Abstract
Gamma-ray binaries are a small subclass of the high mass X-ray binary population which exhibit emission across the whole electromagnetic spectrum. We present radial velocities of 1FGL J1018.6−5856 based on observations obtained with the Southern African Large Telescope (SALT). We combine our measurements with those published in the literature to get a broad phase coverage. The mass function obtained supports a neutron star compact object, although a black hole mass is possible for very low inclination angles. The improved phase coverage allows constraints to be placed on the orbital eccentricity (e = 0.31 ± 0.16), which agrees with estimates from high energy data.
INTRODUCTION
Gamma-ray binaries (GRBis) are a small subclass of high mass X-ray binaries (HMXBs), comprising only six sources: PSR B1259−63 (Johnston et al. 1992) , LS 5039 (Motch et al. 1997) , LS I+61 303 (Gregory & Taylor 1978) , HESS J0632+057 (Aharonian et al. 2007 ), 1FGL J1018.6−5856 ) and LMC P3 (Corbet et al. 2016) . These systems comprise a pulsar or black hole (BH) and a massive O or Be star. GRBis differ from traditional HMXBs (Be X-ray and supergiant X-ray binaries) in that they show a peak in their spectral energy distribution (SED) above 1 MeV, as well as display multiwavelength emission across the whole electromagnetic spectrum. In all but one case (PSR B1259−63) the nature of the compact object is uncertain (it is either a neutron star or BH). Through radio pulse timing, a pulse period of 47.7 ms was found in PSR B1259−63, confirming the nature of the compact object as a pulsar. Pulsations are not detected in any of the other gamma-ray binaries. Furthermore, the large uncertainty in the mass function results in a large uncertainty in the calculated mass of the compact object in the other systems, making it difficult to distinguish between a pulsar and a BH (a minimum compact object mass of 3 M ⊙ would indicate a BH).
INTRODUCTION
ii 1FGL J1018.6−5856 (J1018), the subject of this paper, was discovered at GeV energies when a 16.6 day modulation was detected from the first Fermi/LAT catalog . Follow-up work at optical wavelengths taken with the 1.9 m telescope at the South African Astronomical Observatory (SAAO) and the 2.5 m telescope at Las Campanas Observatory (LCO) revealed Balmer, He i and He ii absorption lines (similar to LS 5039), with the He ii/He i ratio indicating an O6V((f)) spectral type (Fermi LAT Collaboration et al. 2012) . UsingÉchelle spectra obtained with the CTIO 1.5 m telescope, Waisberg & Romani (2015b) performed a radial velocity (RV) analysis of J1018. Using Gaussian fits to the H i, He i and He ii absorption lines, Waisberg & Romani (2015b) found a semi-amplitude (K) range of 12−40 km/s. The maximum determined semi-amplitude (40 km/s) from their work means that the compact object likely has a mass >2.2 M ⊙ , favouring a BH. The RV curve from Waisberg & Romani (2015b) , however, is hindered by a large scatter due to imprecision of the continuum calibration, as well as possible contamination by stellar wind line features. Strader et al. (2015) (S15) performed a follow-up RV study of J1018 using medium resolution spectra obtained with the Goodman High-Throughput Spectrograph onboard the SOAR 4.1 m telescope. They performed a cross-correlation of their spectra against the O6V star, HD 172275, in two wavelength regions with the Hγ line and He ii lines (4542Å and 4686Å). The two RV plots show different systemic velocities, with the He ii RV measurements offset from the Balmer line measurements by +6 km/s. A similar behaviour from RV measurements made from the two line species has been reported in LS 5039, which is believed to be due to the contamination of wind for the He ii and H i (Casares et al. 2005; Sarty et al. 2011) . With a modest phase coverage, S15 performed a circular Keplerian fit to the RV measurements and found semi-amplitudes of 11.4 ± 1.5 km/s and 12.2 ± 2.7 km/s from Hγ and He ii, respectively. S15 found that for inclination angles between 26
INTRODUCTION
• and 64
INTRODUCTION
• a compact object mass of 1.4−2.5 M ⊙ (canonical neutron star mass range) is obtained, while for lower inclination angles (i ≤ 16
INTRODUCTION
• ) a BH is favoured (M X ≥ 5M ⊙ ). In this work, we present newÉchelle optical spectroscopy of J1018 obtained with the High Resolution Spectrograph (HRS) on the Southern African Large Telescope (SALT). The broader phase coverage, combined with radial velocities from S15, allows for better constraints on the orbital parameters and hence mass estimate of the compact object. Figure 1 . Normalized average blue spectrum for the wavelength range used of 1FGL J1018.6−5856 with the strongest lines used for cross-correlation labelled.
OBSERVATIONS AND REDUCTIONS
J1018 was observed 8 times with SALT (Buckley et al. 2006 ) using the HRS (Bramall et al. 2010 (Bramall et al. , 2012 Crause et al. 2014 ) between 06 December 2015 and 20 December 2015. Our observations were done in medium resolution (MR) mode (R ∼ 40000) with the blue and red arms, providing a total wavelength coverage of ∼3700−8900Å. Only spectra with wavelength ranges 4500−5500Å and 5700−6750Å for the blue and red arms, respectively, were useable. All observations were performed with exposure times of 1400 s. Regular calibration sets of ThAr arc lamps, bias and flats were taken throughout the duration of our programme. Primary reductions of the spectra (including overscan correction, bias subtraction and gain correction) were performed with the SALT science pipeline (Crawford 2015) . The subsequent reduction steps which include background subtraction, removal of the blaze function, identification of the arc lines and merging of the orders for the object spectra were carried out using the midas feros (Stahl et al. 1999 ) and echelle (Ballester 1992) packages (see Kniazev et al. 2016 for a detailed description of the reduction steps). The spectra were normalised and corrected for the heliocenter using iraf 1 tasks rvcorrect and dopcor.
RADIAL VELOCITIES
For our analysis we use the blue spectra to measure radial velocities. Figure 1 shows the mean spectrum of J1018 with the strongest absorption lines of He i (5016Å), He ii (4542, 4686 and 5411Å) and Hβ present. For the wavelength range that we have access to, the mean spectrum is consistent with the low resolution spectrum of Fermi LAT Collaboration et al. 2457377.4990 19.5 ± 10.9
RADIAL VELOCITIES
Figure 2. Best-fitting curve to the radial velocities of the He ii lines. The unfilled circles are from Strader et al. (2015) and the filled circles are from this work.
RADIAL VELOCITIES
(2012). To obtain the radial velocities we performed a cross-correlation using the blue arm spectra. We used an iterative process described by Manick et al. (2015) and Foellmi et al. (2003) to generate a high signal-noise ratio (SNR) zero velocity template. First, individual spectra were rectified and the continuum level subtracted. Each spectrum was then converted to a logarithmic wavelength scale and the spectra were grouped according to SNR (highest to lowest). The radial velocity (RV) shifts of each spectrum relative to the spectrum with the highest SNR were measured. Individual spectra were then shifted to the same rest wavelength as the high SNR template using the results of the first cross-correlation iteration, and a mean spectrum created by combining all the shifted spectra. This became our final template spectrum, which was then used to compute the RV shifts. These shifts (converted to velocities) are shown in Table 1 and Fig. 2 (filled circles).
RADIAL VELOCITIES
Different combinations of the line species were considered during the cross-correlation analysis, however For the reasons explained above we use the combined RV values from our work, using the full wavelength range of the blue spectra, and the He ii measurements from S15 to derive the final orbital solutions. A Keplerian model (Fig. 2) was fitted to the combined measurements with the period fixed to that derived from X-ray studies of J1018 (P = 16.544 days, An et al. 2015) . We used the rvlin software package provided by Wright & Howard (2009) to perform the Keplerian fits and obtain the model parameters. The uncertainties in the best-fit parameters were obtained using the boottran bootstrapping routines described in Wang et al. (2012) . We note that if the period is left as a free parameter, a period of P = 16.583 ± 0.042 days is derived, in good agreement with the X-ray period (as well as that derived from gamma-rays, P = 16.549 ± 0.007 days; Coley et al. 2014) . A summary of the orbital parameters is presented in Table 2 .
MASS OF THE COMPACT OBJECT
The derived orbital parameters allow us to use the mass function defined by
MASS OF THE COMPACT OBJECT
where M x and M O are the masses of the compact object and optical companion, respectively, and i is the (2015) and Waisberg & Romani (2015b) . For the parameters listed in Table 2 a mass function of f (M x ) = 0.0027 ± 0.0013 M ⊙ is obtained. From this, the mass of the compact object can be calculated for different inclination angles. Fig. 3 shows the mass-mass plot obtained from the orbital parameters derived from the RV fit. Referring to Fig. 3 , for a lower limit mass of the optical companion (20 M ⊙ ) and a canonical neutron star mass range of 1.4−2.5 M ⊙ , this implies an orbital inclination angle range of ∼50−26 • . For the upper limit optical companion mass of 26.4 M ⊙ , inclination angles between ∼70
MASS OF THE COMPACT OBJECT
• and 32
MASS OF THE COMPACT OBJECT
• are implied. The most massive NS known has a mass of 2.0 M ⊙ (Antoniadis et al. 2013) , which corresponds to inclination angles of i = 33
MASS OF THE COMPACT OBJECT
• and i = 41
MASS OF THE COMPACT OBJECT
• for optical companion mass of 20 M ⊙ and 26.4 M ⊙ , respectively. For the compact object to be a BH (M x ≥ 3.0 M ⊙ ), lower inclination angles of i < 22
MASS OF THE COMPACT OBJECT
• and i < 26
MASS OF THE COMPACT OBJECT
• for optical companion mass of 20 M ⊙ and 26.4 M ⊙ , respectively, are implied.
DISCUSSION
The value of the mass function obtained from this study agrees with that found by Strader et al. (2015) to within the errors. For the mass range of the companion star considered, we also find similar values for the allowed inclination angles for different masses of the compact object (Fig. 3) . We obtained a best-fit Keplerian model of eccentricity e = 0.31 ± 0.16. The probability of this model, given our data, is P (χ 2 > 11.05) ∼85 % (a circular fit results in a probability ∼50%). The analysis of Fermi GeV data revealed a relatively low modulation amplitude which, if the gamma-ray flux is due to anisotropic inverse Compton scattering of stellar photons by energetic electrons, implies low inclination and eccentricity (Chen et al. 2017 ). An & Romani (2017) model the X-ray and gamma-ray light curves and SED of J1018, where the fit of the model to the spike in the modulated X-ray light curve is explained by an orbital eccentricity of e = 0.35 and inclination angle of i ∼ 50
DISCUSSION
• . These implications for the inclination angle from high energy analysis further support a neutron star as the compact object in J1018. Furthermore, the mass of the donor star could be significantly less than the estimates used, which is typical of donors in HMXBs to display different characteristics than those of a main-sequence equivalent (e.g. Coe et al. 2015; Rajoelimanana et al. 2017 and references therein) . This would reduce the mass estimate of the compact object for a given mass function, making a NS even more likely. The wide phase coverage allows us to put constraints on the orbital eccentricity, which is also in agreement (to within errors) with estimates from high energy studies (An & Romani 2017) . Fig. 4 shows the orbit of the compact object around the optical companion as viewed from an inclination angle of i = 0
DISCUSSION
• . This was produced using mass and radius of 22.9 M ⊙ and 9.3 R ⊙ , respectively, for the optical companion (Casares et al. 2005 ) and a compact object of mass 1.4 M ⊙ . From Fig. 4 , the periastron passage of the compact object occurs close to inferior conjunction (INFC). The peak of the X-ray, GeV and TeV emission in J1018 occurs at similar phases (H. E. S. S. Collaboration et al. 2015; An et al. 2015) . If the gamma-ray emission is due to anisotropic inverse Compton scattering of stellar photons, then the GeV peak is expected to occur at phases when the compact object is behind the donor star (superior conjunction, SUPC), while TeV emission is expected to be maximum at INFC (since absorption is maximum at SUPC). The peculiar behaviour of the emission maxima resulting at similar phases is discussed by S15, where a proposed solution is the occurrence of INFC and periastron passage at a similar phase. Fig. 5 shows the updated orbital period vs. eccentricity plot from Townsend et al. (2011) for HMXBs with J1018 included from the derived orbital eccentricity in this work. J1018 lies around the transition zone between supergiant and Be X-ray binary systems, and follows the correlation of the two quantities which was noted by Casares et al. (2012) for the GRBis with known orbital parameters. It was speculated that the correlation between eccentricity and orbital period in GRBis is due to the small separation required for very high energy (TeV) emission to be triggered, which for long period systems requires large eccentricities. With the limited sample, GRBis display the same characteristics as accreting HMXBs, supporting the notion that GRBis may v represent an earlier phase in the evolution of HMXBs.
CONCLUSION
We observed J1018 with SALT and, combining our observations with S15, we obtain orbital phase coverage which allows us to constrain the orbital parameters. In particular, we obtained an eccentricity of e = 0.31±0.16 and a mass function f (M x ) = 0.0027 ± 0.0013 M ⊙ . The eccentricity obtained is in agreement with that implied from high energy studies of the source. For a range of values of the mass of the optical companion of 20 − 26.4 M ⊙ , the mass function obtained gives a mass for the compact object which favours a NS, with a BH possible for very low inclination angles (i 26
CONCLUSION
• ).

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