Low-luminosity stellar wind accretion onto neutron stars in HMXBs

0.87 MB·

by  Konstantin Postnov

Low-luminosity stellar wind accretion onto neutron
stars in HMXBs
7
1
0
2

n
KonstantinPostnov
a ∗†
J SternbergAstronomicalInstitute,MoscowM.V.LomonosovStateUniversity,Universitetskijpr.,

2 13,Moscow119234,Russia
E-mail: [email protected]
]
E
LidaOskinova
H
InstituteforPhysicsandAstronomy,UniversityPotsdam,14476Potsdam,Germany
.
h E-mail: [email protected]
p
- JoseMiguelTorrejón
o
r InstitutoUniversitariodeFísicaAplicadaalasCienciasylasTecnologías,Universidadde
t
s Alicante,03690Alicante,Spain
a
[ E-mail: [email protected]

1
v Featuresandapplicationsofquasi-sphericalsettlingaccretionontorotatingmagnetizedneutron
6
starsinhigh-massX-raybinariesarediscussed.Thesettlingaccretionoccursinwind-fedHMXBs
3
3 whentheplasmacoolingtimeislongerthanthefree-falltimefromthegravitationalcapturera-
0 dius,whichcantakeplaceinlow-luminosityHMXBswithL .4 1036 ergs 1. Webrieflyre-
0 x × −
. viewtheimplicationsofthesettlingaccretion,focusingontheSFXTphenomenon,whichcanbe
1
0 relatedtoinstabilityofthequasi-sphericalconvectiveshellabovetheneutronstarmagnetosphere
7
due to magnetic reconnectionfrom fast temporarily magnetized winds from OB-supergiant. If
1
: a young neutron star in a wind-fed HMXB is rapidly rotating, the propeller regime in a quasi-
v
i spherical hot shell occurs. We show that X-ray spectral and temporal properties of enigmatic
X
γCasBe-starsareconsistentwithfailedsettlingaccretionregimeontoapropellingneutronstar.
r
a The subsequent evolutionary stage of γCas and its analogs should be the XPer-type binaries
comprisinglow-luminosityslowlyrotatingX-raypulsars.
11thINTEGRALConferenceGamma-RayAstrophysicsinMulti-WavelengthPerspective,
10-14October2016
Amsterdam,TheNetherlands
Speaker.
∗
SupportedbyRSFgrant16-12-10519
†
cCopyrightownedbytheauthor(s)underthetermsoftheCreativeCommons
(cid:13)
Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). http://pos.sissa.it/
Low-luminositystellarwindaccretionontoneutronstarsinHMXBs KonstantinPostnov
1. Two regimes ofwindaccretion
In high-mass X-ray binaries (HMXB), a compact stellar remnant (neutron star (NS) or black
hole)accretesfromstellarwindofamassive( 10 20M )earlytypeopticalcompanion[1]. Here
∼ − ⊙
we will consider accretion onto NS only. The stellar wind is captured by the orbiting NS at the
characteristic capture (Bondi) radiusR =2GM /(v2 +v2),wherev istheorbitalNSvelocity,
B x orb w orb
v is the stellar wind velocity at the NS distance from the optical star. Typically, in HMXBs
w
v 108cms 1 v . In the stellar wind, a bow shock is formed at about R . Downstream the
w − orb B
∼ ≫
shock,plasmafallstowardstheNS,andinthecaseofrotatingmagnetizedNS,isstoppedbytheNS
magnetosphereatthecharacteristicdistance R ,whereR isthemagnetosphere(Alfvénic)radius
A A
∼
determined bythepressure balance betweenthefalling plasmaandtheNSmagneticfield. Plasma
enters the NS magnetosphere via different (mostly Rayleigh-Taylor) instabilities, gets frozen into
themagneticfieldandiscanalizedtowardstheNSmagneticpoles. Theaccretionpowerisreleased
near the NS surface with a total luminosity L 0.1M˙c2, where M˙ is the accretion rate onto the
x
∼
NS.
There can be two regimes of wind accretion onto magnetized NS in X-ray binaries: direct
supersonic (Bondi) accretion, when the plasma captured from stellar wind cools down on time
scales shorter than the free-fall time from R , t t (R ), and quasi-spherical subsonic (set-
B cool ff B
≪
tling)accretion, whentheplasmacoolingtimeislongerthanthetimeofmatterfallfromR toR ,
B A
t t (R ). The cooling of plasma is the most efficient via Compton interaction with X-ray
cool ff B
≫
photons produced near the NSsurface, therefore the two regimes occurs at high (direct Bondi su-
personic accretion) and low (subsonic settling accretion) X-ray luminosities. Transition between
these accretion regimes occurs at L 4 1036 erg s 1 [2]. Observational appearances of the set-
x −
≃ ×
tlingaccretionregimearediscussed in[3]. Importantly, inthesettlingaccretion regimetheplasma
entry rate in the NS magnetosphere is determined by the plasma cooling time and is less than
thaninthedirectBondi-Hoyle-Littleton casebythefactor f(u)=(t /t )1/3,whichcanbemuch
cool ff
smallerthanunity.
2. A model ofbright flares inSFXTs
Supergiant FastX-raytransients (SFXTs)areasubclass oftransient hardX-rayHMXBswith
NSsdiscovered by INTEGRAL[4,5,6],which have been actively studied inthe last years. They
arecharacterizedbysporadicpowerfuloutburstsontopofalow-luminosityquiescentstate(see[8]
forasummaryoftheSFXTproperties).
AtsofterX–rays(1-10keV),SFXTsarecaughtatX–rayluminositiesbelowL 1034 ergs 1
q −
∼
most of the time. A luminosity as low as 1032 erg s 1 (1-10 keV) has been observed in some
−
sources, leading toaveryhighobserved dynamic range (ratio between luminosity during outburst
andquiescence) uptosixordersofmagnitude(e.g.IGRJ17544–2619 [7]).
Low X-ray luminosity at quiescence suggests that quasi-spherical settling accretion can be
realized in SFXTs. In the frame of this accretion regime a model [9] was proposed explaining
the origin ofpowerful flares in these sources. Indeed, in the settling accretion regime ahot quasi-
sphericalconvectiveshellisformedabovetheNSmagnetosphere. Themassaccretionratethrough
theshellisdeterminedbytheplasmacoolingtime, M˙ f(u)M˙ (seeabove),andunderstationary
a B
≈
1
Low-luminositystellarwindaccretionontoneutronstarsinHMXBs KonstantinPostnov
external conditions (density and velocity of the stellar wind) the shell can be in a quasi-steady
state. Note that here changes in the wind density (possible clumping) lead to moderate variations
oftheresultingmassaccretionrate, M˙ ρ11/32v 33/16 (hereweassumethemosteffectiveCompton
−w
∼
cooling), while for the high-luminosity Bondi accretion we expect a much stronger dependence,
M˙ ρv 3. In the model [9], a powerful SXFT flare is produced when the entire shell above the
B ∼ −w
NSmagnetosphere isdestabilized andaccretesontotheNSinthefree-falltimescale. Asthemass
oftheshellinthequiescentstateis∆M M˙ t L /f(u)t ,theremustbeacorrelationbetween
B ff q ff
≃ ≈
theenergyemittedinabrightflareandthemeanquiescentluminosity, ∆M L7/9,whichisindeed
q
∼
observed forbrightSFXTflares[9].
ThereasonfortheinstabilityoftheshellcanbemagnetizedstellarwindoftheOB-companion.
Heretwofactorsareimportant: (1)thatamagnetizedwindblobmayhavesmallervelocitythusin-
creasing themassaccretion ratethrough theshelland(2),themostimportantly, thatthetangential
magneticfieldcarriedbythewindblobcaninitiatemagneticreconnectionnearthemagnetospheric
boundary thusopeningthemagneticfieldlinesandenablingfreeplasmaentry. Forefficientrecon-
nectiontooccur,thetimeofplasmaentryinmagnetosphere (i.e.,thecharacteristic instability time
t f(u)t (R ))should becomparable toorlongerthanthemagnetic reconnection time,which
inst ff A
∼
is t =ǫ t ǫ t (R ) (here t t (R ) is the Alfvénic time at the magnetosphere boundary,
rec r A r ff A A ff A
≃ ∼
ǫ <1 is the magnetic reconnection efficiency). Therefore, the condition for reconnection can be
r
writtenas f(u)<ǫ 0.1,whichcannaturallybemetinlow-luminosityHMXBs. Notethatinhigh-
r
∼
luminosity HMXBs the magnetized stellar wind could not largely disturb the NS magnetosphere,
asthe timeofplasma entry inthemagnetosphere inthiscase canbeshorter thanthe reconnection
time.
3. Onthe difference between persistent HMXBsand SFXTs
SeveralmechanismshavebeenproposedtoexplaintheSFXTphenomenon: accretionofdense
stellarwindclumps[10,11],centrifugalinhibitionofaccretionatthemagnetosphere(thepropeller
mechanism)[12,13],andwindaccretioninstabilityatthesettlingregimediscussedintheprevious
Section [9]. Opticalcompanions ofSFXTsandpersistent HMXBsarevery similar(e.g. [11],and
thedifferentbehaviourofNSsinthesebinariesareapparentlyrelatedtothestellarwindproperties.
Recently, [14] performed a comparative study of optical companions in two prototypical sources
of these classes, IGR J17544-2619 (O9I) and Vela X-1 (B0.5Iae). It was found that the wind
termination velocity in IGR J17544-2619 (v 1500 km s 1) is two times as high as in Vela X-
−
∞ ≃
1 ( 700 km s 1). The high wind velocity strongly decreases the captured mass accretion rate,
−
≃
since in the radiation-driven winds from early type supergiants M˙ M˙ /v4 L /v5 , where
B OB OB
∼ ∞ ∼ ∞
L is the star’s luminosity. The authors conclude that if the NS in IGR J17544-2619 is indeed
OB
rapidlyrotating(tensofseconds),thepropellermechanismismostlikely. However,inotherSFXTs
with slowly rotating NSs(e.g., in IGR J16418-4532 with P =1212 s and IGR J16465-4507 with
∗
P =228stheapplicability ofthepropellermechanism maybequestionable.
∗
Wesuggest that the possible key difference between the SFXTsand persistent HMXBsis re-
lated to fast magnetized stellar winds of OB-supergiants in SFXTs. Magnetic fields are directly
observed in about 10% of OB-stars, and the magnetic field can significantly affect the stellar
wind properties [15]. Recently, the cumulative distributions of the waiting-time (time interval
2
Low-luminositystellarwindaccretionontoneutronstarsinHMXBs KonstantinPostnov
between two consecutive X-ray flares), and the duration of the hard X-ray activity (duration of
the brightest phase of an SFXT outburst), as observed by INTEGRAL/IBIS in the energy band
17-50keVdetectedfromthreeSFXTsshowingthemostextremetransientbehaviour(XTEJ1739-
302, IGRJ17544-2619, SAXJ1818.6-1703) were analyzed [16]. A correlation between the total
energy emitted during SFXT outbursts and the time interval covered by the outbursts (defined as
the elapsed time between the firstand the last flarebelonging tothe same outburst as observed by
INTEGRAL) was found. It was suggested that temporal properties of flares and outbursts of the
sources, whichsharecommonproperties regardlessdifferentorbitalparameters,canbeinterpreted
inthemodelofmagnetized stellarwindswithfractalstructure fromtheOB-supergiant stars.
Tofurthertestthishypothesis, itwouldbeimportanttosearchfortracesofmagnetized stellar
windsinspectraoftheopticalcomponents ofSFXTs.
4. Non-accreting NSs inquasi-spherical shells: caseofγ Cas stars
Features of propelling NSs at quasi-spherical settling accretion stage. As mentioned
above, at low accretion rates in HMXBs with rapidly rotating NSs the propeller mechanism can
operate. A young rapidly rotating NSformed after supernova explosion in a HMXBshould spin-
down at the propeller stage before accretion can be possible. Forthe conventional disk accretion,
thedurationofthepropellerstageisdeterminedbytheNSspinperiod P ,sizeoftheNSmagneto-
∗
sphere R andtheNSmagneticfield(dipolemagneticmomentµ),∆t µ 2R3(P ) 1. Fortypical
A P∼ − A ∗ −
parameters, the propeller stage lasts a hundred thousand years. In the settling accretion the NS
spin-sown is mediated by the hot convective shell around the magnetosphere, the propeller stage
duration∆t stronglydependsonthewindvelocityandbinaryorbitalperiod,∆t v7P2 L 1 and
P P∼ w orb −x
canbeverylong[17],oforderofmillionyears,whichiscomparable tothelifetimeoftheoptical
companion. Therefore, a sizable fraction of young rapidly rotating NSs in HMXBs can be at the
propeller stage. This is especially relevant to low-kick NSs originated from e-capture supernovae
(ECSN)[18, 19]. Indeed, in this case the NS remains in the orbital plane of the binary system,
and if the optical companion is the Be-star that acquired fast rotation during the preceding mass
transferstage,theNScanbeimmersedinthelow-velocitydenseequatorial Be-diskoutflow. Then
thequasi-spherical settling accretion ontoNScanberealized.
Observational manifestations of the propeller mechanism in wind-fed sources with settling
accretion wereconsidered in[17]. Thebasic properties ofthe hotmagnetospheric shell supported
by a propelling NS in circular orbit in a binary system around a Be-star predicts the following
observables:
i) the system emits optically thin multi-temperature thermal radiation with the characteristic tem-
peratures attheshellbaseabove 10keV:
∼
T = 2GMX 27[keV](R/109[cm]) 1
A 5 RA ≈ −
R
36[keV]µ 8/15L2/15(M /M )19/15, (4.1)
≈ 3−0 32 X ⊙
whereµ =µ/(1030Gcm3)istheNSdipolemagneticmoment,L =L /(1032ergs 1)istheX-ray
30 32 X −
luminosity oftheshell. Theshellischaracterized byhighplasmadensities 1013 cm 3:
−
∼
ρ 4.4 10 11[gcm 3](L /µ )2/3(M /M )1/3, (4.2)
A − − 32 30 X
≈ × ⊙
3
Low-luminositystellarwindaccretionontoneutronstarsinHMXBs KonstantinPostnov
andemissionmeasures 1055 cm 3:
−
∼
EM=RRRABn2e(r)4πr2dr=4πn2e,AR3Aln(cid:16)RRAB(cid:17)
≈3.7×1054[cm−3]µ430/15L1342/15(MX/M⊙)−2/15ln(cid:16)RRAB(cid:17). (4.3)
ii)thetypicalX-rayluminosityofthesystemisL 1033ergs 1;
X −
∼
iii)noX-raypulsations arepresentandnosignificant X-rayoutbursts areexpectedinthecaseofa
coplanar circular orbitwiththeBe-disk;
iv) the hot shell is convective and turbulent, therefore the observed X-ray emission lines from the
optically thinplasmashouldbebroadened upto 1000kms 1;
−
∼
v)thetypicalsizeofthehotshellis R .R ;
B
∼ ⊙
vi)thecoldmaterial,suchastheBe-diskinthevicinityofthehotshell,shouldgiverisetofluores-
centFeK-line;
vii) the life-time of a NS in the propeller regime in binaries with long orbital periods can be
∼
106yrs,hencesuchsystemsshouldbeobservable amongfaintX-raybinariesintheGalaxy.
EnigmaticX-rayemission fromBe-starγCas.
Theoptically brightest Be-star γCas(B0.5IVpe) is wellseen in the night sky by anaked eye
eveninabigcity. ItisawellknownbinarysystemwithanorbitalperiodofP 204dthatconsists
b
≈
ofanoptical starwithmass M 16M andanunseencompanion withmass M 1M [20,21].
o X
≈ ⊙ ≈ ⊙
The binary orbital plane and the disc of the Be-star are coplanar. The low-mass companion of γ
Cas can be a NS in the propeller stage grazing the outer Be-star disc region. The lack of periodic
pulsations, aswellasproperties ofthehotdensethermalplasmadeducedfromX-rayobservations
(kT 20 keV and n 1013 cm 3, v 1000 km s 1) (see [22] and below), which challenge
hot e − turb −
∼ ∼ ∼
all previously proposed scenarios of X-ray emission from γ Cas [23], are naturally expected in a
hotmagnetospheric shellaroundthepropelling NS.
The propeller model explains both the gross X-ray features of γCas and well matches the
detailed observations of its X-ray variability. Ahot convective shell above the NSmagnetosphere
should displaytimevariability inawiderange,thatdependonthesoundspeed, c ,whichisofthe
s
orderofthefree-falltime,t . Theshortesttime-scaleist R /c R /t (R ) R3/2/√2GM
ff min A s m ff m m
∼ ∼ ∼ ∼
afewseconds, whilethelongest timescale ist R /t (R ) 106[s](v /100kms 1) 3 afew
max B ff B w − −
∼ ∼ ∼
days. These are typical time scales of X-ray variability seen in γCas [22]. The single power-law
spectrum P(f) 1/f over the wide frequency range from 0.1Hz to 10 4Hz, as derived for the
−
∼
X-ray time variability in γCas [22], is common in accreting X-ray binaries [24]and is thought to
ariseinturbulized flowsbeyondthemagnetospheric boundary.
XMM-NewtonandNuSTARobservations ofγ Cas. Weobserved γ Cason 24-07-2014, si-
multaneously withXMM-Newton(ObsID0743600101) andNuSTAR(ObsID30001147002) space
telescopes(P.I.J.M.Torrejón). TheX-raytelescopesacquireddatafor34and30.8ks,respectively,
providing forthe firsttimeacontinuous spectral coverage from 0.3to60keV. Unlike allprevious
X-rayobservations, thelowandhighenergyspectraarestrictlysimultaneous, overlapfrom3to10
keV, with no gaps, and have S/N ratios at high energies which allow to constrain the high energy
4
Low-luminositystellarwindaccretionontoneutronstarsinHMXBs KonstantinPostnov
spectralcomponents, minimizinganypossiblecontribution fromlongtermvariability. Afullanal-
ysis of these data sets is under way and will be presented elsewhere. Here we concentrate mostly
onthehighenergycontinuumandthemostprominentemissionlines. InFig. 1wepresentthedata
setsrebinnedtohaveminimumS/Nratioperbinof5. Besidesthecontinuum,threeemissionlines
areclearlyseen: FeXXVHelikeat6.68keV,FeXXVIH-likeLyαat6.97keV.Thesetransitions
arise in a very hot thermal plasma (see below). A third Fe line at 6.39 keV arises from cold near
neutral Fefluorescence. TheFeKαtransition isnotincluded inthethermal plasmamodels and is
modelled withaGaussian.
InTable 1wepresent the results ofour analysis. Wehave essentially twoways ofdescribing
thewholespectrum: withthreeAPECplasmasat(19.7,9.2and 1keV)andFesubsolarabundance
(0.5Z )–Model2–oronebremsstrahlung(kT=16.7keV)andtwoAPECs(8.5, 1keV)andsolar
⊙
Feabundance – Model 1. Thefirst solution corresponds, essentially, to what has been said before
in previous studies. The sub solar abundance is driven by the fact that the continuum requires a
high temperature and this over predicts the intensity of the Fe hot lines (Fe XXV and Fe XXVI).
ThisFeunderabundance isneitherpredicted norexplained inanyofthecompeting explanations.
The Model 1 solution, in turn, allows Fe solar abundance. The bremmstrahlung component
isthe maincontributor atallenergies and becomes dominant above20keV. Thevolume emission
measures,deducedfromthenormalizationconstantsC inTable1areEM 4.7 1055 cm 3forthe
i −
∼ ×
Bremsstrahlung and 2.2 1055 cm 3 forthe hot APECcomponent, respectively. Forthe Model 2,
−
×
theEMsseemtobemoresimilar,2.6 1055and3.7 1055cm 3fortheAPEC1and2,respectively.
−
× ×
In terms of reduced χ2 Model 1 is better than Model 2 although the difference is not large
(1.237 vs 1.284). The unabsorbed X-ray luminosity observed is L = 1.5 1033 erg s 1. Both
x −
×
models provide acceptable descriptions of the observational data. Except for the Fe line under
abundance for the first solution, the rest of the parameters can be perfectly explained within the
failedquasi-spherical accretion paradigm presented in[17].
01 APEC kT=19.7 keV 01 Bremss kT=16.3 keV
V−1 0. V−1 0.
e APEC kT=9.2 keV e APEC kT=8.5 keV
k k
m s −2−1 10−3 m s −2−1 10−3
c c
ns ns
oto0−4 APEC kT=1.03 keV Fe Kalpha oto0−4 APEC kT=1.02 keV Fe Kalpha
h1 h1
P P
V V
e e
k0−5 k0−5
15 15
χ 0 χ 0
5 5
− −
1 10 1 10
Energy (keV) Energy (keV)
Figure1: CombinedXMM-NewtonandNuSTARX-rayspectraofγCasfittedbythreeAPECplasmaswith
undersolarFeabundance(Model1,leftpanel)andthermalbremsstrahlungandtwoAPECcomponentswith
solarFeabundance(Model2,rightpanel)andparameterslistedinTable1.
5
Low-luminositystellarwindaccretionontoneutronstarsinHMXBs KonstantinPostnov
Parameter Model1 Model2
Nis (1022 cm 2) 0.100 0.004 0.104+0.004
H − ± 0.005
cf 0.71 0.02 0.72 −0.03
± ±
Ndisk (1022 cm 2) 1.12+0.12 1.21 0.17
H − 0.10 ±
−
C 0.036 0.001 ...
Bremss
±
kT (keV) 16.3 0.3 ...
Bremss
±
C ... 0.065+0.017
APEC1 0.008
kT (keV) ... 19.71.−8
APEC1 1.6
Z (Z ) 1 0.48−0.02
Fe
⊙ ±
C 0.053 0.006 0.091+0.009
APEC2 ± 0.017
kT (keV) 8.5 0.5 9.2+0.−5
APEC2 ± 0.4
−
C 0.0026 0.0002 0.0026 0.0003
APEC3
± ±
kT (keV) 1.019 0.016 1.028 0.017
APEC3
± ±
λ (Å) 1.922+0.005 1.922+0.005
FeKα 0.004 0.004
I (10 5 phs 1 cm 2) 17 2− 15 2−
FeKα − − −
± ±
σ (Å) 0.021+0.007 0.021+0.007
FeKα 0.008 0.008
− −
χ2 (dof) 1.237(3129-14) 1.284(3129-14)
r
Table 1: Best parameters from the simultaneous fit to XMM-Newton and NuSTAR data. All errors are
quotedatthe90%confidencelevel.
5. Conclusion
1)Accretion ontoslowlyrotating (P &100s)magnetized NSsinwind-fed HMXBscanpro-
∗
ceedintwodifferentregimes–directBondi-Hoyle-Littletonsupersonicaccretion,whichisrealized
athighX-rayluminosities L &4 1036 ergs 1,andsettlingsubsonicaccretion atlowerluminosi-
x −
×
ties. Inthelastcase,aquasi-spherical convectiveshellisformedaroundthemagnetosphere, which
mediates theangular momentumtransfer to/from theNS[2]. Thesettling accretion modelcanex-
plain the low-states observed in accreting X-ray pulsars, the long-term spin-down in GX 1+4 and
theexistence ofaveryslowlyrotating NSswithoutneedofsuperstrong magneticfields[3].
2) The quasi-spherical shell formed at the settling accretion stage is quasi-stationary, but can
be destabilized by magnetic reconnection if accretion occurs from magnetized stellar wind of the
earlytypesupergiant companion. ThisprovidesthepossibleexplanationtoSFXTbrightflares[9].
If the wind from the OB-supergiant in an HMXB with slowly rotating NS is fast and temporarily
magnetized,theSFXTphenomenoncanbeobserved,whileifthewindisslow,abrighterpersistent
HMXB like Vela X-1 will appear (here the wind magnetization is unimportant since the time for
6
Low-luminositystellarwindaccretionontoneutronstarsinHMXBs KonstantinPostnov
magneticreconnection islongerthanthetimeittakesforplasmatoentertheNSmagnetosphere).
3) The spectral and timing properties of the hot multi-temperature plasma filling the quasi-
sphericalconvectiveshellaboveNSmagnetosphere,whichisformedatthesettlingaccretionstage,
were shownto beconsistent withX-ray spectral properties of enigmatic low-luminosity BeX-ray
binary γ Cas and its analogs [17]. Therefore, γ Cas systems can be propelling NSs inside quasi-
spherical shells at subsonic settling accretion stage from Be-disks. These NSs were formed from
low-kick ECSNsupernovae in Be-binaries. With time, the propelling NSin γ Casand itsanalogs
should spin down and will become slowly rotating X-ray pulsars at the stage of quasi-spherical
settling accretion withmoderateX-rayluminosity likeXPer[25]
Given the significant time duration, the propelling NSs surrounded by hot quasi-spherical
shellscanbepresentamonglow-luminositynon-pulsatingHMXBs. TheirX-rayspectralproperties
as summarized above and their long orbital binary periods can be used to distinguish them from,
forexample,fainthardX-rayemissionfrommagneticcataclysmic variables (e.g. [26]).
Acknowledgements. WorkofKP(settling accretion theory anditsapplications) issupported
by RSF grant 16-12-10519. JMT acknowledges the research grant ESP2014-53672-C3-3-P and
LOacknowledges theDLRgrant50OR1508.
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