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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 C48FF146CE; Fri, 17 Apr 2026 09:06:35 +0200 (CEST) X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on mail.planetcloud-host.com X-Spam-Level: X-Spam-Status: No, score=-0.0 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 ACDF214603 for <stefano@bicar.it>; Fri, 17 Apr 2026 09:05:39 +0200 (CEST) Received: from server.astronomerstelegram.org (localhost [127.0.0.1]) by server.astronomerstelegram.org (Postfix) with ESMTP id C266928808DD for <stefano@bicar.it>; Fri, 17 Apr 2026 03:05:38 -0400 (EDT) To: stefano@bicar.it From: atel@astronomerstelegram.org Subject: ATel 17755, 17756 Message-Id: <20260417070538.C266928808DD@server.astronomerstelegram.org> Date: Fri, 17 Apr 2026 03:05:38 -0400 (EDT) ------------------------------------------------------------------------------ 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 #17755 ATEL #17755 Title: Fermi-LAT detection of renewed high gamma-ray activity from the BL Lac PKS 0537-441 Author: S. Rani (Michigan Technological University), E. Bronzini (INAF-OAS), P. Monti-Guarnieri (University of Trieste and INFN Trieste), T. Lewis (Michigan Technological University) on behalf of the Fermi Large Area Telescope Collaboration Queries: ettore.bronzini@inaf.it Posted: 16 Apr 2026; 19:40 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 renewed gamma-ray activity from a source positionally consistent with the BL Lac object PKS 0537-441, also known as 4FGL J0538.8-4405 (Abdollahi et al. 2022, ApJS, 260, 53), with coordinates R.A. = 84.70984 deg, Dec. = -44.08581 deg (J2000; Johnston et al. 1995, AJ, 110, 880), and redshift z = 0.894 (Peterson et al. 1976, ApJ, 207, L5). Preliminary analysis indicates that this source was in an elevated gamma-ray state on April 15, 2026, with a daily averaged gamma-ray flux (E > 100 MeV) of (1.1 +/- 0.2) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of 5 relative to the average flux reported in the fourth data release of the fourth Fermi-LAT catalogue (4FGL-DR4, Ballet et al. 2023, arXiv:2307.12546) and it is comparable with previously reported peak activity from this source. The corresponding photon index is 1.9 +/- 0.1, indicating a harder spectrum than the 4FGL-DR4 value of 2.14 +/- 0.01. The spectral hardening led to the detection of a high-energy photon (E = 27 GeV) associated with the source at a confidence level p > 0.999. The source has shown sustained elevated emission over multiple days, confirming that the observed activity represents a persistent high state rather than a transient fluctuation. The Fermi-LAT Collaboration has previously reported flaring activity from this source in ATels #2591, #2124 and #1759. Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. A preliminary light curve for 4FGL J0538.8-4405 can be accessed via the Fermi-LAT Light-Curve Repository at <a href= https://fermi.gsfc.nasa.gov/ssc/data/access/lat/LightCurveRepository/source.html?source_name=4FGL_J0538.8-4405>4FGL J0538.8-4405</a>. This source belongs to the list LAT daily monitored sources and therefore a daily light-curve can also be accessed at <a href="https://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/PKS_0537-441">PKS 0537-441</a>. We encourage multifrequency observations of this source. The Fermi-LAT contact person for this source is Gino Tosti (tosti@pg.infn.it). The Fermi LAT is a pair conversion telescope designed to cover the energy band 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=17755 ============================================================================== ATEL #17756 ATEL #17756 Title: Discovery of Radio Pulsation from the Radio-quiet Gamma-ray Pulsar PSR J2238+5903 with Deep FAST Observations Author: Jianli Zhang(NAOC, CAS), Hui Zhu(NAOC, CAS), Guanhong Lin(NAOC, CAS), DeJia Zhou(NAOC, CAS), Songzhan Chen(IHEP, CAS), Wen-Jun Huang(GZU), Zi-Wei Ou(SYSU), P. H. Thomas Tam(SYSU), Sha Wu(IHEP, CAS), Qiang Yuan(PMO, CAS), Yi Zhang(PMO, CAS) Queries: jlzhang@bao.ac.cn Posted: 17 Apr 2026; 04:13 UT Subjects:Radio, Gamma Ray, VHE, Pulsar We report the detection of radio pulsations from the radio-quiet gamma-ray pulsar PSR J2238+5903 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST; [1]). This is the first radio detection of this source, establishing a critical multi-wavelength connection between its gamma-ray and very-high-energy (VHE) gamma-ray emission.<br/> PSR J2238+5903 is spatially associated with 1LHAASO J2238+5900 [2], a VHE gamma-ray source in the first LHAASO catalog. This source is located at RA = 22:38:28.15, Dec = +59:03:44.8 (J2000), with a spin period of P = 0.16274 s derived from gamma-ray observations [3].<br/> The FAST observations were carried out on 2021 August 14 in the L-band (1.0-1.5 GHz) with the 19-beam receiver, with a total on-source integration time of 3000 s. Data were processed using the PRESTO pipeline [4]. A significant periodic signal was detected, fully consistent with the gamma-ray ephemeris, at an optimal dispersion measure of DM = 248.0 pc cm<sup>−3</sup>. The distance to the source is estimated to be ∼7 kpc from the YMW16 Galactic electron density model.<br/> This detection confirms that PSR J2238+5903 is a faint but detectable radio pulsar despite being classified as radio-quiet in gamma-ray studies. Its coincidence with a VHE source provides a unique key to understanding particle acceleration, pulsar wind nebulae, and the origin of VHE gamma-rays associated with pulsars.<br/> References<br/> 1. Nan, R., et al. 2011, Int. J. Mod. Phys. D, 20, 989<br/> 2. Cao, Z., et al. 2024, ApJS, 271, 25<br/> 3. Smith, D. A., et al. 2023, ApJ, 958, 191<br/> 4. Ransom, S. M. 2001, ASP Conf. Ser. 245, 551<br/> ------------------------------------------------------------------------------ Password Certification: Jianli Zhang (jlzhang@bao.ac.cn) https://www.astronomerstelegram.org/?read=17756 ============================================================================== ============================================================================== This is an automatically-generated notice. You can remove yourself permanently from receiving both the Daily Email Digest and Instant Email Notices from The Astronomer's Telegram by following this link: https://www.astronomerstelegram.org?unsubscribe&confirmation=a803d34e7ea924f9a4142bac00763549&address=stefano@bicar.it
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