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Optical follow-up of the gamma-ray flare from the FSRQ 3C 345

ATel #10456; M. Berton (University of Padova), P. Calcidese (Osservatorio Astronomico della Regione Autonoma Valle d'Aosta), S. Chen (University of Padova), E. Congiu (University of Padova)
on 3 Jun 2017; 12:13 UT
Credential Certification: Marco Berton (marco.berton@unipd.it)

Subjects: Optical, AGN, Blazar, Quasar

As a follow-up of the flaring activity of the flat-spectrum radio quasar 3C 345 (z = 0.5928, Lynds, C.R., et al., 1965, ApJ, 142, 1667) in gamma rays, we observed the source in optical with the 81cm telescope of the Osservatorio astronomico della Regione Autonoma Valle d'Aosta (Italy), and the 1.22m telescope of the Asiago Astrophysical Observatory (Italy). The gamma-ray activity of 3C 345 was detected by Fermi/LAT (ATel#10453) on 2017, May 31st and in the following days. We report 3C 345 to have magnitude R = 17.60 +/- 0.22 on 2017, June 2nd (22 UTC). This measurement is not very different than the magnitude measured during the very low activity state of 3C 345 registered in Summer 2016 (R = 18.09 +/- 0.23 on 2017, July 25th, as measured by the Asiago/Schmidt telescope). Spectroscopic observations were carried out with the 1.22m telescope, using a B&C spectrograph, a HeFeAr lamp for wavelength calibration, and the standard star BD+332642 for flux calibration. We obtained three exposures 1800 seconds each on 2017, May 24th, and four exposures 1200 seconds each on 2017, June 3rd (01 UTC). The spectral resolution is ~700 in both cases. The spectra indicate that the rest-frame 3000 Angstroms continuum flux of the source was (1.39 +/- 0.18)E-16 erg/s/cm^2/A on June 3rd, slightly lower than the flux (1.87 +/- 0.22)E-16 erg/s/cm^2/A measured on May 24th. The Mg II line integrated flux instead, measured after the continuum subtraction, went from (2.14 +/- 0.15)E-14 erg/s/cm^2 on May 24th to (1.90 +/- 0.13)E-14 erg/s/cm^2 on June 3rd, which is constant within the error bars. The source therefore does not appear to be simultaneously flaring in optical and in gamma rays. This result, if confirmed, might indicate that the ongoing gamma-ray flare is an example of orphan flare, occurring only in the high energy range (e.g. Bottcher, M., 2005, ApJ, 630, 186). We encourage further multiwavelength observations of the source to monitor this flaring state, and particularly observations of optical spectra. The Asiago Astrophysical Observatory is operated by the Department of Physics and Astronomy 'G. Galilei' of the University of Padova.