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Detection of Galactic Center Source G2 at 3.8 micron during Periapse Passage Around the Central Black Hole

ATel #6110; A. M. Ghez (UCLA), G. Witzel (UCLA), B. Sitarski (UCLA), L. Meyer (UCLA), S. Yelda (UCLA), A. Boehle (UCLA), E. E. Becklin (UCLA), R. Campbell (WMKO), G. Canalizo (UCR), T. Do (Toronto), J. R. Lu (UH), K. Matthews (Caltech), M. R. Morris (UCLA), A. Stockton (UH)
on 2 May 2014; 16:11 UT
Credential Certification: Andrea Ghez (ghez@astro.ucla.edu)

Subjects: Infra-Red, AGN, Black Hole, Transient

Referred to by ATel #: 6285

We report new observations of Galactic Center sources G2 & SgrA* from the W. M. Keck Observatory. Both sources are of great interest and vary temporally; G2 is the putative gas cloud now passing through periapse in its orbit around the black hole at the center of the Milky Way Galaxy and SgrA* is the emission associated with the central black hole. Our observations were obtained on 2014 March 19 & 20 (UT) with the Keck II laser guide star adaptive optics (LGSAO) system and the facility near-infrared camera (NIRC2) through the K'[2.1 µm] and L'[3.8 µm] broadband filters. At this time, G2 was expected to have been at closest approach with a separation from SgrA* of only ~20 mas and, therefore, to be spatially unresolved from SgrA* in our L' observations, which have an angular resolution of ~90 mas. Nevertheless, the two can be disentangled spectrally. In the L'-band, both Sgr A* and G2 contribute to the total flux; however, Sgr A*'s L' flux is estimated and removed based on (1) the K'-flux, where G2 does not contribute significantly, and (2) the well measured and constant K'-L' color of Sgr A*. Each night, roughly 20 interleaved measurements were made at each wavelength (exposure time of 28 and 30 sec at K' and L', respectively), with a duty cycle time of 134 sec for the two wavelengths. Our preliminary estimate of G2's 3.8 µm de-reddened flux density is 1.7 ± 0.2 mJy (or equivalently an observed magnitude of 14.1 ± 0.2 in the L'-band), which is consistent with measurements from earlier years (2002-2013). During these observations, SgrA* was quite faint (3.8 µm de-reddened flux density of 1.5 ± 0.2 mJy, which is 1/30 of the maximum observed at near-infrared wavelengths), allowing G2's flux density to be robustly measured. We conclude that G2, which is currently experiencing its closest approach, is still intact, in contrast to predictions for a simple gas cloud hypothesis and therefore most likely hosts a central star. Keck LGSAO observations of G2 will continue in the coming months to monitor how this unusual object evolves as it emerges from periapse passage.

UCLA Galactic Center Group