IGRJ05414-6858 is a Be-X-ray Binary

ATel #2704; Arne Rau, Patricia Schady, Jochen Greiner, Frank Haberl (all MPE) and Adria Updike (Clemson)
on 29 Jun 2010; 14:04 UT
Credential Certification: Arne Rau (arne@astro.caltech.edu)

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We report on Swift/UVOT and GROND observations following the discovery of the new hard X-ray transient IGR J05414-6858 with INTEGRAL (ATel #2695). Target of Opportunity observations with the Swift satellite started 2010 June 25th 05:39 UT and lasted until 09:06 UT. The analysis of the XRT data revealed a faint X-ray source located 3'0 from the center of the 2'5-uncertainty INTEGRAL location and a possible infrared counterpart (2MASS 05412663-690122) had been reported (ATel #2696). Closer inspection of the 2MASS images revealed the counterpart to be elongated.

Follow-up observations with the 7-channel Gamma-Ray Burst Optical-NearIR Detector (GROND) mounted at the 2.2m MPG/ESO telescope in La Silla were performed on 2010 June 26th 10:51 UT and Jun 29th 10:29 UT. We found that the 2MASS object is composed of three blended point sources located at

A: RA(J2000) = 05:41:26.66, Dec(J2000) = -69:01:23.7
B: RA(J2000) = 05:41:26.63, Dec(J2000) = -69:01:21.7
C: RA(J2000) = 05:41:26.56, Dec(J2000) = -69:01:25.4

with uncertainties of +/- 0"4 in both directions. PSF photometry provided the following AB magnitudes for source A:

June 26th:
J = 16.4 +/- 0.1
H = 16.9 +/- 0.1
K = 17.3 +/- 0.2

June 29th:
g = 15.53 +/- 0.03
r = 15.75 +/- 0.03
i = 16.11 +/- 0.05
z = 16.17 +/- 0.05
J = 16.3 +/- 0.1
H = 16.8 +/- 0.1
K = 17.3 +/- 0.1

These magnitudes were derived by calibrating the images against GROND zero-points and 2MASS field stars and are corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.075mag (Schlegel et al. 1998).

The analysis of the Swift/UVOT images revealed a single point source whose location is consistent with source A (RA(J2000)=05:41:26.61, Dec(J2000) = -69:01:23.4 (with an error of +/-0.5) and the following Galactic foreground extinction corrected AB magnitudes:

V = 15.37 +/- 0.04
B = 15.19 +/- 0.02
U = 15.05 +/- 0.02
UVW1 = 15.05 +/- 0.02
UVM2 = 15.10 +/- 0.02
UVW2 = 15.03 +/- 0.02

(statistical error only, magnitudes can be slightly overestimated due to contamination by source B).

The combined UVOT + GROND spectral energy distribution of source A is best fit by a B1-2III stellar template. At the distance of the LMC (50kpc) the absolute V-band magnitude would be M_V=-3.1, supporting this interpretation.

Archival XMM-Newton observations (obtained 2001 Oct 19th, 9ks exposure) do not show an X-ray counterpart at the XRT position to a 3-sigma limit of 8e-03 cts/s (0.2-4.5 keV EPIC PN). Assuming a power law with photon index 1.0 and Galactic foreground extinction, this count rate translates to a 0.2-10.0 keV flux of ~5e-14 erg/cm^2/s and a luminosity of ~1.5e34 erg/s at LMC distance. The latter is approx. 500 times fainter than observed by INTEGRAL (ATel #2695).

The optical luminosity, spectral energy distribution, and large X-ray amplitude suggest that source A is the optical/NIR counterpart of IGR J05414-6858, and that this transient is a Be-X-ray binary in the LMC that was discovered by INTEGRAL during an outburst.