et al., 2002). More recently Neilsen and Lee (2009)suggested that radiatively/thermally-driven winds coulddeplete the mass of the disk enough to suppress relativisticjets. In a few exceptional cases (e.g. the ‘heartbeat’ state ofGRS 1915+105, Neilsen et al., 2011; IGR J17091�3624,King et al., 2012a), detailed studies have found mass lossrates in excess of ð10� 20ÞMacc! These remarkable results,too, are supported by the results of Ponti et al. (2012), whofind that _Mw is typically at least twice _Macc, and approaches10 _Macc at high Eddington ratio.

If winds are truly as massive as these results suggest, itbegins to seem significant that they are preferentiallylaunched at the same phase of black hole outbursts whenwe observe the disappearance of steady jets and majorchanges in the structure of the accretion flow. Could it bethat disk winds are indeed the mechanism by which jetsare suppressed and state transitions take place, as sug-gested by Neilsen and Lee (2009) and Neilsen et al.(2011)? At present, the data cannot rule out this interpreta-tion, but with careful tracking of _Mw going into this statetransition, it may be possible to shed new light on thisimportant question in the near future.

3. On inferring wind driving mechanisms

As noted in Section 2, in the last few years there havebeen a number of developments that suggest deep connec-tions between the X-ray luminosity or accretion rate, thestate of the accretion flow, and the behavior of accretiondisk winds. The significance of such connections is notentirely clear, however: Miller et al. (2012) (see also Milleret al., 2008) have argued that these some of these connec-tions can be explained in terms of a magnetic field config-uration that changes during outburst, while other authorshave presented interpretations based on radiatively- andthermally-driven winds (Neilsen and Lee, 2009; Ueda

0 20 40 60 80

00.

20.

40.

6

Time (Days Since MJD 55910)

2−20

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MA

XI

Flux

(C

rab)

Fig. 3. Left: MAXI monitoring of the 2011–2012 outburst of 4U 1630�47, witand other wavelengths indicated by downward arrows. Right: Chandra’s high-rof the outburst indicated by Ponti et al. (2012).

et al., 2009; Ueda et al., 2010; Neilsen et al., 2011; Neilsenet al., 2012a; Ponti et al., 2012).

Because the detailed implications of these massive evolv-ing winds depend heavily on their formation physics, it iscritical to draw robust conclusions about the mechanismsthat produce them. To this end, we often take advantageof the fact that the well-known driving mechanisms (radia-tion pressure, Compton heating, and MHD) typically oper-ate in different regimes of density, ionization, and distancefrom the black hole (e.g. Proga and Kallman, 2002; Milleret al., 2008).

For example, since radiation pressure is most commonlytransmitted via UV resonance absorption lines, it may beineffective when the gas has little or no opacity in the UV(e.g. at very high ionization, nJ 103 ergs cm s�1; Progaand Kallman, 2002). In contrast, Compton heating mayproduce highly-ionized outflows, but because they requirea large surface area of gas to be heated to the point thatthe sound speed exceeds the escape speed, thermally-drivenwinds are only expected at large distances from the blackhole (104 � 105 rg; Begelman et al., 1983; Woods et al.,1996). In addition, simulations at varying luminositiesand spectral shapes (e.g. Woods et al., 1996) have shownthat thermal driving tends to produce outflows with smallmass fluxes and/or gas densities (n K 1012 cm�3 for the spe-cific case of GRO J1655�40; Luketic et al., 2010). MHDprocesses like the Blandford–Payne mechanism, however,may produce dense outflows at small radii (Blandfordand Payne, 1982).

3.1. Absorption lines, density, and GRO J1655�40

In principle, then, it is possible to use the observed prop-erties of winds to infer their launching mechanisms. Thedifficulty is that many physical factors influence the observ-ability of lines. For example, the X-ray luminosity sets theionization parameter of the gas:

0.01

0.1

5×10

−3

0.02

0.05

Phot

ons

cm−

2 s−

1 keV

−1

102 5

0.5

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atio

Energy (keV)

h Chandra HETGS and Suzaku observations indicated by upward arrows,esolution spectra reveal a strong accretion disk wind at precisely the phase

5 This was a long-awaited discovery, particularly since it had beenknown for some time that both accretion disks and relativistic jets are

736 J. Neilsen / Advances in Space Research 52 (2013) 732–739

n ¼ Lnr2

; ð1Þ

while the shape of the ionizing spectrum and the gas den-sity determine which ions are visible at any n (Kallmanand Bautista, 2001). The density and geometry of the winddetermine the equivalent hydrogen column density of theabsorber:

NH ¼ nDr ð2Þ

and the column density of each individual ion follows fromNH, the chemical abundances Ai, and the ionization balancexi:

Ni ¼ xiAiNH: ð3Þ

Finally, the ion column densities are folded into the equiv-alent width W k of each line via the curve of growth4:

W k

k¼ pe2

mec2Nikfji: ð4Þ

Here k and fji are the line wavelength and oscillatorstrength, respectively. Given the complexity of the connec-tions between the gas properties, the radiation field, andillumination patterns, it is not immediately obvious howany observed wind is produced. However, for a wind thatis both very highly ionized (nJ 103 ergs cm s�1) and suffi-ciently dense that its implied radius (c.f. Eq. 1) is well insidethe radius where thermal driving is effective (i.e. the Comp-ton radius, R� RC � 1011�12 cm), the natural conclusion isthat MHD processes likely play a role in its launching.

The classic example of this argument in black hole X-raybinaries comes from Miller et al. (2006a), who first pub-lished the extraordinary Chandra HETGS absorption linespectrum of GRO J1655�40 (Fig. 1). Photozioniationanalysis indicated a characteristic ionization parameter ofnJ 104 ergs cm s�1, an order of magnitude too high forline driving to be effective. In addition to many other lines,they detected two Fe XXII absorption lines at 11.77 Aand11.92 Awhose ratio can be used as a density diagnostic.Their analysis and subsequent studies (Miller et al., 2008;Kallman et al., 2009) led to the conclusion that the densitymust have been at least n J 1014 cm�3, placing the absorbersome three orders of magnitude inside the Compton radius,where thermal driving cannot operate. Based on the highoptical depth in the wind and possible saturated lines, Net-zer (2006) argued for a lower density (ne � 1013 cm�3) andionization parameter (n � 103), which would have implieda more distant wind consistent with thermal driving. Milleret al. (2008) subsequently argued that Netzer’s model sig-nificantly overpredicted soft X-ray absorption lines, andaccording to simulations of thermally-driven winds tailoredto GRO J1655�40 (Luketic et al., 2010), even the lowerdensity proposed by Netzer (2006) is an order of magnitudetoo high for thermal driving to be the dominant launchingmechanism. By process of elimination, Miller et al. (2006a,

4 Note that this equation only holds in the optically-thin limit.

2008) and Kallman et al. (2009) concluded that the densewind in GRO J1655�40 must be powered by magneticprocesses.5

3.2. MHD winds in black hole X-ray binaries?

But how are we to understand the disk-wind-jet cou-pling in light of the apparent variations in wind formationphysics between different systems? Radiative/thermal driv-ing has been invoked to explain most winds in black holesystems, but there is convincing evidence for an MHDwind in GRO J1655�40. The continuing debate over theaccretion disk wind launching mechanism begs the ques-tion: is there a single, universal process that governs theinteraction between disks, winds, and jets in black holeX-ray binaries? Can the behavior of inflows and outflowsbe unified in such disparate systems? As our understandingof accretion and ejection physics evolves, it can be instruc-tive to revisit prior observations and the conclusions wedraw from them.

Perhaps the most salient development comes from Rey-nolds (2012), hereafter Reynolds, 2012, who demonstratesthat the well-known launching mechanisms (i.e. Comptonheating, magnetocentrifugal acceleration, and radiationpressure) can be distinguished not only by the ionizationand the density, but also the optical depth of the winds theyproduce. With a focus on Compton-thick winds, the for-bidden regime of parameter space for each mechanism isclearly set out in terms of the optical depth s, the Edding-ton ratio k, the ratio fv of the wind’s terminal velocity tothe escape velocity at the launch radius, and the distanceto the black hole r (in units of gravitational radii rg).

How does this analysis inform the interpretation of thedense, highly-ionized wind in GRO J1655�40, which hada column density N H � 1024 cm�2 (near the Compton-thicklimit; Miller et al., 2006a, 2008; Kallman et al., 2009)? Fol-lowing R12, let us consider each launching mechanism inturn:

1. Thermal driving: As noted above, and by Miller et al.(2006a, 2008) and Luketic et al., 2010, thermal drivingcannot possibly produce such a dense outflow close tothe black hole (r � ð1� 7Þ � 109 cm = 1000� 6800 rg;Kallman et al., 2009).

2. Radiative driving: Again, as argued above following theoriginal work on these data, the wind is far too ionizedfor UV line driving to be effective at launching the wind.But radiation pressure can also act on free electrons, andR12 shows that in this case, the momentum transferredto the electrons is insufficient to drive a wind ifk < 2f 2

v . In other words, the momentum flux in a

governed in part by magnetic processes (e.g. Balbus and Hawley, 1991;Blandford and Znajek, 1977; McKinney, 2006; Mirabel and Rodrıguez,1999 and references therein).

J. Neilsen / Advances in Space Research 52 (2013) 732–739 737

radiatively-driven wind cannot exceed that of the radia-tion field.The soft-state wind of GRO J1655�40 pro-vides an interesting illustration of this constraint.During this particular observation, we estimate the Edd-ington ratio to be about k � 0:06 (Neilsen and Homan,2012). The ratio fv is much harder to determine, sinceonly the line-of-sight velocity can be measured. Kallmanet al. (2009) found a blueshift of � 375 km s�1, which ismuch less than the escape velocity at plausible launchradii (vesc � 5000� 14; 000 km s�1; fv � 0:03� 0:07).Normally, one might suppose that fv � 1 and attributethe small blueshift to a velocity primarily perpendicularto the line of sight, but it is difficult to see how this sce-nario is consistent with the small solid angle of the wind(see below).If we take the velocities at face value andaccept that the wind remains bound to the black hole,we find k � ð6� 40Þ � 2f 2

v , i.e. there is no shortage ofmomentum in the radiation field. However, this is nota sufficient condition to launch the wind, since the radi-ation force on the gas is still required to exceed the forceof gravity (C. Reynolds 2012, private communication).Thus, despite an abundance of momentum flux, radia-tion pressure cannot explain the dense, highly-ionizedwind in GRO J1655�40 (see also Miller et al., 2006a).

3. Magnetocentrifugal driving: Although magnetocentrifu-gal acceleration (e.g. Blandford and Payne, 1982) isnot the only MHD process that can drive winds (e.g.Proga, 2003; see also Miller et al., 2008 and referencestherein), it has been studied in great detail and lendsitself nicely to an analytic constraint on the productionof Compton-thick outflows. As shown by R12, Comp-ton-thick magnetocentrifugal winds can only be pro-duced at radii,

r < 800 -�7 sk

� ��2 Xp

� ��2 g0:1

� ��2

rg; ð5Þ

i.e. where,

sk<

ffiffiffiffiffiffiffiffiffiffiffiffiffi800 rg

-7r

rXp

� �g

0:1

� �: ð6Þ

Here X is the solid angle of the wind, g is the radiative effi-ciency of the accretion flow, and - � 2� 3 is the ratio ofthe size of the acceleration zone of the wind to the launchradius (for more details, see R12 and references therein).-For the dense wind in GRO J1655�40, the observed col-umn density N H ¼ 1024�0:02 cm�2 implies an opticaldepth6 s � 0:67, from which we estimate s=k � 11� 2.Miller et al. (2006a) use upper limits on emission linestrengths to place the tight constraint X < 4p=9. If weallow -J 1 and use the smallest plausible radiusr � 970rg, Eq. 6 for the allowed parameter space for mag-netocentrifugal winds becomes s=k < 2:0. That is, given the

6 Since the vast majority of the electrons in an ionized plasma come fromhydrogen, for our purposes it is reasonable to assume an ionizationfraction very close to 1.

small solid angle of the wind and its relatively large dis-tance from the black hole, the optical depth of the windis at least � 5� too large for it to be launched by magneto-centrifugal processes. However, we stress that other MHDmechanisms (see Miller et al., 2008 and references therein)have not been ruled out.

In short, based on the constraints presented in Reynolds(2012), we find that the dense, highly-ionized wind inGRO J1655�40 cannot be driven by magnetocentrifugaleffects. We therefore confirm the original suggestion ofMiller et al. (2008) that the wind is driven by some otherMHD process, like magnetic pressure (e.g. Proga, 2003).It should be noted that R12 uses a simplified model ofthe wind and driving mechanisms, and that the geometryof observed winds may be somewhat more complicated.For example, as detailed by Giustini and Proga (2012)and references therein, the bulk properties of winds (e.g.density, ionization, velocity, etc.) may be strong, non-monotonic functions of position, even for outflows withsimple streamlines. This is clearly an area where future the-oretical work can continue to improve the accuracy androbustness of inferences from observations. In any case,more detailed study of the remarkable wind in GROJ1655�40 is forthcoming.

Finally, we can return to the question at hand: is there asingle, universal process that governs the interactionbetween disks, winds, and jets in black hole X-ray binaries?To the best of our knowledge, we can trace the origin ofmost winds from stellar-mass black holes back to the radi-ation field of the inner accretion flow (e.g. Lee et al., 2002;Kubota et al., 2007; Neilsen and Lee, 2009; Ueda et al.,2009; Ueda et al., 2010; Neilsen et al., 2011, 2012a). Thisseems to be a compelling argument for the scenario pre-sented in Section 2, in which accretion disk winds play anintegral role in the evolution of black hole outbursts by vir-

tue of their connections to the radiation field.However, given that there is one clear case of a magnet-

ically-driven wind, and no hard evidence that other windsare not driven by magnetic fields, this question should stillgive us pause. Are the results from detailed studies of onewind observed in GRO J1655�40 applicable to the classof black hole binaries as a whole? King et al. (2013) findevidence of a three-way correlation between jet power,wind power, and bolometric luminosity over eight ordersof magnitude in black hole mass. This seems to indicatethat outflows and inflows could be regulated by a commonprocess related to the mass accretion rate, but the underly-ing physics of this regulation is still open for discovery.

4. Conclusions

In the last few years, a significant effort has been devotedto understanding the physics and behavior of accretiondisk winds in black hole X-ray binaries, with importantdevelopments coming from both archival studies andnew observations. By extrapolating from the specific

738 J. Neilsen / Advances in Space Research 52 (2013) 732–739

(exceptional) case of GRO J1655�40 to the ensemble ofwinds studied by Ponti et al. (2012), we have come tounderstand that highly-ionized winds are ubiquitousaround stellar mass black holes, that they may evolve sig-nificantly in outburst, and that they may carry away a sig-nificant fraction of the inflowing gas. It is worth reiteratingthat winds are not necessarily confined to specific states per

se, but appear to evolve continuously during outbursts.While their formation physics is complex and challengingto discern, and while MHD processes may be important,it seems that most (but not all) known accretion disk windsare consistent with radiative or thermal driving (Lee et al.,2002; Kubota et al., 2007; Neilsen and Lee, 2009; Uedaet al., 2009; Ueda et al., 2010; Neilsen et al., 2011; Neilsenet al., 2012a; Diaz Trigo et al., 2012 come to a similar con-clusion for neutron star binaries).

At once driven by and ionized by the luminosity of thecentral engine, these massive outflows may require radia-tion for their existence, but their substantial influencemay ultimately be their undoing. By draining vast quanti-ties of mass from the accretion disk, they may not only sup-press relativistic jets and cause or facilitate state transitionsas seen in GRS 1915+105 (with tentative evidence in othersystems; Ponti et al., 2012), but they may also cripple theability of the disk to launch a wind! In the future, by takingadvantage of our ever-growing understanding of the spec-tral/timing behavior of X-ray binaries in outburst, we willtrack these evolving, massive, ionized outflows in order tocontinue shedding new light on the physics of accretion andejection around black holes.

Acknowledgements

I thank the anonymous referees, whose comments en-hanced the context and clarity of the paper, as well as ChrisReynolds and Jon Miller for comments that substantiallyimproved the discussion of wind driving mechanisms. Thiswork was supported by the National Aeronautics andSpace Administration through the Smithsonian Astrophys-ical Observatory contract SV3-73016 to MIT for supportof the Chandra X-ray center, which is operated by theSmithsonian Astrophysical Observatory for and on behalfof the National Aeronautics and Space Administration un-der contract NAS8-03060, and by NASA through the Ein-stein Fellowship Program, Grant PF2-130097.

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