The periodic very young source EC 53 reached its maximum brightness

ATel #11705; T. Giannini, S. Antoniucci, D. Lorenzetti (INAF - Osservatorio Astronomico di Roma, Monte Porzio, Italy), A. Harutyunyan (Telescopio Nazionale Galileo, La Palma, Spain), D. Licchelli (Universita' del Salento, Lecce, Italy), U. Munari (INAF - Osservatorio Astronomico di Padova, Padova, Italy)
on 11 Jun 2018; 12:09 UT
Credential Certification: Teresa Giannini (teresa.giannini@oa-roma.inaf.it)

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In the framework of our EXor monitoring program dubbed EXORCISM (EXOR OptiCal and Infrared Systematic Monitoring - Antoniucci et al. 2013 PPVI, Lorenzetti et al. 2007 ApJ 665, 1182; Lorenzetti et al. 2009 ApJ 693, 1056), we observed the object EC53 recently signaled by Johnston et al. (ATel #11614) as a strongly embedded source showing a sub-mm luminosity burst, They also provide H- and K-band observations detecting this brightness increase also in the near-IR, in the scattered light by the nebula surrounding a compact source, invisible at those wavelengths. EC 53 (R.A. 18:29:51.2, Dec. +01:16:39.5) is located in the Serpens molecular cloud together with several sources young enough to show a substantial variability. Firstly identified as variable by Hodapp et al. (1996, ApJ 468, 861), EC 53 was recognized as a nebula illuminated by a periodically variable star every approximately 18 months (Hodapp et al. 2012, ApJ, 744, 56). Yoo et al. (2017, ApJ, 849, 69) describing a sub-mm luminosity burst, confirm that periodicity and suggest that the periodic variability is due to accretion instabilities driven by a very close binary companion not directly observable. Our photometry has been obtained on two different dates with the TNG Italian Telescope at La Palma (Canary Island): on 2018 May 10: J=15.6, H=12.2, K=10.6, [J-H]=3.4, [H-K]=1.6 and on 2018 June 8: J=16.1, H=12.3, K=10.9, [J-H]=3.8, [H-K]=1.4. Johnston et al. (ATel #11614) presented a rising brightness whose highest level (on late April 2018) corresponds to about H=12.7, K=11.8, [H-K]=0.9. Our photometry definitely indicates that the near-IR brightness continued to increase until a maximum occurred between the end of May and the first days of June. Such a behavior is well in agreement with the expected periodicity, according to which the maximum luminosity was anticipated somewhere between late May and early July, 2018 (ATel #11614). Since we observed EC 53 at its maximum luminosity, we can provide, for the first time, also J-band photometry. On May 10, at the same telescope, we obtained also a low resolution spectrum in the range 1.4 – 2.4 µm. At variance with usual spectra of eruptive variables (FUors/EXors), it is completely featureless, veiled by the strong continuum.