Swift J045558.9-702001, a new HMXB in the LMC
ATel #5540; G. Vasilopoulos, R. Sturm, F. Haberl, P. Maggi (MPE, Germany), A. Udalski (Warsaw University Astronomical Observatory, Poland)
on 4 Nov 2013; 11:15 UT
Credential Certification: Richard Sturm (rsturm@mpe.mpg.de)
Subjects: X-ray, Binary, Neutron Star
We report a new high-mass X-ray binary (HMXB), found in a Swift observation performed on 2013 October 11 as part of the monitoring of the nova LMC2012 (ObsId: 00049549004). The Swift/XRT count rate was 0.023±0.008 cts s-1, corresponding to a flux of 1.6×10-12 erg cm-2 s-1 (0.3-10 keV) and an unabsorbed luminosity of 5.1×1035 erg s-1 for LMC distance (50 kpc).
To study the temporal variability and derive the spectral properties of the source,
we analysed all the available Swift archival observations that covered its position.
The source was detected at a similar flux level in a total of 59 Swift observations,
57 of which were performed within a time period of 100 days. To improve its position, we requested an on-axis Swift/XRT observation that was performed on 2013 October 22 (ObsId: 00033009001).
We estimate a refined X-ray position of RA=04:55:58.9 and DEC=-70:20:01 (J2000, 1σ uncertainty: 2.2") by calculating the weighted mean of the detections with the best statistics. Thus the source is named Swift J045558.9-702001.
Because of the low statistics, we assume a constant spectral shape, described by an absorbed power law
and fit all spectra simultaneously with C-statistic (542 counts in total).
The column density (NH) was modeled by the sum of two components. The first one accounts for the Galactic absorption and was fixed to a value of NH=9.17×1020 cm-2 taken from the Dickey & Lockman HI map (Dickey & Lockman, 1990, ARAA, 28, 215). The second one is accounting for both the LMC and the intrinsic absorption of the source, and was set free (with half solar metalicity).
The NH for the second component is consistent with 0 (<3×1020cm-2, 3σ confidence level).
The spectrum is hard, with a best-fit photon index of Γ=0.55±0.14,
which is typical for a neutron-star HMXB system (Haberl et al. 2008, A&A 489,327-348).
We derive a mean luminosity of 3.8×1035 erg s-1 [flux: 1.26(0.46)×10-12 erg s-1], a maximum luminosity of 7.3×1035 erg s-1 [flux: 2.45(+1.0-1.5)×10-12 erg s-1], and a total variability of ~5.5.
The likely optical counterpart (2MASS J04555883-7019599) is found at RA=04:55:58.840 and DEC=-70:19:59.98 at an angular separation of 1.5".
It has been classified as B 1-2 star (Massey et al. 2002, ApJS, 141, 81) and is suggested to be the emission-line star [L63] 42 by Howarth (2013, A&A 555, 141).
The OGLE III and OGLE IV counterparts are LMC129.6 23 and LMC530.18 23149, respectively.
The light curve (see OGLE-IV real time monitoring of X-ray variables) reveals low level variability (~0.1 mag). We found no evidence for periodicity by the analysis of the OGLE data.
Given the nature of the proposed counterpart and X-ray spectral index we conclude that the newly detected source is a Be/X-ray binary in the LMC, most likely showing persistent X-ray emission.
We thank the Swift team for accepting and scheduling the target of opportunity observation.
