The First Simultaneous X-Ray/Radio Detection of the First Be/BH System MWC 656

2.5 MB·

by  M. Ribó

Draftversion January26,2017
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
THE FIRST SIMULTANEOUS X-RAY/RADIO DETECTION OF THE FIRST Be/BH SYSTEM MWC 656
M. Ribo´1, P. Munar-Adrover2, J. M. Paredes1, B. Marcote3,1, K. Iwasawa4, J. Moldo´n5,1,6, J. Casares7,8,9, S.
Migliari10, X. Paredes-Fortuny1
1DepartamentdeF´ısicaQu`anticaiAstrof´ısica,InstitutdeCi`enciesdelCosmos(ICCUB),UniversitatdeBarcelona,IEEC-UB,Mart´ıi
Franqu`es1,E08028Barcelona,Spain
2INAF/IAPS-Roma,I-00133Roma,Italy
3 JointInstituteforVLBIERIC(JIVE),Postbus 2,7990AADwingeloo,TheNetherlands
7 4ICREA,InstitutdeCi`enciesdelCosmos(ICCUB),UniversitatdeBarcelona,IEEC-UB,Mart´ıiFranqu`es1,E-08028Barcelona,Spain
1 5JodrellBankCenterforAstrophysics,SchoolofPhysicsandAstronomy,TheUniversityofManchester,Manchester M139PL,UK
0 6ASTRON,TheNetherlands InstituteforRadioAstronomy,Postbus 2,7990AA,Dwingeloo,TheNetherlands
2 7Instituto deAstrof´ısicadeCanarias,E-38200LaLaguna, Tenerife,Spain
8DepartamentodeAstrof´ısica,UniversidaddeLaLaguna, Avda. Astrof´ısicoFranciscoS´anchezs/n,E-38271LaLaguna, Tenerife,Spain
n 9DepartmentofPhysics,Astrophysics,UniversityofOxford,DenysWilkinsonBuilding,KebleRoad,OxfordOX13RH,UKand
a 10EuropeanSpaceAstronomyCentre,Apartado/P.O.Box78,VillanuevadelaCanada,E-28691Madrid,Spain
J Draft version January 26, 2017

5
2 ABSTRACT
MWC 656 is the first known Be/black hole (BH) binary system. Be/BH binaries are important

] in the context of binary system evolution and sources of detectable gravitationalwaves because they
E
are possible precursorsof coalescingneutron star/BHbinaries. X-rayobservations conducted in 2013
H
revealed that MWC 656 is a quiescent high-mass X-ray binary (HMXB), opening the possibility to
. explore X-ray/radio correlations and the accretion/ejection coupling down to low luminosities for
h
BH HMXBs. Here we report on a deep joint Chandra/VLA observation of MWC 656 (and contem-
p
poraneous optical data) conducted in 2015 July that has allowed us to unambiguously identify the
-
o X-ray counterpart of the source. The X-ray spectrum can be fitted with a power law with Γ ∼ 2,
r providing a flux of ≃ 4×10−15 erg cm−2 s−1 in the 0.5–8 keV energy range and a luminosity of
st LX ≃ 3×1030 erg s−1 at a 2.6 kpc distance. For a 5 M⊙ BH this translates into ≃ 5×10−9 LEdd.
a These results imply that MWC 656 is about 7 times fainter in X-rays than it was two years before
[
andreachesthe faintestX-rayluminosities ever detectedin stellar-massBHs. The radiodata provide

1 a detection with a peak flux density of 3.5±1.1 µJy beam−1. The obtained X-ray/radioluminosities
v for this quiescent BH HMXB are fully compatible with those of the X-ray/radio correlations derived
5 from quiescent BH low-mass X-ray binaries. These results show that the accretion/ejection coupling
6 in stellar-mass BHs is independent of the nature of the donor star.
2 Subject headings: binaries: general — stars: emission-line, Be — stars: individual (MWC 656) —
7 X-rays: binaries — X-rays: individual (MWC 656) — black hole physics
0
.
1
1. INTRODUCTION a BH in orbit around a Be star (Casares et al.
0
2014). Its discovery was triggered by the detec-
7 X-ray observations of stellar-mass black holes (BHs)
tion, with AGILE, of the transient gamma-ray source
1 in binary systems have allowed for the study of
: the properties of the accretion disks that surround AGL J2241+4454 (Lucarelli et al. 2010), undetected
v with Fermi/LAT (Alexander & McSwain 2015) but re-
them, as well as their changes over time (see
Xi Remillard & McClintock 2006 for a review). Radio ob- cently reported to show recurrent activity with hints of
long-termvariabilitywithAGILE(Munar-Adrover et al.
r servations of these systems have allowed us to gain
a knowledge of the ejection processes in the form of rel- 2016). The binary nature of MWC 656 (a bright
Be star also named HD 215227) was suggested by a
ativistic jets (Mirabel & Rodr´ıguez 1999; Fender 2001;
60.37dperiodicityinopticalphotometry(Williams et al.
Fender & Mun˜oz-Darias 2016). Simultaneous observa-
2010; Paredes-Fortuny et al. 2012), and later confirmed
tions have revealed the existence of non-linear correla-
through radial velocity studies (Casares et al. 2012). A
tionsbetweentheX-rayandradioluminositiesduringthe
detailed spectroscopic analysis revealed the presence of
so-called low/hard and quiescent states that are consis-
tent with scale-invariant jet models (Corbel et al. 2003; a HeII emission line at λ4686 ˚A that could only be pro-
Gallo et al.2003;Markoff et al.2003;Fender et al.2004; duced in an accretion disk around the invisible compan-
Fender & Belloni 2004; Corbel et al. 2013; Gallo et al. ion to the Be star. A re-analysis of radial velocity data
2014). However, these studies have mainly been con- using an Fe II emission line from the Be circumstellar
ducted for BHs in low-mass X-ray binaries (LMXBs) disk and the HeII emission line from the accretion disk,
because the only confirmed BH in a high-mass X-ray together with an improvedspectral type classificationof
binary (HMXB) in the Galaxy before the discovery of B1.5-B2III forthe Bestar,providedamassforthe com-
MWC 656, namely Cygnus X-1, always displays a high pact object in the range of 3.8–6.9 M⊙, thus confirming
luminosityanddoes notallowusto tracethe correlation its BH nature (Casares et al. 2014).
(see, e.g., Zdziarski 2012). The discovery of the first Be/BH system is relevant
MWC 656 is the first binary system containing in the context of binary system evolution. Although it
2 Rib´o et al.
partially solves the problem of the absence of Be/BH ×10−8
systems(Belczynski & Ziolkowski2009),thediscoveryof 40′′ 5.0
MWC656isobservationallybiasedbythelackofabright
4.5
X-raycounterpart,suggestingthattheremightbealarge
number of hidden Be/BH systems. In addition, simula- 30′′ 4.0
tions have shown that it is very difficult to form Be/BH
binaries with properties similar to those of MWC 656, 3.5
althoughthey canevolveintocloseNS/BHsystemsthat 0) 20′′ MWC656 3.0
willmergeontimescalesofafewGyrandproducegravi- 00
2
tational wavesdetectable with advancedLIGO/Virgo in (J 2.5
nearby galaxies (Grudzinska et al. 2015). Dec 10′′
Munar-Adrover et al.(2014)conducteda14-ksXMM- CXOU-1 2.0
Newton observation of MWC 656 on 2013 June 4 that
1.5
provided the detection between 0.3 and 5.5 keV of a +44◦43′00′′
faint source coincident at the 2.4σ level with MWC 656. 1.0
This revealed that MWC 656 was a new HMXB. The
spectrum analysis required a model fit with two compo- 0.5
nents, a blackbody plus a power law, the latter domi- 43m00s 59s 58s 57s 22h42m56s
nating above ≃0.8 keV. The thermal emission was pro- RA(J2000)
posed to arise from the wind of the Be star, while
Fig.1.—Chandra ACIS-Simageof 50′′×50′′ at the position of
the non-thermal emission arose from the vicinity of the
MWC656(indicatedbythecross)inthe0.5–8.0keVenergyband
BH. The observed non-thermal luminosity was LX = smoothedusingaGaussianinterpolationwitha3′′kernel. Theflux
(1.6+1.0)×1031 erg s−1, or (3.1±2.3)×10−8L for scaleappearsontherightoftheimage. Contourscorrespondtothe
a so−u0r.c9e distance of 2.6± 0.6 kpc, indicating aEBddH in previousXMM-NewtonEPIC-pnimagefromMunar-Adroveretal.
(2014).
deep quiescence (see Plotkin et al. 2013 for reference).
Radio observations with different interferometers, fre-
fective exposure time of 59.1 ks. The data analysis was
quencies, and epochs have provided 3σ upper limits
performedinthe 0.5–8keVenergy bandat whichChan-
to the flux density as low as 30 µJy (Moldo´n 2012;
dra presents higher sensitivity.
Marcote 2015). Recently, Dzib et al. (2015) have re-
We show the Chandra X-ray image of the field
ported a detection with VLA at a peak flux density
of 10 ± 3 µJy beam−1. Observations at TeV energies around MWC 656 in Figure 1, together with the con-
tours of the previous XMM-Newton image reported in
withtheMAGICtelescopeshaveonlyledtoupperlimits
Munar-Adroveret al. (2014). Inspection of the X-ray
(Aleksi´c et al.2015). ForadetailedreviewonMWC656
image superimposing optical positions from the USNO-
see Rib´o (2015).
B1.0 catalog (Monet et al. 2003) reveals a systematic
InthisLetter,wereportonadeepjointChandra/VLA
offset. Using three common sources we have applied
observation (and contemporaneous optical data) of the
a correction of RA = +0.′′5 and Dec = +0.′′1 to
first Be/BH binary MWC 656 that has allowed us to:
the Chandra data (and of RA = +2′.′4 and Dec =
1) resolve the XMM-Newton detection into two sources,
+4.′′9 to the XMM-Newton data using the only com-
one of them unambiguously associated with MWC 656;
mon source). The better angular resolution of Chan-
2)findthatitissignificantlyfainterthantwoyearsbefore
dra has allowed us to resolve the XMM-Newton source
and reaches the faintest quiescent luminosities detected
into two sources. We have used the wavdetect algo-
in stellar-mass BHs; and 3) detect a faint radio counter-
rithm to obtain the significance and positions of both
partandfindthatthe obtainedX-ray/radioluminosities
sources. MWC 656 is detected at the 8.5σ confidence
for this quiescent BH HMXB are fully compatible with
level(c.l.), with equatorialcoordinates(after correction)
thoseoftheX-ray/radiocorrelationsderivedfromquies-
cent BH LMXBs at the low-luminosity end. RA = 22h42m57.s27 ± 0.s01, Dec = 44◦43′18.′′5 ± 0.′′1
(uncertainties are statistical only and at 1σ c.l.). The
2. OBSERVATIONS, DATA ANALYSIS, AND position of MWC 656 from Gaia data at the epoch
RESULTS of the Chandra observation (epoch=2015.56) following
Gaia Collaboration et al.(2016)andGaia Collaboration
2.1. X-rays (2016) is: RA = 22h42m57s.29718 ± 0s.00002, Dec =
MWC 656 was observed with the Chandra X-ray Ob- 44◦43′18.′′2099±0′.′0002. The offset between both po-
servatory ACIS-S camera using the VFAINT data mode sitions is 0.′′4 ± 0′.′1 ± 0.′′1 , and the Chandra
during ∼60 ks, from 2015 July 24 at 21:03 UT to July source is thus the counsttaetrpart of MsyWst C 656.
25 at 14:29 UT (PI: M. Rib´o; Chandra ObsId 16753). The new source, located ∼13.′′5 to the southeast of
The observation covered the orbital phase range 0.01– MWC 656, is detected at a 15.9σ c.l. and has equato-
0.02 (using JD0 =2453243.3from Williams et al. 2010). rialcoordinates(aftercorrection)RA=22h42m57.s969±
The data were analyzed using the Chandra Interac- 0.s005, Dec = 44◦43′07.′′34 ± 0.′′05, and thus is named
tive Analysis of Observations (CIAO) software, v4.71 CXOU J224257.9+444307(CXOU-1 from now on). The
(Fruscione et al. 2006). The event files were reprocessed Chandra image reveals that MWC 656 is fainter than
by means of the chandra repro script. No flares were CXOU-1 (see below), while the shape of the XMM-
detected in the background light curve, yielding an ef- Newton contours reveals that in 2013 June MWC 656
was brighter than CXOU-1.
1 http://cxc.harvard.edu/ciao/index.html With the use of the srcflux script we extracted net
X-ray/radioobservation of MWC 656 3
MWC656 CXOUJ224257.9+444307
ACIS-S ACIS-S
10−6 pow 10−6 pow
bb bb
−1] −1]
V V
ke ke
−1 −1
s s
−2 10−7 −2 10−7
m m
c c
[ [
E) E)
F( F(
10−8 10−8
2.5 pow 2.5 pow
del 2.0 bb del 2.0 bb
mo 1.5 mo 1.5
a/ 1.0 a/ 1.0
at 0.5 at 0.5
d 0.0 d 0.0
1 2 3 4 5 8 1 2 3 4 5 8
E[keV] E[keV]
Fig.2.— Left: MWC 656 Chandra ACIS-S spectrum in the 0.5–8.0 keV energy range (crosses) overplotted with the fitted absorbed
power-lawmodel (bluesolidline)andblackbody model (reddashedline). Thelower panel illustratestheratiobetween theobservational
dataandthecorrespondingmodel(bluecrosseswithcirclesforthepower-lawmodelandredcrossesfortheblackbodymodel). Right: the
samefigureforthenewsourceCXOUJ224257.9+444307.
TABLE 1
Summaryof spectral fitresults
Source Powerlaw Blackbody
Γ F(0.5–8keV) C-statistic kBT F(0.5–8keV) C-statistic
10−15 [ergcm−2 s−1] [keV] 10−15 [ergcm−2 s−1]
MWC656 2.2+−10..39 4.1+−21..35 0.3 0.6±0.3 2.7+−11..51 3.0
CXOUJ224257.9+444307 1.8±0.6 8.0+−22..81 4.4 0.7±0.2 6.0+−21..37 12.5
Note. —Uncertainties areat68%c.l. Fluxvaluesareunabsorbed.
countsforMWC656(22.1±4.9at90%c.l.) andCXOU- at 1σ c.l. The power-law fit for MWC 656 provides a
1 (38.7±6.3), taking into account the background and photon index of 2.2+1.3, while for CXOU-1 we obtain
−0.9
the point-spreadfunctionofthe instrument. Thesemea- 1.8±0.6, thus fully compatible at 1σ c.l. The conclu-
surements were obtained from a region centered at the sion from the analysis of the Chandra data is that both
peakoftheX-rayemission(foundbywavdetect)ofeach sources have a similar spectrum, while the X-ray flux of
sourcewitharadiusof3′′,whilebackgroundcountswere MWC 656is approximatelyhalf of the one measuredfor
obtained from a nearby circular region with a 10′′ ra- CXOU-1. There is no reported evidence for this source
dius, avoiding contamination from nearby sources, al- intheliteratureandthelownumberofphotonsprevents
ways within the 0.5–8.0 keV energy band. us from clearly establishing its nature.
We used XSPEC version12.8.2(Arnaud 1996) to per- WehavetestediftheChandradataforthetwosources
form a spectral analysis of both sources. The same re- producesaspectrumthatneedsatwo-componentmodel.
gions for source and background used to extract net We have combined the spectra of the two sources and
counts were considered. We created response files in find that the data can be reproduced with a single
order to take into account the energy-dependent be- power law with a photon index of 2.1±0.5 (C-statistic
havior of the instrument. We fitted the spectra with 3.7). Therefore, the Chandra data do not support the
two distinct models, an absorbed power law and an ab- two-component model reported in Munar-Adroveret al.
sorbedblackbody,alwaysusingafixedcolumndensityof (2014).
N =1.8×1021 cm−2 (see Munar-Adrover et al. 2014).
H
The low number of counts required the use of the cstat
statistic within XSPEC to estimate the best-fit parame- 2.2. Radio
ters and their associated uncertainties (Cash 1979).
MWC 656 was observed with the Karl G. Jansky
We show the spectra of MWC 656 and CXOU-1, to-
Very Large Array (VLA) of the National Radio As-
gether with the fitted models, in Figure 2. The results
tronomy Observatory (NRAO) on 2015 July 25 from
of the fits are listed in Table 1. Although both sources
05:00 to 11:00 UTC (during the Chandra observation)
canbefittedwitheithermodel,theobtainedC-statistics
in the A configuration. The observation was conducted
revealthatthe power-lawmodelis preferredsinceblack-
at10GHz, with full circularpolarization,using 32spec-
bodymodelsresultinanexcessatbothlowandhighen-
tralwindowswith64channelsof2MHzbandwidtheach,
ergies. The obtained fluxes for the power-law models in
providing a total bandwidth of 4 GHz. The amplitude
the0.5–8keVrangeare4.1+−21..35×10−15 ergcm−2 s−1 for calibrator was 3C 48. The phase calibrator J2255+4202
MWC 656 and8.0+−22..81×10−15 erg cm−2 s−1 for CXOU- was observed in 1:20-min runs interleaved with 6-min
1. Although CXOU-1 appears brighter in Figure 1, the runs on the target source.
fluxes obtained by the spectral analysis are compatible The calibration was conducted using version 4.5.0 of
4 Rib´o et al.
scopic data. Photometric observations were conducted
20′′
with a passband optical filter at the 0.5 m robotic Tele-
scope Fabra Roa Montsec (TFRM, Fors et al. 2013) on
the nights from 2015 July 22 to 27 (except the 24), and
with BVRI filters at the 0.8 m robotic Joan Or´o Tele-
c
19′′ scope (TJO, Colom´e et al. 2010) from 2015 July 28 on-
wards. Both telescopes are located at Observatori As-
tron`omic del Montsec (OAdM, Sant Esteve de la Sarga,
0) MWC656 Catalonia). Whiledetailedresultswillbepublishedelse-
00 where, during the Chandra/VLA observation MWC 656
2
(J 18′′ displayedphotometrycompatiblewithitsalreadyknown
δ
behavior (Paredes-Fortuny et al. 2012; Paredes-Fortuny
2016). Inspection of focused images taken on 2015
November12revealsnoopticalcounterpartforCXOU-1,
implying I >18.5 mag.
c
+44◦43′17′′ MWC 656 was also observed with the fiber-
fed STELLA Echelle Spectrograph (SES) of the
1.2 m robotic STELLA-I (ST) optical telescope
(Strassmeier et al. 2004) at the Observatorio del Teide
57.4s 57.3s 22h42m57.2s (OT, Tenerife, Spain) on the nights of 21–25 July 2015
α(J2000) (except the 22). The setup and the data reduction
Fig. 3.—VLAimageof4′′×4′′centeredattheopticalpositionof were the same as in Aleksi´c et al. (2015). The spectra
show the presence of the double peaked He II λ4686
MWC656(indicated bytheopencross)obtained at10GHzwith
the VLA in its A configuration. The synthesized beam size is of emission line with an equivalent width comparable to
0.35×0.19 arcsec2 in P.A. of 47◦. Contours represent −2.5, 2.5, that reported in Casares et al. (2014). We also detect
and3timesthermsnoiseof1.1µJybeam−1. other emission lines, mainly Hα, Hβ and weak Fe II
lines with comparable strengths to those measured by
the CASA package2 of NRAO (McMullin et al. 2007).
Casares et al.(2012). Therefore,MWC656wasinasim-
The data have been reduced using standard amplitude
ilar optical state as in past observations.
and phase calibration steps. We imaged the data us-
ing the clean procedure with a natural weighting, ob- 3. DISCUSSION AND CONCLUSIONS
taining a synthesized beam of 0.35×0.19 arcsec2 in a The Chandra observation of MWC 656 reported here
Position Angle (P.A.) of 47◦ (north to east). Due to has revealed that the XMM-Newton source presented in
the proximity of a bright quasar affecting the field of
Munar-Adroveret al. (2014) was in fact the superpo-
MWC 656 (see Marcote 2015), we imaged a region of a
sition of two sources: MWC 656 and the new source
fewarcminincludingthisquasar. Weconductedamulti-
CXOU J224257.9+444307 (CXOU-1). Here MWC 656
scale clean, considering the usual point-like components is significantly fainter than it was in the XMM-Newton
and extended components with sizes one to three times
observation. The VLA observation of MWC 656 has led
the synthesizedbeam(see Cornwell2008andRich et al.
to the detection of a faint source. This is the firstsimul-
2008fordetails). Tocheckthe reliabilityoftheobtained
taneousX-ray/radiodetection ofthe firstBe/BHbinary
results we also analyzed the data using standard clean,
system. Here we discuss the implications of these detec-
different weightingschemes, anddifferent time intervals,
tions.
whichalwaysledto compatible resultswithin uncertain- First,itisworthnotingthatduringtheChandra/VLA
ties.
observation MWC 656 was showing optical photometric
TheobtainedimageisshowninFigure3. Afaintradio
and spectroscopic properties compatible with those re-
source with a peak flux density of 3.5±1.1 µJy beam−1
ported in previous works, indicating that there was no
is detected at the optical position of MWC 656 within
significant change in either the Be star and its circum-
uncertainties. Although this is a marginal detection,
stellar disk or the outer part of the BH accretion disk
there are several facts that support the reality of the (He II emission line). In addition, inspection of the
radiosourceasthecounterpartofMWC 656: 1)itisthe MAXI data (Matsuoka et al. 2009) in the direction of
brightest radio source in the image, 2) it is the only one
AGL J2241+4454 reveals no X-ray emission up to now
locatedwithinthe Chandra X-raysourcepositionuncer-
(2009 August to 2016 October). Therefore, MWC 656
tainty,3) itis fully compatible with the opticalposition,
appears to have been in a long quiescent X-ray state, at
and 4) there is a former radio detection of MWC 656
least during the last 7 years.
(Dzib et al. 2015). The X-ray flux of MWC 656 provided by Chandra in
Inspection at the position of CXOU-1 reveals no radio 2015 July is 4.1+2.3 ×10−15 erg cm−2 s−1 in the 0.5–
counterpart,witha3σfluxdensityupperlimitof3.2µJy −1.5
8 keVenergyrange. For comparison,the previousX-ray
for a point-like source.
observation conducted with XMM-Newton in 2013 June
provided a total flux of 4.6+1.3 × 10−14 erg cm−2 s−1
2.3. Optical −1.1
in the 0.3–5.5 keV energy range, which translates into
MWC 656 was observed with different optical facili- 3.5±0.9×10−14 erg cm−2 s−1 in the 0.5–8 keV energy
tiestoobtaincontemporaneousphotometricandspectro- range. Thisfluxisapproximatelyoneorderofmagnitude
larger than the one measured for MWC 656 with Chan-
2 http://casa.nrao.edu dra. However,theXMM-Newtonsourcewasthesuperpo-
X-ray/radioobservation of MWC 656 5
1032
LMXB HMXB
A0620−00 MWC656
1031 GX339−4 CygnusX-1
GS2000+25
XTE J1859+226
1030 GRO J0422+32
CygnusX-1
XTE J1118+480
1] SwiftJ1357.2-0933
−
s 1029 Corbeletal. (2013)
g
[erGHz 1028 Gallo etal. (2014) M X B c orrelation
6 L
8.
L
1027
1026
MWC656 quiescence
1025
1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039
L1−10keV [ergs−1]
Fig.4.—Radiovs. X-rayluminositydiagramincludingthepositionofMWC656(with1σuncertainties)obtainedwiththeChandra/VLA
observation reported in this work and the positions of a few additional significant sources from simultaneous observations. We plot the
X-ray/radiocorrelationsforBHLMXBsfromCorbeletal.(2013;solidredlinewithalightredshadow)andfromGalloetal.(2014;dashed
green line with a light green shadow). To display the luminosity range of LMXBs that follow the correlation we show data on the BH
LMXBsGX339−4(redhexagons), aswellasthetwofaintestsourceseverdetected inradioandX-rayssimultaneously: A0620−00(open
redsquare;Galloetal.2006)andXTEJ1118+480(lightreddiamond;Galloetal.2014). ThefaintestX-raydetectionsofLMXBswithout
radiocounterparts havealsobeenplottedforreference. ThesmallbluedotsindicatetheregionoftheparameterspacewhereCygnusX-1
hasbeendetectedinthelow/hardstate(Galloetal.2012). Thehard-stateradio-quietoutliersfromthecorrelationhavenotbeenplotted.
Thegraydashedlineseparatesthequiescentstateregion(left)andtheotherstates (right)accordingtoPlotkinetal.(2013).
sitionoftwosources. AssumingthatCXOU-1hadacon- star would require too high magnetic fields3 combined
stantfluxof∼8.0×10−15ergcm−2 s−1 inthe0.5–8keV with relatively high electron densities above the energy
energyrange(asdeterminedfromtheChandradata),the threshold of 10 keV (Dulk 1985; Gu¨del 2002). There-
correspondingfluxofMWC656atthetimeoftheXMM- fore,in what follows we consider that the detected radio
Newton observation was ∼ 2.7×10−14 erg cm−2 s−1 in emission in MWC 656 has a synchrotronorigin in a jet.
the same energy range. This would indicate a decrease We show in Figure 4 the radio versus X-ray lumi-
of a factor of ∼7 in the X-ray flux of MWC 656 between nosity diagram for BH X-ray binaries. The simul-
2013 June and 2015 July. taneous Chandra/VLA observation of MWC 656 al-
Considering a distance of 2.6±0.6 kpc to MWC 656 lows us to place the source within the diagram in
(Casares et al. 2014), the Chandra X-ray luminosity of a reliable way4. MWC 656 is one of the faintest
the source in the 0.5–8 keV energy range is L = stellar-mass BHs ever detected in X-rays, together
X
(3.3+2.4)×1030 erg s−1 (including the flux and distance with XTE J1118+480, GS 2000+25, XTE J1859+226,
unce−r2t.a0inties). Considering a BH mass in the range of GRO J0422+32, A0620−00, and Swift J1357.2−0933,
3.8–6.9 M⊙ this translates into (4.9+−33..92)×10−9 LEdd. (2G00a3ll,o2e0t06a;l.P2lo0t1k4i;nMetillaelr.-2J0o1n6e)s,eatnadl.th20e1f1a;inGteasltlooenteailn.
In the 1–10 keV energy range these values are: L =
(2.5+2.6)×1030 erg s−1 = (3.7+4.0)×10−9 L . TXhese X-raysalsodetected in radio. The obtained luminosities
−1.7 −2.7 Edd for this quiescent BH HMXB are fully compatible with
luminosities and the obtained photon index are fully
those of the X-ray/radio correlations derived from qui-
compatible with a BH in deep quiescence (Plotkin et al.
escent BH LMXBs at the low-luminosity end. Together
2013).
with Cygnus X-1 at the high-luminosity end, it is now
The simultaneous VLA observation of MWC 656 has
yielded a detection with a peak flux density of 3.5 ±
1.1 µJy beam−1. The accreting BH in MWC 656 could 3 We note the following: 1) the rapid rotation of Be stars pre-
vents the existence of high magnetic fields in these objects; and
easily produce it through synchrotron emitting elec- 2) magnetism is less present in massive binaries than in isolated
trons in a jet, as seen in many X-ray binaries (e.g., massivestars(Sch¨olleretal.2014;Neineretal.2015).
Fender & Mun˜oz-Darias2016andreferencestherein). In 4 The position of MWC 656 in Figure 3 of Dzibetal. (2015)
is based on their VLA data obtained in 2015 Feb-Apr and the
contrast, the production of such flux density by gyro-
non-simultaneous XMM-Newton data obtained in 2013 June. In
synchrotron radiation in the magnetic field of the Be addition, our Chandra observation shows that the XMM-Newton
flux corresponds to two sources and that the X-ray luminosity of
MWC656issignificantlyvariable.
6 Rib´o et al.
clear, for the first time, that the accretion/ejection cou- ment and operated by the Institute for Space Studies
pling instellar-massBHsis independent ofthe natureof of Catalonia (IEEC). This work has made use of data
the donor star. from the European Space Agency (ESA) mission Gaia
Finally,giventheX-rayvariabilityfoundinMWC656, (http://www.cosmos.esa.int/gaia), processed by the
future simultaneous X-ray/radio observations should al- Gaia DataProcessingandAnalysisConsortium(DPAC,
low us to trace the motion of the source in the radio http://www.cosmos.esa.int/web/gaia/dpac/consortium).
versus X-ray luminosity diagram, and directly check the Funding for the DPAC has been provided by national
slopeofthe correlationatthe low-luminosityendforthe institutions, in particular the institutions partici-
first time in HMXBs. pating in the Gaia Multilateral Agreement. This
research has made use of the MAXI data provided
by RIKEN, JAXA, and the MAXI team. This re-
We thank the anonymous reviewer for providing
search has made use of NASA’s Astrophysics Data
suggestions that helped to improve the original version
System; the SIMBAD database and the VizieR
of the manuscript. We thank S. Corbel for providing
catalogue access tool (Ochsenbein et al. 2000), op-
the data for LMXBs and E. Gotthelf for useful dis-
erated at CDS, Strasbourg, France; and Astropy, a
cussions on the Chandra astrometry. The scientific
community-developed core Python package for As-
results reported in this article are based to a significant
tronomy (Astropy Collaboration et al. 2013). We
degree on observations made by the Chandra X-ray
acknowledge support by the Spanish Ministerio de
Observatory. This research has made use of software
Econom´ıa y Competitividad (MINECO/FEDER,
provided by the Chandra X-ray Center (CXC) in the
UE) under grants AYA2013-47447-C3-1-P, AYA2013-
application package CIAO. This research has made use
47447-C3-2-P,AYA2013-42627, AYA2016-76012-C3-1-P,
of the XSPEC software. The National Radio Astron-
FPA2015-69210-C6-2-R, MDM-2014-0369 of ICCUB
omy Observatory is a facility of the National Science
(Unidad de Excelencia ‘Mar´ıa de Maeztu’), SEV-2015-
Foundation operated under cooperative agreement by
0548 of IAC (Centro de Excelencia ‘Severo Ochoa’),
Associated Universities, Inc. The Common Astronomy
and the Catalan DEC grant 2014 SGR 86. P.M.A.
Software Applications, CASA, is a software produced
acknowledges partial support through the ASI grant
andmaintainedbytheNRAO.Theauthorsacknowledge
no. I/028/12/0. J.C. acknowledges support by the
support of the TFRM team for preparing and carrying
Leverhulme Trust through the Visiting Professorship
out the optical photometric observations. The Joan
Grant VP2-2015-046.
Or´o Telescope (TJO) of the Montsec Astronomical
Facilities: CXO(ACIS), VLA.
Observatory (OAdM) is owned by the Catalan Govern-
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