Mon. Not. R. Astron. Soc. 000, 000 000 (0000) Printed 28 February 2017 (MN LATEX style file v2.2) An evolving synchrotron jet spectrum in Swift J1357.2 0933 in quiescence David M. Russell 1, Ahlam Al Qasim 1, Fraser Lewis 2,3 and Richard M. Plotkin 4 1 New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE 2 Faulkes Telescope Project, School of Physics, and Astronomy, Cardiff University, The Parade, Cardiff, CF24 3AA, Wales, UK 3 Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK 4 International Centre for Radio Astronomy Research Curtin University, GPO Box U1987, Perth, WA 6845, Australia 28 February 2017 ABSTRACT We present six years of optical monitoring of the black hole candidate X-ray binary Swift J1357.2 0933. On these long timescales, the quiescent light curve is dominated by high amplitude variability spanning 2 magnitudes, with an increasing trend of the mean flux from 2012 to 2017 which is steeper than in any other X-ray binary found to date. Such an increase in the optical flux between outbursts is expected from disc instability models, but the high amplitude variability is not. Previous studies have shown that the quiescent spectral, polarimetric and rapid variability properties of Swift J1357.2 0933 are consistent with synchrotron emission from weak compact jets. We collect all optical infrared data taken during quiescence since 2006 from transient surveys and the literature. The optical/infrared spectral energy distributions are generally well described by a power law which evolves from a steep, red slope (α < 1) before and soon after the 2011 outburst, to one which is flat (α 0) since 2013. We find that the changing spectrum is responsible for the brightening, and identify a transition between these two regimes, between June 2012 and March 2013. The spectral break in the jet spectrum shifts from infrared wavelengths before this transition to optical wavelengths after the transition. The result is an increase in the u-band flux of a factor of 14, an increase in the I-band flux of a factor of 4 6, and almost no change at all in the infrared flux. Swift J1357.2 0933 is a valuable source to study black hole jet physics at very low accretion rates, and possibly the only quiescent source in which the jet properties can be regularly monitored. Key words: accretion, accretion discs, black hole physics, ISM: jets and outflows, X-rays: binaries 1 INTRODUCTION It has been known for more than a decade that accreting black holes can launch jets at very low accretion rates, when the X-ray luminosities are less than 10 5 of the Eddington luminosity (Hjellming et al. 2000; Gallo, Fender & Pooley 2003; Gallo, Fender & Hynes 2005; Gallo et al. 2006; Markoff et al. 2008). Radio emission has been detected from jets released by low-mass X-ray binaries (LMXBs) in quiescence (with X-ray luminosities 10 30 10 33.5 erg s 1 ), in a growing number of systems, all hosting black hole candidates (Gallo et al. 2005, 2006, 2014; Miller-Jones et al. 2011, 2015; Strader et al. 2012; Chomiuk et al. 2013; Dzib, Massi, & Jaron 2015; Tetarenko et al. 2016). LMXBs spend E-mail: dave.russell@nyu.edu most of their time in this state of quiescence between outbursts, and therefore presumably they (at least in the black hole systems) probably launch jets for most of thier lifetime. However, long-term radio studies of LMXB jets in quiescence do not exist, because they are far too faint to monitor with current radio facilities. Even V404 Cyg, which is one of our closest black hole (BH) candidates and is also fairly luminous in quiescence (at a distance of 2.4 kpc and an X-ray luminosity of 10 33 erg s 1 ; Miller-Jones et al. 2009), has an average flux density of only 0.3 mjy and less than a dozen or so detections in quiescence (Hjellming et al. 2000; Gallo et al. 2005; Miller-Jones et al. 2009; Hynes et al. 2009; Markoff et al. 2015; Rana et al. 2016; Bernardini et al. 2016b; Plotkin et al. 2017). Most BH systems are much fainter than V404 Cyg, possessing radio flux densities on the order of µjy, with only one or two detections of a source
2 D. M. Russell et al. made over decades (e.g. Gallo et al. 2006, 2014; Ribó et al. 2017). The long-term behaviour and properties of quiescent jets from Galactic BHs remain elusive. At optical wavelengths, long-term monitoring studies of quiescent LMXBs have revealed the ellipsoidal modulation of the companion star in some systems, leading to measurements of the fundamental parameters such as the masses and orbital inclinations of the systems (e.g. Greene, Bailyn & Orosz 2001; Casares & Jonker 2014; Shahbaz, Watson & Dhillon 2014). In other LMXBs, the long-term quiescent optical light curve is dominated by flickering, flares and/or variability from the accretion flow (e.g. Yang et al. 2012; Koljonen et al. 2016; Wu et al. 2016) and some exhibit a combination of the above contributions (e.g. Zurita et al. 2003; Shahbaz et al. 2005; Cantrell et al. 2010; MacDonald et al. 2014; Bernardini et al. 2016a). Theoretically, the disc instability model (DIM; e.g. Lasota 2001; Dubus, Hameury & Lasota 2001; Hameury et al. 2017) predicts that between outbursts, the temperature and surface density of the accretion disc increases as matter builds up in the disc. This phenomenon has only recently been seen in LMXBs with long-term optical monitoring; in H1705 250 there is a hint at an increase in the optical flux over six years of monitoring in quiescence (Yang et al. 2012), and in GS 1354 64 (BW Cir) and GRS 1124 68 (Nova Muscae 1991) there is a quantifiably more significant rise in the optical flux (Koljonen et al. 2016; Wu et al. 2016). In one BH system Swift J1357.2 0933, a deep radio observation yielded a 3σ rms upper limit of 3.9 µjy during quiescence, whereas synchrotron emission, likely from the jet was detected at optical infrared (OIR) wavelengths (Plotkin et al. 2016). While OIR synchrotron emission has been detected in a number of BH LMXBs during quiescence (e.g. Gallo et al. 2007; Gelino, Gelino & Harrison 2010; Froning et al. 2011; Russell et al. 2013) and one neutron star system (Baglio et al. 2013), in Swift J1357.2 0933 this synchrotron emission appears to dominate the quiescence OIR spectrum. High amplitude, short-timescale optical variability (Shahbaz et al. 2013), a red or flat spectral energy distribution (SED; Shahbaz et al. 2013; Plotkin et al. 2016) and evidence for intrinsic polarization (Russell et al. 2016) are all unique properties of Swift J1357.2 0933, that cannot be produced by the accretion disc, advection dominated flows or the companion star, and have led to the conclusion that a jet is continuously launched during quiescence. Shahbaz et al. (2013) found that the OIR SED was red and variable in quiescence, with a steep spectral index of α = 1.4 ± 0.1 (where F ν ν α ); Plotkin et al. (2016) reported a flat infrared spectrum (α 0), that turned over to a steeper slope at shorter wavelengths in the optical UV regime, consistent with the spectral break in the jet spectrum between optically thin and optically thick, partially self-absorbed synchrotron emission. This break has been identified in a number of LMXBs during outbursts (e.g. Corbel & Fender 2002; Migliari et al. 2010; Gandhi et al. 2011; Russell et al. 2013, 2014; Koljonen et al. 2015; Diaz Trigo et al. 2017), but Swift J1357.2 0933 represents the first robust measurement of a jet break for a quiescent X-ray binary. The discrepancy between the infrared SEDs of Shahbaz et al. (2013) and Plotkin et al. (2016) suggests that there is long-term evolution of the jet spectrum in Swift J1357.2 0933. Here, we present six years of optical monitoring of Figure 2. caption the source, and collect all OIR data available, to investigate the long-term behaviour of the OIR flux and SED of Swift J1357.2 0933. In addition to the jet 2 DATA COLLECTION 2.1 Faulkes Telescope monitoring We have conducted a long-term monitoring campaign of Swift J1357.2 0933 with the two, robotic 2-m Faulkes Telescopes (North, at Haleakala on Maui, Hawaii, USA, and South, at Siding Spring, Australia) since its outburst in 2011. Our observations are part of an ongoing monitoring campaign of 40 low-mass X-ray binaries (Lewis et al. 2008). Most of our observations were made using the Bessell I-band filter, with some in Bessell R-band in 2011 2012, and some SDSS g, r, i, z consecutive observations in 2016 and 2017. The latter were made specifically to investigate the optical SED and measure the spectral index. Both Faulkes Telescopes are equipped with cameras with a pixel scale of 0.30 arcsec pixel 1 and a field of view of 10 arcmin, except in 2011 February, in which the cameras had 0.28 arcsec pixel 1 and 4.8 arcmin field of view. Flux calibration was achieved using the Sloan Digital Sky Surveys (SDSS) magnitudes of several stars in the field, from SDSS Data Release 12 (Alam et al. 2015). For Bessell I-band and R-band, we converted the SDSS magnitudes of the field stars to I magnitudes adopting the colour transformations in Table 3 of Jordi, Grebel & Ammon (2006). We reported one of our I-band magnitudes in Russell et al. (2016) that was quasi-simultaneous with near-infrared polarimetric observations; all other Faulkes data are new to this paper.
Swift J1357.2 0933 in quiescence 3 Figure 1. caption log(flux; mjy) α 0.5 0-0.5-1 -1.5-2 -2.5 1.5 1 0.5 0-0.5-1 -1.5-2 -2.5-3 54000 54500 55000 55500 56000 56500 57000 57500 outburst MJD Figure 4. caption V-band I,i -band quiescence I,i fit 2.2 Archival data from transient surveys 2.3 Optical photometry with the Faulkes Telescope(s) / LCOGT 3 RESULTS AND ANALYSIS 4 DISCUSSION & CONCLUSIONS Acknowledgements. The Faulkes Telescopes are maintained and operated by the Las Cumbres Observatory (LCO). REFERENCES Alam S., et al., 2015, ApJS, 219, 12 Baglio M. C., D Avanzo P., Muñoz-Darias T., Breton R. P., Campana S., 2013, A&A, 559, A42 Bernardini F., Russell D. M., Shaw A. W., Lewis F., Charles P. A., Koljonen K. I. I., Lasota J. P., Casares J., 2016a, ApJ, 818, L5 Bernardini F., Russell D. M., Kolojonen K. I. I., Stella L., Hynes R. I., Corbel S., 2016b, ApJ, 826, 149 Cantrell A. G., et al., 2010, ApJ, 710, 1127 Casares J., Jonker P. G., 2014, SSRv, 183, 223 Chomiuk L., Strader J., Maccarone T. J., Miller-Jones J. C. A., Heinke C., Noyola E., Seth A. C., Ransom S., 2013, ApJ, 777, 69 Corbel S., Fender R. P., 2002, ApJ, 573, L35 Diaz Trigo M., Migliari S., Miller-Jones J. C. A., Rahoui F., Russell D. M., Tudor V., 2017, A&A, in press (arxiv:1611.06988) Dubus G., Hameury J.-M., Lasota J.-P., 2001, A&A, 373, 251 Dzib S. A., Massi M., Jaron F., 2015, A&A, 580, L6 Froning C. S., et al., 2011, ApJ, 743, 26 Gallo E., Fender R. P., Pooley G. G., 2003, MNRAS, 344, 60 Gallo E., Fender R. P., Hynes R. I., 2005, MNRAS, 356, 1017 Gallo E., Fender R. P., Miller-Jones J. C. A., Merloni A., Jonker P. G., Heinz S., Maccarone T. J., van der Klis M., 2006, MNRAS, 370, 1351
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