Chandra and XMM-Newton Observations of the Gamma-ray Binary 1FGL J1018.6-5856

ATel #3228; G. G. Pavlov (Penn State Univ.), Z. Misanovic (Monash Univ.), O. Kargaltsev (Univ. of Florida), G. P. Garmire (Penn State Univ.)
on 21 Mar 2011; 21:09 UT
Credential Certification: Derek B. Fox (dfox@astro.psu.edu)

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The Fermi LAT source 1FGL J1018.6-5856 has been recently identified as a high-mass gamma-ray binary (Corbet et al., ATel #3221). The source was observed with Chandra ACIS-I (ObsID 11831) on 2010-08-17 from 01:17 to 04:04 UTC, which corresponds to the binary phase interval from 0.312 to 0.319 (phase uncertainty +/-0.024; zero phase corresponds to the gamma-ray flux maximum at the reference epoch MJD 55403.3+/-0.4, period 16.58+/-0.04 d, according to ATel #3221). The target was imaged 2.3 arcmin off-axis on the ACIS-I3 chip, the centroid coordinates are

RA(2000) = 10:18:55.618, decl.(2000) = -58:56:46.09

(statistical uncertainty 0.04 arcsec per coordinate, at the 68% level). In the 9.926 ks useful exposure time we collected 674 counts in the 2" radius aperture, 0.3-8 keV band, which corresponds to the source count rate of 0.0679+/-0.0026 cps. The source spectrum fits the absorbed power-law model with

N_H = 0.64(+0.19,-0.17) 10^{22} cm^{-2}
Gamma = 1.36(+0.17,-0.16)
Norm = 1.85(+0.53,-0.41) 10^{-4} photons/cm^2/s/keV at 1 keV
Flux(0.3-10 keV) = (1.45+/-0.15) 10^{-12} erg/s/cm^2

(reduced chi^2 = 0.89 for 38 dof; errors at 68% level; the flux is aperture corrected).

The field of 1FGL J1018.6-5856 was observed with XMM-Newton (ObsID 0604700101) on 2009-08-22 from 16:44 to 22:33 UTC, which corresponds to the binary phase interval from 0.638 to 0.653 (phase uncertainty 0.055). In the useful exposures of 20.41, 19.24 and 14.85 ks for the MOS1, MOS2 and pn detectors we found 746, 789 and 1787 counts in the 25" radius aperture, which corresponds to the background-subtracted count rates of 0.0340+/-0.0013, 0.0383+/-0.0015 and 0.1091+/-0.0029 cps, respectively. The joint fit of the MOS1+MOS2+pn spectra with the absorbed power-law model gives

N_H = 0.646(+0.048,-0.039) 10^{22} cm^{-2}
Gamma = 1.62(+0.059,-0.050)
Norm = 1.43(+0.12,-0.10) 10^{-4} photons/cm^2/s/keV at 1 keV
Flux(0.3-10 keV) = 0.892(-0.016,+0.039) 10^{-12} erg/s/cm^2

(reduced chi^2 = 0.93 for 136 dof, errors at 68% level, the flux is aperture corrected). Notice that XMM-Newton detected a factor of 1.6 lower flux and perhaps a softer spectrum, but the same hydrogen column density, which is a factor of 2 lower than the HI column density through the entire Galaxy in this direction (l = 284.3 deg, b=-1.7 deg). The inferred spectral parameters are very close to those of the LS 5039 and LSI +61 303 high-mass gamma-ray binaries. The X-ray emission is likely produced by collision of the high-mass star wind with the pulsar wind or magnetosphere of a neutron star companion, similar to the high-mass binary RSR B1259-63/SS 2883.

The 2MASS image of this region shows a star of magnitudes

J = 10.436+/-0.022, H = 10.144+/-0.022, K = 10.016+/-0.019

at R.A.(2000) = 10:18:55.604, decl.(2000) = -58:56:45.95

(differs from the Chandra position by only 0.18 arcsec), the high-mass companion of the compact object. The hydrogen column density determined from the X-ray data corresponds to the color index E(B-V) ~ 1.1. For an O6V star (ATel #3221), the observed magnitudes and extinction correspond to the distance of 6-12 kpc, larger than the distances to the similar systems LS 5039 and LSI +61 303. For d = 9 kpc, the 0.3-10 keV unabsorbed luminosity of our target was 1.8 and 1.3 10^{34} erg/s at the epochs of the Chandra and XMM-Newton observations, respectively, while the average 0.1-100 GeV luminosity is 3.2 10^{36} erg/s. Although the source is projected close to the center of the 10 kyr old radio SNR G284.3-1.8 (MSH 10-53; Milne et al. 1989, Proc. ASA, 8, 2), the SNR is apparently closer to the Sun (2.9 kpc, according to Ruiz & May 1986, ApJ, 309, 667). The binary is also projected onto the TeV source HESS J1018-589 (de Ona Wilhelmi et al., contribution to COSPAR 2010), but the actual source of the TeV emission has not been established yet.