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Return-Path: <atel@astronomerstelegram.org> Delivered-To: stefano@bicar.it Received: by mail.planetcloud-host.com (Postfix, from userid 99) id 50FBA14606; Thu, 26 Feb 2026 08:18:14 +0100 (CET) X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on mail.planetcloud-host.com X-Spam-Level: X-Spam-Status: No, score=1.0 required=5.0 tests=RCVD_IN_DNSWL_BLOCKED, RCVD_IN_VALIDITY_RPBL_BLOCKED,RCVD_IN_VALIDITY_SAFE_BLOCKED,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=disabled version=3.4.0 Received-SPF: Pass (sender SPF authorized) identity=mailfrom; client-ip=174.142.199.243; helo=server.astronomerstelegram.org; envelope-from=atel@astronomerstelegram.org; receiver=stefano@bicar.it Received: from server.astronomerstelegram.org (astronomerstelegram.org [174.142.199.243]) by mail.planetcloud-host.com (Postfix) with ESMTP id 39F3814528 for <stefano@bicar.it>; Thu, 26 Feb 2026 08:17:35 +0100 (CET) Received: from server.astronomerstelegram.org (localhost [127.0.0.1]) by server.astronomerstelegram.org (Postfix) with ESMTP id 7AB2A2880967 for <stefano@bicar.it>; Thu, 26 Feb 2026 02:17:34 -0500 (EST) To: stefano@bicar.it From: atel@astronomerstelegram.org Subject: ATel 17699, 17700, 17701 Message-Id: <20260226071734.7AB2A2880967@server.astronomerstelegram.org> Date: Thu, 26 Feb 2026 02:17:34 -0500 (EST) ------------------------------------------------------------------------------ The Astronomer's Telegram is free to read, free to publish, free to use. Thanks to the support of our patrons, we can continue to keep it free. https://www.patreon.com/astronomerstel ------------------------------------------------------------------------------ The Astronomer's Telegram http://www.astronomerstelegram.org ------------------------------------------------------------------------------ Posted: Within the last 24 hours ============================================================================== ATEL #17699 ATEL #17699 Title: Fermi J1820-1648: extended GeV duration and improved LAT localization Author: C. C. Cheung (NRL), P. Jean (IRAP, Toulouse), on behalf of the Fermi Large Area Telescope Collaboration Queries: Teddy.Cheung@nrl.navy.mil Posted: 25 Feb 2026; 19:36 UT Subjects:Gamma Ray, >GeV, Transient Following the Large Area Telescope (LAT) detection of Fermi J1820-1648 in the 1-week interval from 2026 February 9.0-16.0 (ATel #17688), we searched for additional GeV emission over an extended time window from February 2.0-23.0. Preliminary analysis in 1-day time bins indicates that this source was detected (>3 sigma) on February 6, 7, 8, and 16, giving a total gamma-ray duration of 11 days. Taking the LAT data from 2026 February 6.0-17.0, the average gamma-ray flux (E>100 MeV) is (1.5+/-0.2) X 10^-6 photons cm^-2 s^-1 with a single power-law photon index of 2.3+/-0.1 (statistical uncertainties only). The duration and spectrum are consistent with those of previous gamma-ray novae detected by Fermi-LAT. The best-fit location is RA = 275.09 deg, Dec. = -16.83 deg (J2000) with a 95% containment radius, r95 = 0.08 deg (errors are statistical only) that is smaller than that originally reported (r95 = 0.11 deg). The highly-reddened optical transient detected on 2026 February 22.5087 by Sokolovsky et al.) ATel #17698) is within the improved LAT localization. Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is C.C. Cheung (Teddy.Cheung _at_ nrl.navy.mil). The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden. ------------------------------------------------------------------------------ Password Certification: Teddy Cheung (Teddy.Cheung@nrl.navy.mil) https://www.astronomerstelegram.org/?read=17699 ============================================================================== ATEL #17700 ATEL #17700 Title: SOAR spectroscopic classification of the Galactic plane gamma-ray transient Fermi J1820-1648 Author: P. Craig, I. Molina, J. Strader, L. Chomiuk (Michigan State University), E. Aydi, K. Sokolovsky, D. Perla, V. Antoniou, T. Maccaroone, E. Jarvela, and M. Fausnaugh (Texas Tech University) Queries: eaydi@ttu.edu Posted: 25 Feb 2026; 19:40 UT Subjects:Optical, Gamma Ray, Cataclysmic Variable, Nova, Star, Transient, Variables We report optical spectroscopic follow-up observations of the candidate optical counterpart (ATel #17698) to the Galactic-plane gamma-ray transient Fermi J1820-1648, discovered by Fermi-LAT on 2026-02-18.7 (ATels #17688, #17699). On 2026-02-25.4 we obtained a low-resolution spectrum of the source using the Goodman spectrograph (Clemens et al. 2004, SPIE, 5492, 331) on the 4.1-m SOAR telescope. The spectrum shows broad, double-peaked emission lines of Halpha, O I, and H I. The full width at zero intensity (FWZI) of Halpha is approximately 4500 km/s. The near-infrared lines are significantly stronger relative to Halpha, consistent with substantial reddening. This is also emphasized by the lack of continuum and emission lines at wavelengths shorter than 6000 Angstrom. The overall spectral characteristics are consistent with those of a highly reddened classical nova observed after optical maximum. This classification strongly supports the association between the optical source and the gamma-ray transient, as classical novae are now well-established Galactic gamma-ray sources (e.g., Craig et al. 2026, MNRAS 546, 2270). Determining a reliable distance to this nova will be crucial. Highly reddened and distant novae are not commonly detected by Fermi-LAT, and if this system lies at a large distance, it may represent an unusually gamma-ray luminous Galactic nova. This ATel is based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministerio da Ciencia, Tecnologia, Inovacoes e Comunicacoes do Brasil (MCTIC/LNA), the U.S. National Science Foundation's National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). ------------------------------------------------------------------------------ Password Certification: Elias Aydi (eaydi@ttu.edu) https://www.astronomerstelegram.org/?read=17700 ============================================================================== ATEL #17701 ATEL #17701 Title: Fermi-LAT detection of enhanced gamma-ray activity from the BCU MG2 J164800+2224 Author: Ettore Bronzini (INAF-OAS), on behalf of the Fermi Large Area Telescope Collaboration Queries: ettore.bronzini@inaf.it Posted: 25 Feb 2026; 21:06 UT Subjects:Gamma Ray, >GeV, AGN, Blazar The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed enhanced gamma-ray activity from a source positionally consistent with the Blazar Candidate of Uncertain Type MG2 J164800+2224, also known as 4FGL J1648.0+2221 (Abdollahi et al. 2022, ApJS, 260, 53), R.A. = 252.00640 deg, Dec. = +22.40921 deg (J2000; Petrov&Taylor 2011, AJ, 142, 3, 89), and redshift z=0.822 (The Sloan Digital Sky Survey 2017, ApJS, 233, 25). Preliminary analysis indicates that this source was in an elevated gamma-ray emission state on February 24th, 2026 , with a daily averaged gamma-ray flux (E>100MeV) of (0.4+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of about 60 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT catalog (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546). The corresponding photon index is 2.4+/-0.3, and is consistent with the 4FGL-DR4 value of 2.6+/-0.1 within the uncertainties. The Fermi-LAT Collaboration has previously reported a flaring activity from this source in ATel #16227. Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for BCU MG2 J164800+2224 can be accessed via the Fermi-LAT Light-Curve Repository. We encourage multifrequency observations of this source. For this source, the Fermi-LAT contact person is Adithiya Dinesh (adinesh@ucm.es). The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden. ------------------------------------------------------------------------------ Password Certification: Ettore Bronzini (ettore.bronzini@inaf.it) https://www.astronomerstelegram.org/?read=17701 ============================================================================== ============================================================================== This is an automatically-generated notice. 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