MAXI J1813-095: GROND discovery of a candidate optical/NIR counterpart

ATel #11332; Arne Rau (MPE Garching)
on 22 Feb 2018; 13:14 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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The new transient MAXI J18130-095 (Kawase et al. 2018, ATel #11323) was observed simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) at the MPG 2.2m telescope at the ESO La Silla Observatory (Chile). Observations were obtained on February 22nd 09:18 UT with integrations of ~8.3min in the optical bands and ~10.7min in the NIR bands.

Two sources are detected within the Swift/XRT error circle (Kennea et al. 2018, ATel #11326), both also included in PanSTARRS DR1 (https://panstarrs.stsci.edu).

The GROND positions are:

Source A = PSO J181334.004-093159.456
RA(J2000) = 18:13:34.015
Dec(J2000) = -09:31:59.20
Source B = PSO J181333.964-093158.045
RA(J2000) = 18:13:33.970
Dec(J2000) = 09:31:57.94

with uncertainties of 0.05" and 0.07" in RA and Dec, respectively.

We measure the following AB magnitudes (calibrated against PanSTARRS in griz and against 2MASS in JHK, not corrected for Galactic foreground reddening):

Source A:
g' = 21.55 +/- 0.11
r' = 19.69 +/- 0.02
i' = 18.65 +/- 0.01
Z' = 17.92 +/- 0.01
J = 17.14 +/- 0.07
H = 16.74 +/- 0.05
K = 16.68 +-/ 0.06

Source B:
g' = >23.3
r' = 20.04 +/- 0.02
i' = 18.63 +/- 0.01
Z' = 17.68 +/- 0.01
J = 16.44 +/- 0.08
H = 15.76 +-/ 0.07
K = 15.74 +-/ 0.07

While the GROND photometry of Source B is consistent with the catalogued PanSTARRS values, Source A has brightened by ~1 magnitude in i and z with respected to the PanSTARRS observations (g, r, and y band photometry for this source is not included in the PanSTARRS DR1 catalog). We therefore suggest that Source A is the likely counterpart of MAXI J1813-095.