The Distance of the Gamma-ray Binary 1FGL J1018.6-5856
Vanessa J. Napoli, M. Virginia McSwain, Amber N. Marsh Boyer, Rachael M. Roettenbacher
aa r X i v : . [ a s t r o - ph . H E ] S e p PASP, in press
The Distance of the γ -ray Binary 1FGL J1018.6–5856 Vanessa J. Napoli , M. Virginia McSwain, Amber N. Marsh Boyer, and Rachael M.Roettenbacher Department of Physics, Lehigh University, 16 Memorial Drive E, Bethlehem, PA 18015;[email protected], [email protected], [email protected],[email protected]
ABSTRACT
The recently discovered γ -ray binary 1FGL J1018.6–5856 has a proposedoptical/near-infrared (OIR) counterpart 2MASS 10185560–5856459. We presentStr¨omgren photometry of this star to investigate its photometric variability andmeasure the reddening and distance to the system. We find that the γ -ray binaryhas E ( B − V ) = 1 . ± .
04 and d = 5 . +4 . − . kpc. . While E ( B − V ) is consistentwith X-ray observations of the neutral hydrogen column density, the distance issomewhat closer than some previous authors have suggested. Subject headings: gamma rays: stars – stars: individual (1FGL J1018.6–5856,2MASS 10185560–5856459)
1. Introduction
The
Fermi
Large Area Telescope source 1FGL J1018.6–5856 (Abdo et al. 2010) wasrecently discovered to have modulation of its 100 MeV–200 GeV emission with a 16 . ± .
04d period (Corbet et al. 2011a).
Swift observations identified a variable X-ray source witha position consistent with the location of the γ -ray source, and radio flux variations werealso found using the Australia Telescope Compact Array (Corbet et al. 2011a). Pavlov et al. present address: The Catholic University of America, Physics Department, 620 Michigan Ave. NE,Washington, DC 20064 present address: University of Michigan, Department of Astronomy and Astrophysics, 500 Church St.,Ann Arbor, MI 48109-1042 Chandra and
XMM-Newton , finding absorbed power lawspectra that are variable in both flux and hardness. The observed variability from radioto GeV energies implies that 1FGL J1018.6–5856 is a member of the elite group of “ γ -raybinaries”, which are high mass X-ray binaries that also exhibit rare γ -ray emission.The proposed optical and near-infrared (OIR) counterpart of the high energy source is2MASS 10185560–5856459. Corbet et al. (2011a) find a spectral type of O6 V((f)) for thestar, similar to the γ -ray binary LS 5039 (McSwain et al. 2004). Otherwise, knowledge of theoptical star in this system is quite limited. In this letter, we present Str¨omgren photometry of2MASS 10185560–5856459 to investigate the optical variability of the source, the interstellarreddening E ( B − V ), and the distance.
2. Observations
We observed 2MASS 10185560–5856459 using the Cerro Tololo Inter-American Obser-vatory (CTIO) 0.9m telescope, operated by the SMARTS consortium. We used the SITe2048 CCD in unbinned, quad readout mode with a plate scale of 0.401 ′′ /pixel. Observationswere taken between UT dates 2011 May 20-26 and 2011 June 17-23 using the Str¨omgren b and y filters.Bias images and sky flats were taken at the start of every night. We observed fourstandard stars (Cousins 1987) at a minimum of three different air masses each night. HD79039, HD 80484, HD 128726, HD 157795 were used on the May run, and HD 104664,HD 105498, HD 156623, and HD 157795 were used during June. The target was observedonce each night in both filters with exposure times of 400–700 s and 200–500 s in b and y ,respectively.The data were reduced using standard quadproc and cosmicray routines in IRAF . Weused an aperture of 7 ′′ to determine the instrumental magnitudes of the target and standards,and we calibrated the apparent magnitudes using the method of McSwain (2004). Due topoor photometric conditions on most nights, we found that only data from UT dates 2011May 21, May 26, and June 23 were well calibrated. Our quoted errors include both theinstrumental and transformation coefficient errors as described by McSwain (2004). Theresulting apparent magnitudes of 2MASS 10185560–5856459 are listed in Table 1. We find IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Asso-ciation of Universities for Research in Astronomy (AURA) under cooperative agreement with the NationalScience Foundation.
3. Reddening and Distance Measurements
We used the mean by magnitudes, with errors added in quadrature to be conservative,to determine the Str¨omgren by fluxes according to Gray (1998). We converted the J HK s magnitudes of 2MASS 10185560–5856459 (Skrutskie et al. 2006) to fluxes using the cali-bration of Cohen et al. (2003). These OIR fluxes constitute the observed spectral energydistribution (SED) of the optical star in the γ -ray binary.Corbet et al. (2011b) present an optical spectrum of the star, which they classify asO6 V((f)). The lack of emission in the H α or He II λ III λ T eff = 38 ,
900 K and surface gravity log g = 3 .
92 andinterpolated within the grid of Tlusty OSTAR2002 model fluxes (Lanz & Hubeny 2003) toproduce a model SED for 2MASS 10185560–5856459. We binned the model SED to 50˚A bins to remove small scale line features.Using a grid of values for the reddening, E ( B − V ), and a ratio of total-to-selectiveextinction R = 3 .
1, we applied the Galactic reddening model of Fitzpatrick (1999) to comparereddened model SEDs to the observed stellar fluxes. The ratio of the observed stellar fluxto the reddened model provides the stellar angular size, θ = R ⋆ /d (Gray 1992). Using theestimated stellar radius R ⋆ = 10 . R ⊙ (Martins et al. 2005), the distance d can thus bedetermined. We determined the best fit, E ( B − V ) = 1 . +0 . − . and d = 5 . ± . . The formal error in E ( B − V ) is theoffset from the best-fit value that increases the rms by 2 . / N, where N = 5 is thenumber of wavelength points within the fit region. Our formal error in d is determinedby the range of allowable E ( B − V ) values. This reddened model SED is shown with theobserved fluxes in Figure 1.Another potential source of error is the lack of precisely measured T eff and log g for2MASS 10185560–5856459. Based on the absence of strong emission lines, the main sequencenature of the hot star is well constrained and we allow a range of 3 . ≤ log g ≤ . T eff is less well constrained, so we allow a 10% error in this quantity to provide a more generousrange in the allowed E ( B − V ) and d . For each T eff and log g model, we determined R ⋆ T eff and log g which results in the more realistic error bars E ( B − V ) = 1 . ± .
04 and d = 5 . +4 . − . kpc.The observed neutral hydrogen column density, from Chandra observations, is nH =0 . +0 . − . × atoms cm − (Pavlov et al. 2011). Their XMM-Newton observation was bestfit using somewhat smaller errors for nH , 0 . +0 . − . × atoms cm − . Using the relation nH/E ( B − V ) = 5 . × atoms cm − mag − (Bohlin et al. 1978), the Chandra detectionspredict a possible range for the optical reddening of 0 . < E ( B − V ) < .
43. Thus ourmeasurement of E ( B − V ) is consistent with their results for nH from Chandra .Our distance measurement also agrees well with the value from Corbet et al. (2011b),who estimate 5 ± ± E ( B − V ) available due to our Str¨omgren photometry of the source. With our improveddistance, the unabsorbed luminosities of the Chandra and
XMM-Newton observations shouldbe revised downward to 36% of the values provided by Pavlov et al. (2011).We are grateful to Arturo Gomez and the SMARTS consortium for their help schedulingand supporting these observations. VJN is supported by the National Science Foundationunder REU site grant No. PHY-0849416. RMR is supported by a NASA Harriett G. Jenk-ins Pre-doctoral Fellowship and a Sigma Xi Grant-in-Aid of Research. This work is alsosupported by an institutional grant from Lehigh University. This publication makes use ofdata products from the Two Micron All Sky Survey, which is a joint project of the Univer-sity of Massachusetts and the Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administration and the NationalScience Foundation.
Facilities:
CTIO:0.9m ()
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This preprint was prepared with the AAS L A TEX macros v5.2. b ) b ∆ b exposure ( y ) y ∆ y a O6 V((f))Mass ( M ⊙ ) b R ⊙ ) b T eff (K) b g (cm s − ) b a Corbet et al. (2011a) b Martins et al. (2005) 7 –Fig. 1.— OIR spectral energy distribution of 2MASS 10185560–5856459. The reddenedTlusty model SED with T eff = 38 ,
900 K, log g = 3 .
92, and E ( B − V ) = 1 .
34 is also shown,normalized to the best-fit distance d = 5 ..