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

X-ray observations of stellar-mass black holes (BHs) in binary systems have allowed for the study of the properties of the accretion disks that surround them, as well as their changes over time (see Remillard & McClintock 2006 for a review). Radio observations of these systems have allowed us to gain knowledge of the ejection processes in the form of relativistic jets (Mirabel & Rodríguez 1999; Fender 2001; Fender & Muñoz-Darias 2016). Simultaneous observations have revealed the existence of non-linear correlations between the X-ray and radio luminosities during the so-called low/hard and quiescent states that are consistent with scale-invariant jet models (Corbel et al. 2003; Gallo et al. 2003; Markoff et al. 2003; Fender et al. 2004; Fender & Belloni 2004; Corbel et al. 2013; Gallo et al. 2014). However, these studies have mainly been conducted for BHs in low-mass X-ray binaries (LMXBs) because the only confirmed BH in a high-mass X-ray binary (HMXB) in the Galaxy before the discovery of MWC 656, namely Cygnus X-1, always displays a high luminosity and does not allow us to trace the correlation (see, e.g., Zdziarski 2012).

MWC 656 is the first binary system containing a BH in orbit around a Be star (Casares et al. 2014). Its discovery was triggered by the detection, with AGILE, of the transient gamma-ray source AGL J2241+4454 (Lucarelli et al. 2010), undetected with Fermi/LAT (Alexander & McSwain 2015) but recently reported to show recurrent activity with hints of long-term variability with AGILE (Munar-Adrover et al. 2016). The binary nature of MWC 656 (a bright Be star also named HD 215227) was suggested by a 60.37 d periodicity in optical photometry (Williams et al. 2010; Paredes-Fortuny et al. 2012), and later confirmed through radial velocity studies (Casares et al. 2012). A detailed spectroscopic analysis revealed the presence of a He ii emission line at λ4686 Å that could only be produced in an accretion disk around the invisible companion to the Be star. A re-analysis of radial velocity data using an Fe ii emission line from the Be circumstellar disk and the He ii emission line from the accretion disk, together with an improved spectral type classification of B1.5-B2 III for the Be star, provided a mass for the compact object in the range of 3.8–6.9 M_⊙, thus confirming its BH nature (Casares et al. 2014).

The discovery of the first Be/BH system is relevant in the context of binary system evolution. Although it partially solves the problem of the absence of Be/BH systems (Belczynski & Ziolkowski 2009), the discovery of MWC 656 is observationally biased by the lack of a bright X-ray counterpart, suggesting that there might be a large number of hidden Be/BH systems. In addition, simulations have shown that it is very difficult to form Be/BH binaries with properties similar to those of MWC 656, although they can evolve into close NS/BH systems that will merge on timescales of a few Gyr and produce gravitational waves detectable with advanced LIGO/Virgo in nearby galaxies (Grudzinska et al. 2015).

Munar-Adrover et al. (2014) conducted a 14-ks XMM-Newton observation of MWC 656 on 2013 June 4 that provided the detection between 0.3 and 5.5 keV of a faint source coincident at the 2.4σ level with MWC 656. This revealed that MWC 656 was a new HMXB. The spectrum analysis required a model fit with two components, a blackbody plus a power law, the latter dominating above ≃0.8 keV. The thermal emission was proposed to arise from the wind of the Be star, while the non-thermal emission arose from the vicinity of the BH. The observed non-thermal luminosity was ${L}_{{\rm{X}}}=({1.6}_{-0.9}^{+1.0})\times {10}^{31}$ erg s⁻¹, or (3.1 ± 2.3) × 10⁻⁸ L_Edd for a source distance of 2.6 ± 0.6 kpc, indicating a BH in deep quiescence (see Plotkin et al. 2013 for reference).

Radio observations with different interferometers, frequencies, and epochs have provided 3σ upper limits to the flux density as low as 30 μJy (Moldón 2012; Marcote 2015). Recently, Dzib et al. (2015) have reported a detection with VLA at a peak flux density of 10 ± 3 μJy beam⁻¹. Observations at TeV energies with the MAGIC telescopes have only led to upper limits (Aleksić et al. 2015). For a detailed review on MWC 656 see Ribó 2015.

In this Letter, we report on a deep joint Chandra/VLA observation (and contemporaneous optical data) of the first Be/BH binary MWC 656 that has allowed us to: 1) resolve the XMM-Newton detection into two sources, one of them unambiguously associated with MWC 656; 2) find that it is significantly fainter than two years before and reaches the faintest quiescent luminosities detected in stellar-mass BHs; and 3) detect a faint radio counterpart and find that the obtained X-ray/radio luminosities for this quiescent BH HMXB are fully compatible with those of the X-ray/radio correlations derived from quiescent BH LMXBs at the low-luminosity end.

2.1. X-rays

MWC 656 was observed with the Chandra X-ray Observatory ACIS-S camera using the VFAINT data mode during ∼60 ks, from 2015 July 24 at 21:03 UT to July 25 at 14:29 UT (PI: M. Ribó; Chandra ObsId 16753 ). The observation covered the orbital phase range 0.01–0.02 (using JD₀ = 2453243.3 from Williams et al. 2010). The data were analyzed using the Chandra Interactive Analysis of Observations (CIAO) software, v4.7¹² (Fruscione et al. 2006). The event files were reprocessed by means of the chandra_repro script. No flares were detected in the background light curve, yielding an effective exposure time of 59.1 ks. The data analysis was performed in the 0.5–8 keV energy band at which Chandra presents higher sensitivity.

We show the Chandra X-ray image of the field around MWC 656 in Figure 1, together with the contours of the previous XMM-Newton image reported in Munar-Adrover et al. (2014). Inspection of the X-ray image superimposing optical positions from the USNO-B1.0 catalog (Monet et al. 2003) reveals a systematic offset. Using three common sources we have applied a correction of R.A. = +05 and decl. = +01 to the Chandra data (and of R.A. = +24 and decl. = +49 to the XMM-Newton data using the only common source). The better angular resolution of Chandra has allowed us to resolve the XMM-Newton source into two sources. We have used the wavdetect algorithm to obtain the significance and positions of both sources. MWC 656 is detected at the 8.5σ confidence level (c.l.), with equatorial coordinates (after correction) R.A. = ${22}^{{\rm{h}}}{42}^{{\rm{m}}}57\buildrel{\rm{s}}\over{.} 27\pm 0\buildrel{\rm{s}}\over{.} 01$, decl. = $44^\circ 43^{\prime} 18\buildrel{\prime\prime}\over{.} 5\pm 0\buildrel{\prime\prime}\over{.} 1$ (uncertainties are statistical only and at 1σ c.l.). The position of MWC 656 from Gaia data at the epoch of the Chandra observation (epoch = 2015.56) following Gaia Collaboration et al. (2016) and Gaia Collaboration (2016) is: R.A. = ${22}^{{\rm{h}}}{42}^{{\rm{m}}}57\buildrel{\rm{s}}\over{.} 29718\ \pm 0\buildrel{\rm{s}}\over{.} 00002$, decl. = $44^\circ 43^{\prime} 18\buildrel{\prime\prime}\over{.} 2099\pm 0\buildrel{\prime\prime}\over{.} 0002$. The offset between both positions is 04 ± 01_stat ± 01_syst, and the Chandra source is thus the counterpart of MWC 656.

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The new source, located ∼135 to the southeast of MWC 656, is detected at a 15.9σ c.l. and has equatorial coordinates (after correction) $\mathrm{R.A.}={22}^{{\rm{h}}}{42}^{{\rm{m}}}57\buildrel{\rm{s}}\over{.} 969\pm 0\buildrel{\rm{s}}\over{.} 005$, $\mathrm{decl.}=44^\circ 43^{\prime} 07\buildrel{\prime\prime}\over{.} 34\pm 0\buildrel{\prime\prime}\over{.} 05$, and thus is named CXOU J224257.9+444307 (CXOU-1 from now on). The Chandra image reveals that MWC 656 is fainter than CXOU-1 (see below), while the shape of the XMM-Newton contours reveals that in 2013 June MWC 656 was brighter than CXOU-1.

With the use of the srcflux script we extracted net counts for MWC 656 (22.1 ± 4.9 at 90% c.l.) and CXOU-1 (38.7 ± 6.3), taking into account the background and the point-spread function of the instrument. These measurements were obtained from a region centered at the peak of the X-ray emission (found by wavdetect) of each source with a radius of 3'', while background counts were obtained from a nearby circular region with a 10'' radius, avoiding contamination from nearby sources, always within the 0.5–8.0 keV energy band.

We used XSPEC version 12.8.2 (Arnaud 1996) to perform a spectral analysis of both sources. The same regions for source and background used to extract net counts were considered. We created response files in order to take into account the energy-dependent behavior of the instrument. We fitted the spectra with two distinct models, an absorbed power law and an absorbed blackbody, always using a fixed column density of N_H = 1.8 × 10²¹ cm⁻² (see Munar-Adrover et al. 2014). The low number of counts required the use of the cstat statistic within XSPEC to estimate the best-fit parameters and their associated uncertainties (Cash 1979).

We show the spectra of MWC 656 and CXOU-1, together with the fitted models, in Figure 2. The results of the fits are listed in Table 1. Although both sources can be fitted with either model, the obtained C-statistics reveal that the power-law model is preferred since blackbody models result in an excess at both low and high energies. The obtained fluxes for the power-law models in the 0.5–8 keV range are ${4.1}_{-1.5}^{+2.3}\,\times {10}^{-15}$ erg cm⁻² s⁻¹ for MWC 656 and ${8.0}_{-2.1}^{+2.8}\times {10}^{-15}$ erg cm⁻² s⁻¹ for CXOU-1. Although CXOU-1 appears brighter in Figure 1, the fluxes obtained by the spectral analysis are compatible at 1σ c.l. The power-law fit for MWC 656 provides a photon index of ${2.2}_{-0.9}^{+1.3}$, while for CXOU-1 we obtain 1.8 ± 0.6, thus fully compatible at 1σ c.l. The conclusion from the analysis of the Chandra data is that both sources have a similar spectrum, while the X-ray flux of MWC 656 is approximately half of the one measured for CXOU-1. There is no reported evidence for this source in the literature and the low number of photons prevents us from clearly establishing its nature.

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Table 1.  Summary of Spectral Fit Results

Source	Power Law			Blackbody
	Γ	$F(0.5$–$8\,\mathrm{keV})$10−15 [erg cm−2 s−1]	C-statistic		${k}_{{\rm{B}}}T$ [keV]	F(0.5–8 keV)10−15 [erg cm−2 s−1]	C-statistic
MWC 656	${2.2}_{-0.9}^{+1.3}$	${4.1}_{-1.5}^{+2.3}$	0.3		0.6 ± 0.3	${2.7}_{-1.1}^{+1.5}$	3.0
CXOU J224257.9+444307	1.8 ± 0.6	${8.0}_{-2.1}^{+2.8}$	4.4		0.7 ± 0.2	${6.0}_{-1.7}^{+2.3}$	12.5

Note. Uncertainties are at 68% c.l. Flux values are unabsorbed.

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We have tested if the Chandra data for the two sources produces a spectrum that needs a two-component model. We have combined the spectra of the two sources and find that the data can be reproduced with a single power law with a photon index of 2.1 ± 0.5 (C-statistic 3.7). Therefore, the Chandra data do not support the two-component model reported in Munar-Adrover et al. (2014).

2.2. Radio

MWC 656 was observed with the Karl G. Jansky Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO) on 2015 July 25 from 05:00 to 11:00 UTC (during the Chandra observation) in the A configuration. The observation was conducted at 10 GHz, with full circular polarization, using 32 spectral windows with 64 channels of 2 MHz bandwidth each, providing a total bandwidth of 4 GHz. The amplitude calibrator was 3C 48. The phase calibrator J2255+4202 was observed in 1:20-min runs interleaved with 6-min runs on the target source.

The calibration was conducted using version 4.5.0 of the CASA package¹³ of NRAO (McMullin et al. 2007). The data have been reduced using standard amplitude and phase calibration steps. We imaged the data using the clean procedure with a natural weighting, obtaining a synthesized beam of $0.35\times 0.19\,{\mathrm{arcsec}}^{2}$ in a Position Angle (P.A.) of 47° (north to east). Due to the proximity of a bright quasar affecting the field of MWC 656 (see Marcote 2015), we imaged a region of a few arcmin including this quasar. We conducted a multi-scale clean, considering the usual point-like components and extended components with sizes one to three times the synthesized beam (see Cornwell 2008 and Rich et al. 2008 for details). To check the reliability of the obtained results we also analyzed the data using standard clean, different weighting schemes, and different time intervals, which always led to compatible results within uncertainties.

The obtained image is shown in Figure 3. A faint radio source with a peak flux density of 3.5 ± 1.1 μJy beam⁻¹ is detected at the optical position of MWC 656 within uncertainties. Although this is a marginal detection, there are several facts that support the reality of the radio source as the counterpart of MWC 656: 1) it is the brightest radio source in the image, 2) it is the only one located within the Chandra X-ray source position uncertainty, 3) it is fully compatible with the optical position, and 4) there is a former radio detection of MWC 656 (Dzib et al. 2015).

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Inspection at the position of CXOU-1 reveals no radio counterpart, with a 3σ flux density upper limit of 3.2 μJy for a point-like source.

2.3. Optical

The Chandra observation of MWC 656 reported here has revealed that the XMM-Newton source presented in Munar-Adrover et al. (2014) was in fact the superposition of two sources: MWC 656 and the new source CXOU J224257.9+444307 (CXOU-1). Here MWC 656 is significantly fainter than it was in the XMM-Newton observation. The VLA observation of MWC 656 has led to the detection of a faint source. This is the first simultaneous X-ray/radio detection of the first Be/BH binary system. Here we discuss the implications of these detections.

First, it is worth noting that during the Chandra/VLA observation MWC 656 was showing optical photometric and spectroscopic properties compatible with those reported in previous works, indicating that there was no significant change in either the Be star and its circumstellar disk or the outer part of the BH accretion disk (He ii emission line). In addition, inspection of the MAXI data (Matsuoka et al. 2009) in the direction of AGL J2241+4454 reveals no X-ray emission up to now (2009 August to 2016 October). Therefore, MWC 656 appears to have been in a long quiescent X-ray state, at least during the last 7 years.

The X-ray flux of MWC 656 provided by Chandra in 2015 July is ${4.1}_{-1.5}^{+2.3}\times {10}^{-15}$ erg cm⁻² s⁻¹ in the 0.5–8 keV energy range. For comparison, the previous X-ray observation conducted with XMM-Newton in 2013 June provided a total flux of ${4.6}_{-1.1}^{+1.3}\times {10}^{-14}$ erg cm⁻² s⁻¹ in the 0.3–5.5 keV energy range, which translates into 3.5 ± 0.9 × 10⁻¹⁴ erg cm⁻² s⁻¹ in the 0.5–8 keV energy range. This flux is approximately one order of magnitude larger than the one measured for MWC 656 with Chandra. However, the XMM-Newton source was the superposition of two sources. Assuming that CXOU-1 had a constant flux of ∼8.0 × 10⁻¹⁵ erg cm⁻² s⁻¹ in the 0.5–8 keV energy range (as determined from the Chandra data), the corresponding flux of MWC 656 at the time of the XMM-Newton observation was ∼2.7 × 10⁻¹⁴ erg cm⁻² s⁻¹ in the same energy range. This would indicate a decrease of a factor of ∼7 in the X-ray flux of MWC 656 between 2013 June and 2015 July.

Considering a distance of 2.6 ± 0.6 kpc to MWC 656 (Casares et al. 2014), the Chandra X-ray luminosity of the source in the 0.5–8 keV energy range is ${L}_{{\rm{X}}}=({3.3}_{-2.0}^{+2.4})\times {10}^{30}$ erg s⁻¹ (including the flux and distance uncertainties). Considering a BH mass in the range of 3.8–6.9 M_⊙ this translates into $({4.9}_{-3.2}^{+3.9})\times {10}^{-9}$ L_Edd. In the 1–10 keV energy range these values are: ${L}_{{\rm{X}}}=({2.5}_{-1.7}^{+2.6})\times {10}^{30}$ erg s⁻¹=$({3.7}_{-2.7}^{+4.0})\,\times {10}^{-9}$ L_Edd. These luminosities and the obtained photon index are fully compatible with a BH in deep quiescence (Plotkin et al. 2013).

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 easily produce it through synchrotron emitting electrons in a jet, as seen in many X-ray binaries (e.g., Fender & Muñoz-Darias 2016 and references therein). In contrast, the production of such flux density by gyro-synchrotron radiation in the magnetic field of the Be star would require high magnetic fields¹⁴ combined with relatively high electron densities above the energy threshold of 10 keV (Dulk 1985; Güdel 2002). Therefore, in what follows we consider that the detected radio emission in MWC 656 has a synchrotron origin in a jet.

We show in Figure 4 the radio versus X-ray luminosity diagram for BH X-ray binaries. The simultaneous Chandra/VLA observation of MWC 656 allows us to place the source within the diagram in a reliable way.¹⁵ MWC 656 is one of the faintest stellar-mass BHs ever detected in X-rays, together with XTE J1118+480, GS 2000+25, XTE J1859+226, GRO J0422+32, A0620−00, and Swift J1357.2−0933, (Gallo et al. 2014; Miller-Jones et al. 2011; Gallo et al. 2003; Gallo et al. 2006; Plotkin et al. 2016), and the faintest one in X-rays also detected in radio. The obtained luminosities for this quiescent BH HMXB are fully compatible with those of the X-ray/radio correlations derived from quiescent BH LMXBs at the low-luminosity end. Together with Cygnus X-1 at the high-luminosity end, it is now clear, for the first time, that the accretion/ejection coupling in stellar-mass BHs is independent of the nature of the donor star.

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Finally, given the X-ray variability found in MWC 656, future simultaneous X-ray/radio observations should allow us to trace the motion of the source in the radio versus X-ray luminosity diagram, and directly check the slope of the correlation at the low-luminosity end for the first time in HMXBs.

We thank the anonymous reviewer for providing suggestions that helped to improve the original version of the manuscript. We thank S. Corbel for providing the data for LMXBs and E. Gotthelf for useful discussions on the Chandra astrometry. The scientific results reported in this article are based to a significant degree on observations made by the Chandra X-ray Observatory. This research has made use of software provided by the Chandra X-ray Center (CXC) in the application package CIAO. This research has made use of the XSPEC software. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Common Astronomy Software Applications, CASA, is a software produced and maintained by the NRAO. The authors acknowledge support of the TFRM team for preparing and carrying out the optical photometric observations. The Joan Oró Telescope (TJO) of the Montsec Astronomical Observatory (OAdM) is owned by the Catalan Government and operated by the Institute for Space Studies of Catalonia (IEEC). This work has made use of data from the European Space Agency (ESA) mission Gaia (http://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, http://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research has made use of the MAXI data provided by RIKEN, JAXA, and the MAXI team. This research has made use of NASA's Astrophysics Data System; the SIMBAD database and the VizieR catalogue access tool (Ochsenbein et al. 2000), operated at CDS, Strasbourg, France; and Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013). We acknowledge support by the Spanish Ministerio de Economía y Competitividad (MINECO/FEDER, UE) under grants AYA2013-47447-C3-1-P, AYA2013-47447-C3-2-P, AYA2013-42627, AYA2016-76012-C3-1-P, FPA2015-69210-C6-2-R, MDM-2014-0369 of ICCUB (Unidad de Excelencia "María de Maeztu"), SEV-2015-0548 of IAC (Centro de Excelencia "Severo Ochoa"), and the Catalan DEC grant 2014 SGR 86. P.M.A. acknowledges partial support through the ASI grant No. I/028/12/0. J.C. acknowledges support by the Leverhulme Trust through the Visiting Professorship Grant VP2-2015-046.

Facilities: CXO(ACIS) - Chandra X-ray Observatory satellite, VLA - .