Optical spectrum of the BL Lac object 4FGL J1824.1+5651 (4C +56.27)

ATel #16925; Simona Paiano (INAF/IASF-PA), Renato Falomo (INAF/OAPD), Aldo Treves (Universita' dell'Insubria, INAF/OABrera), Riccardo Scarpa (IAC, Universidad de la Laguna)
on 28 Nov 2024; 18:04 UT
Credential Certification: Simona Paiano (simona.paiano@inaf.it)

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The BL Lac object 4FGLJ1824.1+5651 (4C +56.27) exhibited an enhanced gamma-ray activity on Nov. 16, 2024, of a factor of ~14 with respect to the average flux (ATEL#16913). On August 07, 2018 we obtained an optical spectrum of the source in the range 4100-7700 Ang with the Gran Telescopio Canarias+OSIRIS and R1000B grism (g = 18.5, exposure time = 3000 s). The spectrum (S/N~120) is characterized by a power law continuum (spectral index = 0.43). We clearly detect a broad emission line at 4662 Ang (EW = 3.5 Ang) attributed to MgII 2800, and narrow emissions at 6203 Ang (EW= 1.6 Ang) due to [OII] 3727, and at 5703 Ang and 6439 Ang due to [NeV] 3426 and [NIII] 3869, respectively. This yields a redshift z=0.6645, confirming the values given by Lawrence et al. (1986,1996) and Shaw et al. (2013). In addition to the emission lines, we also find a MgII absorption doublet at z = 0.601. Our spectrum corresponds to a state weaker by a factor of ~2.5 with respect to that reported by Shaw et al. (2013). The dereddened spectrum (E(B-V)=0.05) can be found in the online database ZBLLAC (https://web.oapd.inaf.it/zbllac/downloadpdf.php?pdfname=https://web.oapd.inaf.it/zbllac/pdf/3FGLJ1824p56.pdf&sourcename=4FGLJ1824.1+5651&date=7).