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Radio emission from the X-ray pulsar Her X-1: a jet launched by a strong magnetic field neutron star?

Radio emission from the X-ray pulsar Her X-1: a jet launched by a strong magnetic field neutron... MNRAS 473, L141–L145 (2018) doi:10.1093/mnrasl/slx180 Advance Access publication 2017 November 13 Radio emission from the X-ray pulsar Her X-1: a jet launched by a strong magnetic field neutron star? 1‹ 1 1 2 J. van den Eijnden, N. Degenaar, T. D. Russell, J. C. A. Miller-Jones, 1 3 4 5 R. Wijnands, J. M. Miller, A. L. King and M. P. Rupen Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, Perth, WA 6845, Australia Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA KIPAC, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA Herzberg Astronomy and Astrophysics Research Centre, 717 White Lake Road, Penticton, BC, V2A 6J9, Canada Accepted 2017 November 6. Received 2017 November 3; in original form 2017 October 8 ABSTRACT Her X-1 is an accreting neutron star (NS) in an intermediate-mass X-ray binary. Like low- mass X-ray binaries (LMXBs), it accretes via Roche lobe overflow, but similar to many high-mass X-ray binaries containing a NS; Her X-1 has a strong magnetic field and slow spin. Here, we present the discovery of radio emission from Her X-1 with the Very Large Array. During the radio observation, the central X-ray source was partially obscured by a warped disc. We measure a radio flux density of 38.7 ± 4.8 µJy at 9 GHz but cannot constrain the spectral shape. We discuss possible origins of the radio emission, and conclude that coherent emission, a stellar wind, shocks and a propeller outflow are all unlikely explanations. A jet, as seen in LMXBs, is consistent with the observed radio properties. We consider the implications of the presence of a jet in Her X-1 on jet formation mechanisms and on the launching of jets by NSs with strong magnetic fields. Key words: accretion, accretion discs – stars: neutron – pulsars: individual: Her X-1 – X-rays: binaries. The companion star of Her X-1 has a mass of 2.2 M (Reynolds 1 INTRODUCTION et al. 1997; Leahy & Abdallah 2014). At the simplest level, accreting Her X-1 is an extensively-studied accreting X-ray pulsar, discovered NSs are classified based on the donor star mass into low-mass X-ray with the UHURU satellite (Tananbaum et al. 1972). The pulsar has binaries (LMXBs,  1M ), high-mass X-ray binaries (HMXBs, a low spin period of 1.24 s and is in a binary system with an orbital  10 M ) and the rare intermediate-mass X-ray binaries (IMXBs) period of 1.7 d (Leahy & Abdallah 2014). Her X-1 was the first in between. Her X-1 is an IMXB but combines characteristics from accreting neutron star (NS) where a cyclotron line was discovered both other classes: as in LMXBs, it accretes through Roche lobe (Trumper ¨ et al. 1978), with an energy of ∼40 keV. Although this overflow and an accretion disc (Scott & Leahy 1999), while most energy varies with time and X-ray flux (e.g. Staubert et al. 2016), it HMXBs accrete from the wind or circumstellar disc of the donor. In provides a direct measurement of the pulsar magnetic field of a few addition, Her X-1 has the strong magnetic field and low spin that are times 10 G. typically seen in HMXBs; NS LMXBs instead tend to have weaker Her X-1 shows peculiar variability in X-rays over a 35-d cy- magnetic fields of B  10 G and if pulsations are seen, these are cle, originating from the precession of a warped accretion disc typically at millisecond periods (Patruno & Watts 2012). (Scott & Leahy 1999, see also Fig. 1): the cycle starts in the bright Another observational difference between HMXBs and LMXBs Main High (MH) state, offering an unobscured view of the central is the presence of radio emission and inferred jets. LMXBs very X-ray source. This is followed with the Low State (LS), where the commonly show synchrotron emission from jets, which is corre- X-ray flux drops ∼99 per cent and only reflection off the face of lated with the X-ray emission from the accretion flow (Migliari & the companion and an accretion disc corona are visible (Abdallah Fender 2006; Gusinskaia et al. 2017; Tudor et al. 2017), similar & Leahy 2015). This LS is interspersed by the Short High (SH) to accreting black holes (BHs; Merloni, Heinz & di Matteo 2003; state, reaching a few tens of per cent of the original MH state flux. Falcke, Kording ¨ & Markoff 2004). On the contrary, in NS HMXBs This variability is geometric, and the central X-ray source does not jets have only been observed in Cir X-1, a young NS that might intrinsically vary on the 35-d time-scale. have a high-mass donor (Johnston, Soria & Gibson 2016). As jet formation is still poorly understood, it is unclear which properties of NS LMXBs and HMXBs could explain this apparent system- atic difference: the spin period, magnetic field, or the presence of E-mail: a.j.vandeneijnden@uva.nl 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L141/4622954 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L142 J. van den Eijnden et al. an accretion disc might all play a vital role. As Her X-1 shares characteristics of both classes, it can help understand the difference between their jet launching abilities. In this letter, we present the discovery of radio emission from Her X-1. We present the observations and results in Sections 2 and 3, and afterwards discuss the origin of the emission and implications for our understanding of jet formation in LMXBs and HMXBs. We also consider the possibilities for future observations. 2 OBSERVATIONS 2.1 Radio We observed Her X-1 with the Karl G. Jansky Very Large Array (VLA) on 2013 June06 (MJD 56449) from 02:12:01 to 03:26:38 UT, for a total of ∼54 min of on-source observing time (project ID: 13A-352, PI: Degenaar). We observed the target in X-band between 8 and 10 GHz in two basebands, while the ar- ray was in C-configuration, yielding a synthesized beam of 3.24 arcsec × 1.8 arcsec (position angle 8.57 ). We used J1331+305 Figure 1. MAXI /GSC and Swift/BAT daily X-ray light curves of Her X-1 and J1635+3808 (5.3 from the target) as the primary and secondary around the radio epoch on MJD 56449. The 35-d cyclic variability, due to calibrators, respectively. precession of the warped accretion disc, is clearly visible. The VLA epoch The observation was calibrated and imaged following standard is shown by the dashed line. procedures with the Common Astronomy Software Applications package (CASA) v4.7.2 (McMullin et al. 2007). We did not encounter any significant RFI or calibration issues. Using CASA’s multifre- quency, multiscale CLEAN task, we imaged Stokes I and V to make a source model of the field. With Briggs weighting and setting the robustness parameter to 0 to balance sensitivity and resolution, we −1 reached an RMS noise of 4.8 µJy beam . We fit a point source in the image plane by forcing the fit of an elliptical Gaussian with the FWHM and orientation of the synthesized beam. In addition, we also individually imaged the 8–9 and 9–10 GHz basebands with the same approach as the full band. As we did not observe a polarization calibrator, beam squint can affect our circular polarization estimates away from the pointing centre by a few per cent. 2.2 X-rays We examined the X-ray properties of Her X-1 during the VLA epoch in order to obtain a simultaneous X-ray flux and determine the source’s phase in the 35-d precession cycle. To measure the X-ray flux, we extracted the MAXI/Gas Slit Camera (GSC; Matsuoka et al. 2009) spectrum for the MJD of the VLA obser- vation from the MAXI website (http://maxi.riken.jp). We extracted the spectrum for the full MJD to ensure a sufficient number of counts for a basic characterization of the spectrum. We also obtained the MAXI/GSC and Swift/Burst Alert Telescope (BAT) (Krimm et al. 2013) long-term X-ray light curves of Her X-1. Fig. 1 shows Figure 2. VLA image of Her X-1 at 9 GHz. The black cross indicates the the MAXI and Swift light curves, clearly showing the 35-d cycle and best known position, from the infrared 2MASS survey. In the bottom left revealing that Her X-1 was in the first LS of its precession cycle. corner, we show the half-power contour of the synthesized beam. Finally, we also downloaded the MAXI spectrum on MJD 56437, the peak of the prior MH state, to estimate the unobscured X-ray flux. Abdallah 2014) and defining the radio luminosity L = 4πνS d , R ν 28 −1 this corresponds to L = (1.6 ± 0.2) × 10 erg s . The source is also detected in the 8–9 and 9–10 GHz bands separately at 3 RESULTS 42.2 ± 6.8 and 36.2 ± 6.8 µJy, respectively. However, the low significance means we do not well constrain the radio spectrum 3.1 Radio with α =−0.7 ± 5.3, where S ∝ ν . h m s s Her X-1 is detected at a flux density S = 38.7 ± 4.8 µJy at We measured a position of RA = 16 57 49 .792 ± 0 .027 and 9 GHz, with a significance of 8σ . A zoom of the target field is Dec. =+35 20 32.578 ± 0.225, where the uncertainties equal showninFig. 2. For a distance of d = 6.1 kpc (e.g. Leahy & the synthesized beam size divided by the signal-to-noise of the MNRASL 473, L141–L145 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L141/4622954 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Radio emission and jets in Her X-1 L143 detection. This position is consistent within the 1σ errors with the of the radio emission in GX 1+4 cannot be unambiguously inferred. best known position of Her X-1, from the infrared 2MASS survey Other symbiotic X-ray binaries have not been targeted by radio (Skrutskie et al. 2006), which is shown in Fig. 2 as well, and with campaigns and it is thus unknown whether radio emission occurs the lower accuracy positions at other wavelengths. Hence, this is in more sources of this type. Given the current upper limits or unlikely to be a background source. lack of observations, new, deep radio observations are needed to infer whether Her X-1 is an outlier among slow pulsars in LMXBs or whether radio emission occurs more commonly among such 3.2 X-rays sources. Her X-1 also shares characteristics with many of the NS HMXBs: We fit the two downloaded MAXI 2–20 keV spectra of Her X-1 a strong magnetic field ( 10 G) and a slow spin. Radio detec- to determine the flux on the MJD of the radio observation and at tions of HMXBs are relatively rare (Duldig et al. 1979; Nelson the height of the previous MH state. As the latter is only a short & Spencer 1988; Fender & Hendry 2000; Migliari, Miller-Jones (∼120 s) exposure, both spectra contain few photons (∼145 and & Russell 2011): the NS Cir X-1 launches resolved radio jets 40 photons, respectively) and are only suitable for a very sim- (Tudose et al. 2006), and likely is an HMXB (see Johnston ple fit. In both cases, we used XSPEC to fit an absorbed (TBABS) et al. 2016, for a recent discussion). However, no magnetic field blackbody (BBODYRAD) spectrum. We fix the N in both cases, as estimate is known for Cir X-1. Two other NS HMXBs have been the data quality is not sufficient to determine it directly. We set detected in radio. Most notably, the wind-accreting HMXB GX 301- N = 1.0 × 10 for the obscured LS (Inam & Baykal 2005)and 2 was detected over multiple radio epochs (Pestalozzi et al. 2009). 20 −2 N = 1.7 × 10 cm in the HS (Furst ¨ et al. 2013). This yields −11 −1 −2 However, the flux levels were consistent with those expected from 0.5–10 keV X-ray fluxes of ∼9 × 10 erg s cm during the ra- −9 −1 −2 the stellar wind, and the claimed transient outflow component in dio observation and ∼3 × 10 erg s cm during the MH state. the emission has not been confirmed (Migliari et al. 2011). Addi- The latter is slightly lower than the typical range of X-ray fluxes of −9 −8 −1 −2 tionally, the Be/X-ray binary A 1118-61, consisting of a NS and Her X-1 in the MH state of 5 × 10 to 10 erg s cm (Staubert a Be companion, was detected in only one out of eight observa- et al. 2016). This difference might be due to the short exposure of tions by Duldig et al. (1979). Due to the crowded field, this de- the MAXI spectrum, combined with the dips often seen during the tection might not be related to the Be/X-ray binary. Both these MH state (Igna & Leahy 2011). (possible) detections are thus not conclusive about the presence of ajet. There exist numerous radio non-detections of NS HMXBs. How- 4 DISCUSSION ever, for most of these sources, the radio upper limits (ranging We present the first radio detection of the IMXB Her X-1, at a from hundreds of µJy to mJy levels; Duldig et al. 1979; Nelson & flux density of S = 38.7 ± 4.8 µJy. Her X-1 has been the subject Spencer 1988; Fender & Hendry 2000) are not constraining com- of multiple radio searches, but similar to most NS HMXBs, was pared with NS LMXBs and deep observations with current genera- hitherto never detected. Coe & Crane (1980) observed Her X-1 tion radio telescopes might reveal these sources. Only the Be/X-ray every day of an entire 35-d precession cycle. The source was not binaries A 0535+26 (Tudose et al. 2010) and X Per, and the wind- detected in any of the observations, with 3σ upper limits of 9 mJy. accreting NS HMXB 4U 2206+54 (Migliari & Fender 2006)have Nelson & Spencer (1988) observed Her X-1 twice in a large sample 27 −1 radio luminosity upper limits of 5 × 10 erg s , below our Her study of X-ray binaries and cataclysmic variables, reaching a 5σ X-1 measurement. However, these sources were observed at much upper limit of 1.3 mJy. In this discussion, we will first compare the lower (more than an order of magnitude) X-ray luminosity, making radio properties of Her X-1 with different classes of accreting NSs. any direct comparison with Her X-1 difficult. Subsequently, we will discuss the origin of the radio emission and implication for future research. 4.2 The emission mechanism and physical origin Three radio emission mechanisms are relatively unlikely to explain 4.1 ComparisonwithNSLMXBs andHMXBs our detection of Her X-1. First, thermal emission would require too Radio detections and jets are ubiquitous in disc-accreting, weak high densities of emitting material on too large scales. Secondly, we magnetic field NS LMXBs (Migliari & Fender 2006; Gusinskaia imaged Stokes V in addition to Stokes I and did not detect the target, et al. 2017; Tudor et al. 2017). While L and L do appear setting a 3σ upper limit on the circular polarization of 37 per cent. X R to be related for these types of NS systems, no universal re- Coherent emission should be highly circularly polarized and can lation has emerged (Tudor et al. 2017). Most relevant for the thus be excluded. Finally, free–free emission from a strong stellar comparison with Her X-1 are the handful of LMXBs containing wind, as observed in the HMXB GX 301-2 (Pestalozzi et al. 2009), is a slow pulsar. With the exception of one (see below), none of unlikely: while a wind might be present (Leahy 2015), its strength these sources have been detected in the radio. 2A 1822-371 and implies a flux density over two orders of magnitude lower than 4U 1626-67 have unconstraining upper limits on their radio flux our detection (Wright & Barlow 1975). On the contrary, syn- of 200 µJy (Fender & Hendry 2000). GRO 1744-28 (The Bursting chrotron emission is consistent with the observed radio properties. Pulsar) does have deep ATCA upper limits during it small 2017 out- In the following, we will discuss possible physical origins of such burst (Russell et al. 2017). Finally, for the mildly recycled 11-Hz synchrotron emission. pulsar IGR J1748-2466 no radio upper limits are known. First, synchrotron-emitting shocks could occur in the interaction As stated, a single slow pulsar in an LMXB has been detected: the between the disc and the magnetosphere or in the accretion col- symbiotic X-ray binary GX 1+4 was recently discovered in radio umn on to the magnetic poles. However, the Compton limit on the (van den Eijnden et al. 2017). In this type of source, the NS accretes brightness temperature of 10 K sets a lower limit on the size of from the stellar wind of an evolved low-mass companion. The origin the emitting region of 7.5 × 10 km. We can estimate the size of MNRASL 473, L141–L145 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L141/4622954 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L144 J. van den Eijnden et al. the magnetosphere R by rewriting equation (1) from Cackett et al. strength and such behaviour has not been observed in Her X-1 in (2009): its regular, 35-d cyclic behaviour. Finally, we might observe a compact, synchrotron-emitting radio 4/7 −4/14 B f F ang bol 9 jet, similar to those seen in NS LMXBs with weaker (10 G) R = k 5 −9 −1 −2 1.2 × 10 G η 10 erg s cm magnetic fields. If we compare Her X-1’s radio properties with −8/7 −12/7 −4/7 the NS LMXB sample in the L /L diagram (see e.g. Gusinskaia X R M R D R (1) et al. 2017; Tudor et al. 2017, for recent versions), we see that 1.4M 10 km 5 kpc these are consistent with several accreting millisecond X-ray pulsar where k is a geometry factor relating spherical and disc accretion, (AMXP) observations if we assume the LS flux. While an interesting typically assumed to be 0.5 for disc accretion, B is the magnetic field comparison, as AMXPs have ∼3–4 orders of magnitude weaker strength, f is the anisotropy correction factor, η is the accretion magnetic fields and faster spins, we should actually again use the ang efficiency, F is the bolometric flux, and M, R and D are the mass, estimated unobscured (i.e. MH state) flux. In that comparison, the bol radius and distance of the NS. We use B ∼ 3 × 10 G (Staubert L of Her X-1 is three to ten times lower than hard-state and several et al. 2016), k = 0.5, f = 1, η = 0.1, M = 1.4 M , R = 10 km soft-state Atoll sources at similar L , and more similar to that of ang X (Leahy 2004)and D = 6.1 kpc (Leahy & Abdallah 2014). jet-quenched sources (e.g. Gusinskaia et al. 2017). As R scales inversely with flux, the maximum magnetospheric As jet formation is poorly understood, the cause of jet quenching size can be estimated with the LS 2–10 keV X-ray flux without is puzzling. It is observed in all BH LMXBs if and when they transi- −11 −1 −2 a bolometric correction (e.g. 9 × 10 erg s cm ): this yields tion into the soft spectral state (e.g. Gallo, Fender & Pooley 2003), R ≈ 1.7 × 10 km, smaller than the minimum emission region size. but the picture is more ambiguous in accreting NSs: only in a handful As the low flux during the LS of Her X-1 originates from a geometric of sources is quenching observed (see e.g. Miller-Jones et al. 2010; effect, it is actually more accurate to use the MH state bolometric Gusinskaia et al. 2017). If jet formation requires large scale-height −9 −1 −2 flux; for the measured MH state flux of 3 × 10 erg s cm , R poloidal magnetic fields, quenching might be explained as follows: is even smaller at ∼0.7 × 10 km. Hence, shocks can be excluded in the hard spectral state, the accretion disc might be truncated away as well, assuming that they indeed occur at the magnetosphere and from the compact object (e.g. Done, Gierlinski & Kubota 2007)asa not further out in the accretion flow. radiatively inefficient accretion flow (RIAF; Narayan & Yi 1995)or Another possibility is that we observe a propeller-driven outflow: corona replaces the inner disc, providing the required fields. As the if the magnetosphere spins faster than the disc where the magnetic disc moves inwards during the transition to softer spectral states, the and gas pressure are equal, it creates a centrifugal barrier that can RIAF disappears or the corona is cooled, breaking the jet formation either trap the disc (D’Angelo & Spruit 2010) or expel the mate- mechanism. rial (Illarionov & Sunyaev 1975; Campana et al. 1998). The latter If the above scenario indeed underlies jet quenching, it is difficult has, for instance, recently been inferred through X-ray monitoring to reconcile with the low L in Her X-1: there, the strong stellar in several strong magnetic field accreting NSs (e.g. two Be/X-ray magnetic field prevents the disc from moving inwards. However, this binaries; Tsygankov et al. 2016) and might explain the recent radio stellar field might instead hamper the initial formation of a RIAF detection of GX 1+4 (van den Eijnden et al. 2017). For a given NS or corona, also effectively quenching the jet. Alternatively, the jet magnetic field and spin period, one can estimate the maximum L formation might be partially suppressed as the disc pressure cannot for which the magnetosphere can still create this centrifugal barrier dominate and twist the strong magnetic field (Massi & Kaufman as (e.g. Campana et al. 2002): Bernado´ 2008), or as the magnetic field prevents the formation of a boundary layer at the NS surface, which might play a role in NS jet 2 −7/3 B P 37 7/2 formation (Livio 1999). L ≈ 4 × 10 k X,max 10 G 1s −2/3 5 M R 4.3 Implications for future research −1 erg s (2) 1.4M 10 km Out of the considered origins for the radio emission (shocks, a pro- where P is the pulsar spin and all other parameters are al- peller outflow and a compact jet), a jet appears most compatible with ready defined. For a magnetic field of ∼3 × 10 G, a spin both the correlated X-ray and radio properties and with the known period of 1.24 s and standard NS parameters, we estimate the properties of the Her X-1 system. The presence of a jet in Her X-1 37 −1 L ≈ 1.9 × 10 erg s for Her X-1. automatically implies that (the combination of) a strong magnetic X,max To assess whether a magnetic propeller could be at play in Her field and a slow spin do not completely impede jet formation (as X-1, we need to compare this maximum X-ray luminosity with suggested by, e.g. Massi & Kaufman Bernado 2008). This would the correct L of Her X-1. During the LS radio epoch, L ≈ imply that our understanding of jet formation, in the presence of X X 35 −1 4 × 10 erg s , comfortably below the upper limit for the pro- a strong NS magnetic field, needs to be revisited. Additionally, it peller effect. However, the actual, unobscured X-ray luminosity is opens up the possibility of observing radio emission from several the more accurate probe of the relevant physical properties (i.e. the currently undetected sources: for instance, the LMXBs containing mass accretion rate that balances the magnetic pressure). During slow X-ray pulsars and Be/X-ray binaries accreting from a (small) 37 −1 the prior MH state, Her X-1 reached L ≈ 1.2 × 10 erg s be- disc would be prime targets for such studies, as current genera- tween 2–10 keV. With a bolometric correction, the X-ray luminos- tion radio telescopes (e.g. VLA, ATCA) reach sensitivities orders of 37 −1 ity of Her X-1’s MH state typically reaches (2.5–5) × 10 erg s magnitude below the current typical upper limits for these sources. (Staubert et al. 2016). This is of comparable magnitude as the L In order to confidently confirm a jet nature of the radio emis- X,max estimate, although it should be noted that not every MH state reaches sion in Her X-1, new observations are necessary. A measurement of the same luminosity (e.g. Staubert et al. 2016) and equation (2) is the radio spectral index, combined with a linear polarization mea- merely an estimate. However, in other accreting NSs propellers have surement, and a search for extended structure or a jet-break in the been linked to a simultaneous decrease in X-ray flux and pulsation spectrum, could reveal the emission mechanism. If a jet is indeed MNRASL 473, L141–L145 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L141/4622954 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Radio emission and jets in Her X-1 L145 present, this opens up interesting possibilities to better understand Krimm H. A. et al., 2013, ApJS, 209, 14 Leahy D. 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Radio emission from the X-ray pulsar Her X-1: a jet launched by a strong magnetic field neutron star?

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MNRAS 473, L141–L145 (2018) doi:10.1093/mnrasl/slx180 Advance Access publication 2017 November 13 Radio emission from the X-ray pulsar Her X-1: a jet launched by a strong magnetic field neutron star? 1‹ 1 1 2 J. van den Eijnden, N. Degenaar, T. D. Russell, J. C. A. Miller-Jones, 1 3 4 5 R. Wijnands, J. M. Miller, A. L. King and M. P. Rupen Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, NL-1098 XH Amsterdam, the Netherlands International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, Perth, WA 6845, Australia Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA KIPAC, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA Herzberg Astronomy and Astrophysics Research Centre, 717 White Lake Road, Penticton, BC, V2A 6J9, Canada Accepted 2017 November 6. Received 2017 November 3; in original form 2017 October 8 ABSTRACT Her X-1 is an accreting neutron star (NS) in an intermediate-mass X-ray binary. Like low- mass X-ray binaries (LMXBs), it accretes via Roche lobe overflow, but similar to many high-mass X-ray binaries containing a NS; Her X-1 has a strong magnetic field and slow spin. Here, we present the discovery of radio emission from Her X-1 with the Very Large Array. During the radio observation, the central X-ray source was partially obscured by a warped disc. We measure a radio flux density of 38.7 ± 4.8 µJy at 9 GHz but cannot constrain the spectral shape. We discuss possible origins of the radio emission, and conclude that coherent emission, a stellar wind, shocks and a propeller outflow are all unlikely explanations. A jet, as seen in LMXBs, is consistent with the observed radio properties. We consider the implications of the presence of a jet in Her X-1 on jet formation mechanisms and on the launching of jets by NSs with strong magnetic fields. Key words: accretion, accretion discs – stars: neutron – pulsars: individual: Her X-1 – X-rays: binaries. The companion star of Her X-1 has a mass of 2.2 M (Reynolds 1 INTRODUCTION et al. 1997; Leahy & Abdallah 2014). At the simplest level, accreting Her X-1 is an extensively-studied accreting X-ray pulsar, discovered NSs are classified based on the donor star mass into low-mass X-ray with the UHURU satellite (Tananbaum et al. 1972). The pulsar has binaries (LMXBs,  1M ), high-mass X-ray binaries (HMXBs, a low spin period of 1.24 s and is in a binary system with an orbital  10 M ) and the rare intermediate-mass X-ray binaries (IMXBs) period of 1.7 d (Leahy & Abdallah 2014). Her X-1 was the first in between. Her X-1 is an IMXB but combines characteristics from accreting neutron star (NS) where a cyclotron line was discovered both other classes: as in LMXBs, it accretes through Roche lobe (Trumper ¨ et al. 1978), with an energy of ∼40 keV. Although this overflow and an accretion disc (Scott & Leahy 1999), while most energy varies with time and X-ray flux (e.g. Staubert et al. 2016), it HMXBs accrete from the wind or circumstellar disc of the donor. In provides a direct measurement of the pulsar magnetic field of a few addition, Her X-1 has the strong magnetic field and low spin that are times 10 G. typically seen in HMXBs; NS LMXBs instead tend to have weaker Her X-1 shows peculiar variability in X-rays over a 35-d cy- magnetic fields of B  10 G and if pulsations are seen, these are cle, originating from the precession of a warped accretion disc typically at millisecond periods (Patruno & Watts 2012). (Scott & Leahy 1999, see also Fig. 1): the cycle starts in the bright Another observational difference between HMXBs and LMXBs Main High (MH) state, offering an unobscured view of the central is the presence of radio emission and inferred jets. LMXBs very X-ray source. This is followed with the Low State (LS), where the commonly show synchrotron emission from jets, which is corre- X-ray flux drops ∼99 per cent and only reflection off the face of lated with the X-ray emission from the accretion flow (Migliari & the companion and an accretion disc corona are visible (Abdallah Fender 2006; Gusinskaia et al. 2017; Tudor et al. 2017), similar & Leahy 2015). This LS is interspersed by the Short High (SH) to accreting black holes (BHs; Merloni, Heinz & di Matteo 2003; state, reaching a few tens of per cent of the original MH state flux. Falcke, Kording ¨ & Markoff 2004). On the contrary, in NS HMXBs This variability is geometric, and the central X-ray source does not jets have only been observed in Cir X-1, a young NS that might intrinsically vary on the 35-d time-scale. have a high-mass donor (Johnston, Soria & Gibson 2016). As jet formation is still poorly understood, it is unclear which properties of NS LMXBs and HMXBs could explain this apparent system- atic difference: the spin period, magnetic field, or the presence of E-mail: a.j.vandeneijnden@uva.nl 2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L141/4622954 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L142 J. van den Eijnden et al. an accretion disc might all play a vital role. As Her X-1 shares characteristics of both classes, it can help understand the difference between their jet launching abilities. In this letter, we present the discovery of radio emission from Her X-1. We present the observations and results in Sections 2 and 3, and afterwards discuss the origin of the emission and implications for our understanding of jet formation in LMXBs and HMXBs. We also consider the possibilities for future observations. 2 OBSERVATIONS 2.1 Radio We observed Her X-1 with the Karl G. Jansky Very Large Array (VLA) on 2013 June06 (MJD 56449) from 02:12:01 to 03:26:38 UT, for a total of ∼54 min of on-source observing time (project ID: 13A-352, PI: Degenaar). We observed the target in X-band between 8 and 10 GHz in two basebands, while the ar- ray was in C-configuration, yielding a synthesized beam of 3.24 arcsec × 1.8 arcsec (position angle 8.57 ). We used J1331+305 Figure 1. MAXI /GSC and Swift/BAT daily X-ray light curves of Her X-1 and J1635+3808 (5.3 from the target) as the primary and secondary around the radio epoch on MJD 56449. The 35-d cyclic variability, due to calibrators, respectively. precession of the warped accretion disc, is clearly visible. The VLA epoch The observation was calibrated and imaged following standard is shown by the dashed line. procedures with the Common Astronomy Software Applications package (CASA) v4.7.2 (McMullin et al. 2007). We did not encounter any significant RFI or calibration issues. Using CASA’s multifre- quency, multiscale CLEAN task, we imaged Stokes I and V to make a source model of the field. With Briggs weighting and setting the robustness parameter to 0 to balance sensitivity and resolution, we −1 reached an RMS noise of 4.8 µJy beam . We fit a point source in the image plane by forcing the fit of an elliptical Gaussian with the FWHM and orientation of the synthesized beam. In addition, we also individually imaged the 8–9 and 9–10 GHz basebands with the same approach as the full band. As we did not observe a polarization calibrator, beam squint can affect our circular polarization estimates away from the pointing centre by a few per cent. 2.2 X-rays We examined the X-ray properties of Her X-1 during the VLA epoch in order to obtain a simultaneous X-ray flux and determine the source’s phase in the 35-d precession cycle. To measure the X-ray flux, we extracted the MAXI/Gas Slit Camera (GSC; Matsuoka et al. 2009) spectrum for the MJD of the VLA obser- vation from the MAXI website (http://maxi.riken.jp). We extracted the spectrum for the full MJD to ensure a sufficient number of counts for a basic characterization of the spectrum. We also obtained the MAXI/GSC and Swift/Burst Alert Telescope (BAT) (Krimm et al. 2013) long-term X-ray light curves of Her X-1. Fig. 1 shows Figure 2. VLA image of Her X-1 at 9 GHz. The black cross indicates the the MAXI and Swift light curves, clearly showing the 35-d cycle and best known position, from the infrared 2MASS survey. In the bottom left revealing that Her X-1 was in the first LS of its precession cycle. corner, we show the half-power contour of the synthesized beam. Finally, we also downloaded the MAXI spectrum on MJD 56437, the peak of the prior MH state, to estimate the unobscured X-ray flux. Abdallah 2014) and defining the radio luminosity L = 4πνS d , R ν 28 −1 this corresponds to L = (1.6 ± 0.2) × 10 erg s . The source is also detected in the 8–9 and 9–10 GHz bands separately at 3 RESULTS 42.2 ± 6.8 and 36.2 ± 6.8 µJy, respectively. However, the low significance means we do not well constrain the radio spectrum 3.1 Radio with α =−0.7 ± 5.3, where S ∝ ν . h m s s Her X-1 is detected at a flux density S = 38.7 ± 4.8 µJy at We measured a position of RA = 16 57 49 .792 ± 0 .027 and 9 GHz, with a significance of 8σ . A zoom of the target field is Dec. =+35 20 32.578 ± 0.225, where the uncertainties equal showninFig. 2. For a distance of d = 6.1 kpc (e.g. Leahy & the synthesized beam size divided by the signal-to-noise of the MNRASL 473, L141–L145 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L141/4622954 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Radio emission and jets in Her X-1 L143 detection. This position is consistent within the 1σ errors with the of the radio emission in GX 1+4 cannot be unambiguously inferred. best known position of Her X-1, from the infrared 2MASS survey Other symbiotic X-ray binaries have not been targeted by radio (Skrutskie et al. 2006), which is shown in Fig. 2 as well, and with campaigns and it is thus unknown whether radio emission occurs the lower accuracy positions at other wavelengths. Hence, this is in more sources of this type. Given the current upper limits or unlikely to be a background source. lack of observations, new, deep radio observations are needed to infer whether Her X-1 is an outlier among slow pulsars in LMXBs or whether radio emission occurs more commonly among such 3.2 X-rays sources. Her X-1 also shares characteristics with many of the NS HMXBs: We fit the two downloaded MAXI 2–20 keV spectra of Her X-1 a strong magnetic field ( 10 G) and a slow spin. Radio detec- to determine the flux on the MJD of the radio observation and at tions of HMXBs are relatively rare (Duldig et al. 1979; Nelson the height of the previous MH state. As the latter is only a short & Spencer 1988; Fender & Hendry 2000; Migliari, Miller-Jones (∼120 s) exposure, both spectra contain few photons (∼145 and & Russell 2011): the NS Cir X-1 launches resolved radio jets 40 photons, respectively) and are only suitable for a very sim- (Tudose et al. 2006), and likely is an HMXB (see Johnston ple fit. In both cases, we used XSPEC to fit an absorbed (TBABS) et al. 2016, for a recent discussion). However, no magnetic field blackbody (BBODYRAD) spectrum. We fix the N in both cases, as estimate is known for Cir X-1. Two other NS HMXBs have been the data quality is not sufficient to determine it directly. We set detected in radio. Most notably, the wind-accreting HMXB GX 301- N = 1.0 × 10 for the obscured LS (Inam & Baykal 2005)and 2 was detected over multiple radio epochs (Pestalozzi et al. 2009). 20 −2 N = 1.7 × 10 cm in the HS (Furst ¨ et al. 2013). This yields −11 −1 −2 However, the flux levels were consistent with those expected from 0.5–10 keV X-ray fluxes of ∼9 × 10 erg s cm during the ra- −9 −1 −2 the stellar wind, and the claimed transient outflow component in dio observation and ∼3 × 10 erg s cm during the MH state. the emission has not been confirmed (Migliari et al. 2011). Addi- The latter is slightly lower than the typical range of X-ray fluxes of −9 −8 −1 −2 tionally, the Be/X-ray binary A 1118-61, consisting of a NS and Her X-1 in the MH state of 5 × 10 to 10 erg s cm (Staubert a Be companion, was detected in only one out of eight observa- et al. 2016). This difference might be due to the short exposure of tions by Duldig et al. (1979). Due to the crowded field, this de- the MAXI spectrum, combined with the dips often seen during the tection might not be related to the Be/X-ray binary. Both these MH state (Igna & Leahy 2011). (possible) detections are thus not conclusive about the presence of ajet. There exist numerous radio non-detections of NS HMXBs. How- 4 DISCUSSION ever, for most of these sources, the radio upper limits (ranging We present the first radio detection of the IMXB Her X-1, at a from hundreds of µJy to mJy levels; Duldig et al. 1979; Nelson & flux density of S = 38.7 ± 4.8 µJy. Her X-1 has been the subject Spencer 1988; Fender & Hendry 2000) are not constraining com- of multiple radio searches, but similar to most NS HMXBs, was pared with NS LMXBs and deep observations with current genera- hitherto never detected. Coe & Crane (1980) observed Her X-1 tion radio telescopes might reveal these sources. Only the Be/X-ray every day of an entire 35-d precession cycle. The source was not binaries A 0535+26 (Tudose et al. 2010) and X Per, and the wind- detected in any of the observations, with 3σ upper limits of 9 mJy. accreting NS HMXB 4U 2206+54 (Migliari & Fender 2006)have Nelson & Spencer (1988) observed Her X-1 twice in a large sample 27 −1 radio luminosity upper limits of 5 × 10 erg s , below our Her study of X-ray binaries and cataclysmic variables, reaching a 5σ X-1 measurement. However, these sources were observed at much upper limit of 1.3 mJy. In this discussion, we will first compare the lower (more than an order of magnitude) X-ray luminosity, making radio properties of Her X-1 with different classes of accreting NSs. any direct comparison with Her X-1 difficult. Subsequently, we will discuss the origin of the radio emission and implication for future research. 4.2 The emission mechanism and physical origin Three radio emission mechanisms are relatively unlikely to explain 4.1 ComparisonwithNSLMXBs andHMXBs our detection of Her X-1. First, thermal emission would require too Radio detections and jets are ubiquitous in disc-accreting, weak high densities of emitting material on too large scales. Secondly, we magnetic field NS LMXBs (Migliari & Fender 2006; Gusinskaia imaged Stokes V in addition to Stokes I and did not detect the target, et al. 2017; Tudor et al. 2017). While L and L do appear setting a 3σ upper limit on the circular polarization of 37 per cent. X R to be related for these types of NS systems, no universal re- Coherent emission should be highly circularly polarized and can lation has emerged (Tudor et al. 2017). Most relevant for the thus be excluded. Finally, free–free emission from a strong stellar comparison with Her X-1 are the handful of LMXBs containing wind, as observed in the HMXB GX 301-2 (Pestalozzi et al. 2009), is a slow pulsar. With the exception of one (see below), none of unlikely: while a wind might be present (Leahy 2015), its strength these sources have been detected in the radio. 2A 1822-371 and implies a flux density over two orders of magnitude lower than 4U 1626-67 have unconstraining upper limits on their radio flux our detection (Wright & Barlow 1975). On the contrary, syn- of 200 µJy (Fender & Hendry 2000). GRO 1744-28 (The Bursting chrotron emission is consistent with the observed radio properties. Pulsar) does have deep ATCA upper limits during it small 2017 out- In the following, we will discuss possible physical origins of such burst (Russell et al. 2017). Finally, for the mildly recycled 11-Hz synchrotron emission. pulsar IGR J1748-2466 no radio upper limits are known. First, synchrotron-emitting shocks could occur in the interaction As stated, a single slow pulsar in an LMXB has been detected: the between the disc and the magnetosphere or in the accretion col- symbiotic X-ray binary GX 1+4 was recently discovered in radio umn on to the magnetic poles. However, the Compton limit on the (van den Eijnden et al. 2017). In this type of source, the NS accretes brightness temperature of 10 K sets a lower limit on the size of from the stellar wind of an evolved low-mass companion. The origin the emitting region of 7.5 × 10 km. We can estimate the size of MNRASL 473, L141–L145 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L141/4622954 by Ed 'DeepDyve' Gillespie user on 16 March 2018 L144 J. van den Eijnden et al. the magnetosphere R by rewriting equation (1) from Cackett et al. strength and such behaviour has not been observed in Her X-1 in (2009): its regular, 35-d cyclic behaviour. Finally, we might observe a compact, synchrotron-emitting radio 4/7 −4/14 B f F ang bol 9 jet, similar to those seen in NS LMXBs with weaker (10 G) R = k 5 −9 −1 −2 1.2 × 10 G η 10 erg s cm magnetic fields. If we compare Her X-1’s radio properties with −8/7 −12/7 −4/7 the NS LMXB sample in the L /L diagram (see e.g. Gusinskaia X R M R D R (1) et al. 2017; Tudor et al. 2017, for recent versions), we see that 1.4M 10 km 5 kpc these are consistent with several accreting millisecond X-ray pulsar where k is a geometry factor relating spherical and disc accretion, (AMXP) observations if we assume the LS flux. While an interesting typically assumed to be 0.5 for disc accretion, B is the magnetic field comparison, as AMXPs have ∼3–4 orders of magnitude weaker strength, f is the anisotropy correction factor, η is the accretion magnetic fields and faster spins, we should actually again use the ang efficiency, F is the bolometric flux, and M, R and D are the mass, estimated unobscured (i.e. MH state) flux. In that comparison, the bol radius and distance of the NS. We use B ∼ 3 × 10 G (Staubert L of Her X-1 is three to ten times lower than hard-state and several et al. 2016), k = 0.5, f = 1, η = 0.1, M = 1.4 M , R = 10 km soft-state Atoll sources at similar L , and more similar to that of ang X (Leahy 2004)and D = 6.1 kpc (Leahy & Abdallah 2014). jet-quenched sources (e.g. Gusinskaia et al. 2017). As R scales inversely with flux, the maximum magnetospheric As jet formation is poorly understood, the cause of jet quenching size can be estimated with the LS 2–10 keV X-ray flux without is puzzling. It is observed in all BH LMXBs if and when they transi- −11 −1 −2 a bolometric correction (e.g. 9 × 10 erg s cm ): this yields tion into the soft spectral state (e.g. Gallo, Fender & Pooley 2003), R ≈ 1.7 × 10 km, smaller than the minimum emission region size. but the picture is more ambiguous in accreting NSs: only in a handful As the low flux during the LS of Her X-1 originates from a geometric of sources is quenching observed (see e.g. Miller-Jones et al. 2010; effect, it is actually more accurate to use the MH state bolometric Gusinskaia et al. 2017). If jet formation requires large scale-height −9 −1 −2 flux; for the measured MH state flux of 3 × 10 erg s cm , R poloidal magnetic fields, quenching might be explained as follows: is even smaller at ∼0.7 × 10 km. Hence, shocks can be excluded in the hard spectral state, the accretion disc might be truncated away as well, assuming that they indeed occur at the magnetosphere and from the compact object (e.g. Done, Gierlinski & Kubota 2007)asa not further out in the accretion flow. radiatively inefficient accretion flow (RIAF; Narayan & Yi 1995)or Another possibility is that we observe a propeller-driven outflow: corona replaces the inner disc, providing the required fields. As the if the magnetosphere spins faster than the disc where the magnetic disc moves inwards during the transition to softer spectral states, the and gas pressure are equal, it creates a centrifugal barrier that can RIAF disappears or the corona is cooled, breaking the jet formation either trap the disc (D’Angelo & Spruit 2010) or expel the mate- mechanism. rial (Illarionov & Sunyaev 1975; Campana et al. 1998). The latter If the above scenario indeed underlies jet quenching, it is difficult has, for instance, recently been inferred through X-ray monitoring to reconcile with the low L in Her X-1: there, the strong stellar in several strong magnetic field accreting NSs (e.g. two Be/X-ray magnetic field prevents the disc from moving inwards. However, this binaries; Tsygankov et al. 2016) and might explain the recent radio stellar field might instead hamper the initial formation of a RIAF detection of GX 1+4 (van den Eijnden et al. 2017). For a given NS or corona, also effectively quenching the jet. Alternatively, the jet magnetic field and spin period, one can estimate the maximum L formation might be partially suppressed as the disc pressure cannot for which the magnetosphere can still create this centrifugal barrier dominate and twist the strong magnetic field (Massi & Kaufman as (e.g. Campana et al. 2002): Bernado´ 2008), or as the magnetic field prevents the formation of a boundary layer at the NS surface, which might play a role in NS jet 2 −7/3 B P 37 7/2 formation (Livio 1999). L ≈ 4 × 10 k X,max 10 G 1s −2/3 5 M R 4.3 Implications for future research −1 erg s (2) 1.4M 10 km Out of the considered origins for the radio emission (shocks, a pro- where P is the pulsar spin and all other parameters are al- peller outflow and a compact jet), a jet appears most compatible with ready defined. For a magnetic field of ∼3 × 10 G, a spin both the correlated X-ray and radio properties and with the known period of 1.24 s and standard NS parameters, we estimate the properties of the Her X-1 system. The presence of a jet in Her X-1 37 −1 L ≈ 1.9 × 10 erg s for Her X-1. automatically implies that (the combination of) a strong magnetic X,max To assess whether a magnetic propeller could be at play in Her field and a slow spin do not completely impede jet formation (as X-1, we need to compare this maximum X-ray luminosity with suggested by, e.g. Massi & Kaufman Bernado 2008). This would the correct L of Her X-1. During the LS radio epoch, L ≈ imply that our understanding of jet formation, in the presence of X X 35 −1 4 × 10 erg s , comfortably below the upper limit for the pro- a strong NS magnetic field, needs to be revisited. Additionally, it peller effect. However, the actual, unobscured X-ray luminosity is opens up the possibility of observing radio emission from several the more accurate probe of the relevant physical properties (i.e. the currently undetected sources: for instance, the LMXBs containing mass accretion rate that balances the magnetic pressure). During slow X-ray pulsars and Be/X-ray binaries accreting from a (small) 37 −1 the prior MH state, Her X-1 reached L ≈ 1.2 × 10 erg s be- disc would be prime targets for such studies, as current genera- tween 2–10 keV. With a bolometric correction, the X-ray luminos- tion radio telescopes (e.g. VLA, ATCA) reach sensitivities orders of 37 −1 ity of Her X-1’s MH state typically reaches (2.5–5) × 10 erg s magnitude below the current typical upper limits for these sources. (Staubert et al. 2016). This is of comparable magnitude as the L In order to confidently confirm a jet nature of the radio emis- X,max estimate, although it should be noted that not every MH state reaches sion in Her X-1, new observations are necessary. A measurement of the same luminosity (e.g. Staubert et al. 2016) and equation (2) is the radio spectral index, combined with a linear polarization mea- merely an estimate. However, in other accreting NSs propellers have surement, and a search for extended structure or a jet-break in the been linked to a simultaneous decrease in X-ray flux and pulsation spectrum, could reveal the emission mechanism. If a jet is indeed MNRASL 473, L141–L145 (2018) Downloaded from https://academic.oup.com/mnrasl/article-abstract/473/1/L141/4622954 by Ed 'DeepDyve' Gillespie user on 16 March 2018 Radio emission and jets in Her X-1 L145 present, this opens up interesting possibilities to better understand Krimm H. A. et al., 2013, ApJS, 209, 14 Leahy D. A., 2004, ApJ, 613, 517 Her X-1 itself. For instance, observations at different states during Leahy D. A., 2015, ApJ, 800, 32 its 35-d cycle could independently confirm that precession causes Leahy D. A., Abdallah M. H., 2014, ApJ, 793, 79 this cycle, as the jet likely emits from further out without being Livio M., 1999, Phys. Rep., 311, 225 obscured. Additionally, a jet might be a better tracer of the mass Massi M., Kaufman Bernado´ M., 2008, A&A, 477, 1 accretion rate on to the NS than the X-rays, if it is indeed not in- Matsuoka M. et al., 2009, PASJ, 61, 999 fluenced by obscuration. That could possibly also allow a more McMullin J. P., Waters B., Schiebel D., Young W., Golap K., 2007, in Shaw detailed study of Her X-1’s rare off state, wherein barely any X-ray R. A., Hill F., Bell D. J., eds, ASP Conf. Ser. Vol. 376, Astronomical Data emission is observed for extended periods of time. Analysis Software and Systems XVI. Astron. Soc. Pac., San Francisco, p. 127 Merloni A., Heinz S., di Matteo T., 2003, MNRAS, 345, 1057 ACKNOWLEDGEMENTS Migliari S., Fender R. P., 2006, MNRAS, 366, 79 Migliari S., Miller-Jones J. C. A., Russell D. M., 2011, MNRAS, 415, 2407 JvdE and TDR acknowledge the hospitality of ICRAR Curtin, where Miller-Jones J. C. A. et al., 2010, ApJ, 716, L109 part of this research was carried out, and support from the Leids Narayan R., Yi I., 1995, ApJ, 452, 710 Kerkhoven-Bosscha Fonds. JvdE and ND are supported by a Vidi Nelson R. F., Spencer R. E., 1988, MNRAS, 234, 1105 grant from the Netherlands Organization for Scientific Research Patruno A., Watts A. L., 2012, preprint (arXiv:1206.2727) (NWO) awarded to ND. TDR is supported by a Veni grant from the Pestalozzi M., Torkelsson U., Hobbs G., Lopez-S ´ anchez ´ A. R., 2009, A&A, NWO. JCAM-J is the recipient of an Australian Research Council 506, L21 Future Fellowship (FT140101082). Reynolds A. P., Quaintrell H., Still M. 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Journal

Monthly Notices of the Royal Astronomical Society Letters – Oxford University Press

Published: Nov 13, 2017

Keywords: accretion, accretion discs; stars: neutron; pulsars: individual: Her X-1; X-rays: binaries

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