OPTICAL EMISSION OF THE BLACK HOLE X-RAY TRANSIENT MAXI J1659−152 DURING QUIESCENCE

Galactic black hole X-ray binary systems manifest themselves as X-ray novae and most of them are discovered via their sudden dramatic increase in X-ray brightness. Such an X-ray outburst is due to a change of the amount of material accreted from the companion star by the central compact object. In addition to X-ray follow-up observations following the evolution of the outburst, multi-wavelength observations play an important role in understanding the physics of these systems. In particular, reprocessing in the accretion disk and the companion star will also generate optical emission. We therefore expect to see a dramatic change in optical brightness when an X-ray nova is in outburst.

Optical observations provide the data necessary to determine the orbital period, the evolutionary state of the secondary, the binary mass function, and mass of the compact object itself. In combination with X-ray spectral and timing studies, these provide a detailed picture of the accretion process and the nature of the compact object. Although the optical counterpart of an X-ray nova is usually discovered during an outburst, observations when the source returns to quiescence are also extremely important. While the optical emission of an X-ray nova is very faint in quiescence, it is the best time to study the nature of the companion star and the geometry of the binary system, and to measure the mass function via radial velocity measurements.

The X-ray transient MAXI J1659−152 was first discovered with the Swift Burst Alert Telescope (BAT) on 2010 September 25 as a gamma-ray burst (GRB 100925A; Mangano et al. 2010). Independent discovery made with the Gas Slit Camera (GSC; Mihara et al. 2011) on board the Monitor of All-sky X-ray Image (MAXI; Matsuoka et al. 2009) suggested that it is a previously unknown Galactic X-ray transient. The optical counterpart of MAXI J1659−152 was discovered with the Swift UltraViolet/Optical Telescope (UVOT) immediately after the BAT trigger (Marshall 2010). Subsequent multi-wavelength observations from radio to hard X-ray showed that MAXI J1659−152 is a black hole binary candidate. In particular, frequent X-ray dips with a recurrent time of 2.4 hr were found in RXTE, Swift, and XMM-Newton observations (Kennea et al. 2011; Kuulkers et al. 2012). This strongly suggests that MAXI J1659−152 is a highly inclined system with an orbital period of 2.4 hr, the shortest among all the black hole X-ray binaries.

MAXI J1659−152 returned to quiescence as observed with Chandra on 2011 May 3 (Jonker et al. 2012), but it underwent a mini-outburst from 2011 May 6 (Yang & Wijnands 2011a, 2011b; Jonker et al. 2012). The source has finally settled down in the quiescent state from 2011 mid-August based on a Chandra monitoring program (Jonker et al. 2012).

The optical counterpart of MAXI J1659−152 has been monitored regularly with Swift UVOT and ground-based telescopes during the outburst (Russell et al. 2010; Yang et al. 2011; Yang & Wijnands 2011a, 2011b). In general, the optical activity tracked the X-ray outburst (Russell et al. 2010, 2011; Kong et al. 2011). The optical counterpart of MAXI J1659−152 is not detected in the Digitized Sky Survey indicating that the quiescent magnitude is >21. A tentative quiescent counterpart was reported by Kong et al. (2010) using the Pan-STARRS 1 (PS1) data. The source was only marginally seen in the image with an r'-band magnitude of ∼22.4. This was challenged by Kennea et al. (2011) and Kuulkers et al. (2012) noting that it is too bright for MAXI J1659−152.

In this Letter, we report our new deep optical imaging observation of MAXI J1659−152 in quiescence using the Canada–France–Hawaii Telescope (CFHT) aimed at measuring an accurate quiescent optical magnitude. We also re-analyzed the PS1 data for comparison.

2.1. CFHT

MAXI J1659−152 was observed with the MegaPrime/MegaCam at the CFHT on 2012 March 23. We checked the MAXI GSC daily data and the source was not detected in soft X-ray, indicating that it was likely in quiescence. However, we caution that if there is an X-ray mini-outburst, MAXI J1659−152 will not be detected with MAXI due to its low sensitivity. The MegaCam has an array of 36 CCDs, giving a total of 1° × 1° field of view. We obtained two r'-band images under a seeing condition of ∼08 with an exposure time of 980 s each. The two images were separated by about 30 minutes. The raw images were processed with Elixir² for bias, flat field, overscan, bad pixel mask, and zero point.

We next corrected the CFHT images for the astrometry. By using 10 stars surrounding MAXI J1659−152, we used the IRAF task ccmap to compare with the USNO point source catalog. The resulting registration errors are 021 in R.A. and 0226 in declination.

By examining the CFHT image together with images taken during the outburst (Russell et al. 2010; Kong et al. 2011; see also Figure 1), the optical counterpart of MAXI J1659−152 is clearly a variable (see Figure 1). We further compared the CFHT position of MAXI J1659−152 with the European very long baseline interferometry Network position (Paragi et al. 2010); the offset between the two positions is 0043, indicating that it is the true optical counterpart. We then performed aperture photometry using the task phot in IRAF. In addition to our target, we also measured the optical magnitude for a few nearby stars for checking (see Figure 1).

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2.2. PS1

In Table 1, we list the r'-band magnitudes of MAXI J1659−152 and a couple of check stars of the CFHT and PS1 observations. Given that our new CFHT observations were taken more than seven months after the end of an X-ray outburst confirmed by Chandra (Jonker et al. 2012) and there is no unusual X-ray activity shown in the MAXI GSC data, MAXI J1659−152 is likely in the quiescent state although we cannot totally rule out a mini-outburst or a very long optical decay. The r'-band magnitude of MAXI J1659−152 varied between 23.62 ± 0.06 and 23.81 ± 0.06 in 30 minutes. The two check stars near MAXI J1659−152 did not show any variability, indicating that MAXI J1659−152 is likely a variable star.

For the PS1 data, MAXI J1659−152 is marginally visible with an r'-band magnitude of 22.8 ± 0.3 (see Figure 1). This is roughly consistent with the quick-look analysis reported by Kong et al. (2010). We examined all the PS1 images in this field and the observation reported here has the best seeing and image quality among all the images. This may explain why MAXI J1659−152 was only detected in this observation. Furthermore, the CFHT images indicate that MAXI J1659−152 can be fainter than the PS1 measurement. It could happen that the PS1 observation was taken during a relatively bright state in the quiescence.

From both the CFHT and PS1 observations, we can conclude that the optical counterpart of MAXI J1659−152 shows variability during or near quiescence. In Figure 1, we also show an R-band image taken near the end of an outburst with the 1 m telescope at the Lulin Observatory in Taiwan (Kong et al. 2011). MAXI J1659−152 is clearly much fainter during our CFHT observations. Optical variability is quite common for black hole X-ray transients in quiescence. For example, GRS 1124−684, A0620-00, J0422+32, GS 2000+25, and V404 Cyg show flarings on timescales of a few minutes to 60 minutes with a typical amplitude of 0.1–0.6 mag (Zurita et al. 2003; Shahbaz et al. 2003, 2010). BW Cir (= GS 1354−64) is another quiescent black hole that exhibits large (0.5–1 mag) optical variability (Casares et al. 2009). The origin of optical flaring during quiescence is not well understood. Possible mechanisms include X-ray irradiation of the accretion disk (Hynes et al. 2004), magnetic reconnection events (Zurita et al. 2003), and direct synchrotron emission from an advective-dominated flow (Shahbaz et al. 2003). Simultaneous X-ray and optical observations are required to understand their nature (e.g., Hynes et al. 2004).

In addition to optical flaring activity, the orbital variation can contribute to the observed variability. MAXI J1659−152 is shown to have a 2.4 hr orbital period based on the dip-like events in the X-ray light curves (Kuulkers et al. 2012). In particular, the two CFHT observations are separated by about 30 minutes, equivalent to about 0.2 orbital phase. Variability is therefore not unexpected. The 0.2 mag difference is also consistent with other black hole X-ray transients with measured optical orbital modulation (e.g., see Figure 1 of Zurita et al. 2003). Future optical monitoring observations will be crucial to reveal the optical orbital modulation.

Our new CFHT observations strongly suggest that the proposed optical counterpart of MAXI J1659−152 is real and it shows noticeable variability that could be from flaring or orbital modulation. Even though we adopt the faintest measurement from our CFHT observations (r' = 23.8), it is in contrast to the suggestion that the quiescent magnitude is >27 (Kuulkers et al. 2012). We note that the orbital modulation cannot produce such a huge optical variability (>3 mag) unless the compact object has strong radiation to evaporate the companion like black-widow-type pulsars (e.g., Kong et al. 2012). The suggested quiescent optical magnitude is based on a relationship between the optical outburst amplitude of an X-ray transient and the orbital period found by Shahbaz & Kuulkers (1998; hereafter SK98). With an orbital period of 2.4 hr, the V-band magnitude difference between the outburst and quiescence should be as large as 11 mag, yielding V > 27 in the quiescence. Taking the inclination effect into account, the quiescent V magnitude is >26.2 (Kuulkers et al. 2012). By using previous PS1 measurement in the r' band (Kong et al. 2010) and the limiting magnitudes of the PS1 3π survey, Kuulkers et al. (2012) derived an expected V-band magnitude of V ≳ 22.8; although it cannot rule out a much fainter object, a difference of at least 3 mag indicates that MAXI J1659−152 unlikely follows the SK98 relation. However, we caution that the SK98 relation is an empirical relation based on a sample of 11 black hole transients and there are many factors (e.g., the stellar type of the companion star, the inclination effect on the disk emission, and the peak mass transfer rate) affecting the correlation. Hence, scatter on the correlation is not unexpected.

To estimate the V-band magnitude, we first assume the spectrum of the companion. Given that MAXI J1659−152 is likely an M2 or M5 dwarf (Kuulkers et al. 2012; Jonker et al. 2012), the colors will become B − V = 1.52 (M2 dwarf) or B − V = 1.61 (M5 dwarf). We then correct the colors for the reddening assuming the Galactic values provided by Schlafly & Finkbeiner (2011). Using the transformation of Tonry et al. (2012), we get a reddened r' − R color of 0.3 for both cases. The reddened V−R is 1.9 and 2.2 for an M2 dwarf and an M5 dwarf, respectively. This implies that V − r' is 1.6 (1.9) for an M2 dwarf (M5 dwarf). Using the observed r'-band magnitude (23.8) from our CFHT observation, we have V = 25.4 for an M2 dwarf and V = 25.7 for an M5 dwarf. These are consistent with the above limits derived by Kuulkers et al. (2012). In any case, the estimated quiescent V-band magnitude is within 1 mag difference for a high-inclination (80°) system although the difference could be as large as 2 mag for a lower inclination (65°) system.

Based on our new CFHT measurement, although the estimated V-band magnitude is still brighter, it is not a large offset as proposed by Kuulkers et al. (2012). As we mentioned above, there are other factors causing scatter on the SK98 correlation. For instance, the black hole X-ray binary XTE J1118+480 has an orbital period of 4.08 hr and the outburst amplitude is about 6–7 mag. According to SK98, the amplitude is at least 9.4 mag. However, XTE J1118+480 is underluminous in outbursts; the outburst X-ray spectra are hard in contrast to typical high/soft state of X-ray transients in outbursts, and the X-ray-to-optical flux ratio is low (∼5; see Zurita et al. 2006 for a discussion). Unlike XTE J1118+480, the X-ray spectral behaviors and X-ray-to-optical flux ratio of MAXI J1659−152 are similar to a typical X-ray transient in outburst (Kennea et al. 2011). Follow-up photometric and spectroscopic observations of MAXI J1659−152 in the future will be able to provide better constraints. On the other hand, the quiescent X-ray luminosity of MAXI J1659−152 as measured with Chandra is an order of magnitude higher than that derived from the orbital period–X-ray luminosity correlation (Jonker et al. 2012). Given that both optical and X-ray brightnesses are higher than expected, one simple explanation is that MAXI J1659−152 is not in a true quiescent state. It is worth noting that some short orbital period black hole systems have a long decay time. For example, GRO J0422+32 reached its optical quiescent state 760 days after an outburst (Garcia et al. 1996). Further optical and X-ray observations will confirm this.

We can now use the absolute magnitude of the companion to estimate the distance to the source. The absolute V-band magnitude of an M2 dwarf and an M5 dwarf is 10 and 11.8, respectively. Since V−R is 1.5 (1.8) for an M2 (M5) dwarf, this implies that M_R is 8.5 (10). By using the transformation of Tonry et al. (2012), M_r becomes 8.77 (10.27). Comparing with the reddening (A_r = 1.4; Schlafly & Finkbeiner 2011) corrected r'-band magnitude, the distance is 5.3 kpc (M2 dwarf) or 2.7 kpc (M5 dwarf). If we further assume the accretion disk contributes 50% of the visible light (see Jonker et al. 2012), then the distance would be between 7.5 kpc (M2 dwarf) and 3.8 kpc (M5 dwarf). If A_r is as large as 1.7 (Schlegel et al. 1998), then the distance to MAXI J1659−152 will be 4.6–6.5 kpc and 2.3–3.3 kpc for an M2 dwarf and an M5 dwarf, respectively. Comparing with previous estimations (e.g., Kennea et al. 2011; Kaur et al. 2012; Kuulkers et al. 2012; Jonker et al. 2012), an M2 dwarf companion star is more likely. This is also consistent with the conclusion made by Jonker et al. (2012). On the other hand, if MAXI J1659−152 has an M5 dwarf companion star and a distance of ∼2 kpc, the observed X-ray luminosity will be consistent with the orbital period–X-ray luminosity correlation (Jonker et al. 2012). Future optical spectroscopy during quiescence will be crucial to determine the stellar type of the companion star and hence the distance to the system. In addition, an ellipsoidal light curve modeling during quiescence will allow us to derive the inclination angle of the system. These observations will also test if the SK98 correlation holds for MAXI J1659−152. Combining with a deep X-ray observation during the quiescent state, we could constrain the orbital period–X-ray luminosity correlation.

We thank an anonymous referee for comments that have improved this paper. Ground-based observations were obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada–France–Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. Access to the CFHT was made possible by the Institute of Astronomy and Astrophysics, Academia Sinica, National Tsing Hua University, and National Science Council, Taiwan.

The Pan-STARRS1 Surveys (PS1) have been made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg, and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, and the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate. We thank the PS1 Builders and PS1 operations staff for construction and operation of the PS1 system and access to the data products provided.

This project is supported by the National Science Council of the Republic of China (Taiwan) through grant NSC100-2628-M-007-002-MY3 and NSC100-2923-M-007-001-MY3. A.K.H.K. gratefully acknowledges support from a Kenda Foundation Golden Jade Fellowship.

Facilities: CFHT - Canada-France-Hawaii Telescope, PS1 - Panoramic Survey Telescope and Rapid Response System Telescope #1 (Pan-STARRS)