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2025ApJ...988L..34J - Astrophys. J., 988, L34 (2025/July-3)

EP240801a/XRF 240801B: An X-Ray Flash Detected by the Einstein Probe and the Implications of Its Multiband Afterglow.

JIANG S.-Q., XU D., VAN HOOF A.P.C., LEI W.-H., LIU Y., ZHOU H., CHEN Y., FU S.-Y., YANG J., LIU X., ZHU Z.-P., FILIPPENKO A.V., JONKER P.G., POZANENKO A.S., GAO H., WU X.-F., ZHANG B., LAMB G.P., DE PASQUALE M., KOBAYASHI S., BAUER F.E., SUN H., PUGLIESE G., AN J., D'ELIA V., FYNBO J.P.U., ZHENG W., CASTRO-TIRADO A.J., YIN Y.-H.I., ZOU Y.-C., DELLER A.T., PANKOV N.S., VOLNOVA A.A., MOSKVITIN A.S., SPIRIDONOVA O.I., OPARIN D.V., RUMYANTSEV V., BURKHONOV O.A., EGAMBERDIYEV S.A., KIM V., KRUGOV M., TATARNIKOV A.M., INASARIDZE R., LEVAN A.J., MALESANI D.B., RAVASIO M.E., QUIROLA-VASQUEZ J., VAN DALEN J.N.D., SANCHEZ-SIERRAS J., MATA SANCHEZ D., LITTLEFAIR S.P., CHACON J.A., TORRES M.A.P., CHRIMES A.A., SARIN N., MARTIN-CARRILLO A., DHILLON V., YANG Y., BRINK T.G., DAVIES R.L., YANG S., ARYAN A., CHEN T.-W., KONG A.K.H., LI W.-X., LI R.-Z., MAO J., PEREZ-GARCIA I., FERNANDEZ-GARCIA E.J., ANDREWS M., FARAH J., FAN Z., PADILLA GONZALEZ E., HOWELL D.A., HARTMANN D., HU J.-W., JAKOBSSON P., LI C.-K., LING Z.-X., McCULLY C., NEWSOME M., SCHNEIDER B., TINYANONT K.S., SUN N.-C., TERRERAN G., TANG Q.-W., WANG W.-X., XU J.-J., YUAN W.-M., ZHANG B.-B., ZHAO H.-S. and ZHANG J.

Abstract (from CDS):

We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on 2024 August 1 by the Einstein Probe (EP) satellite with a weak contemporaneous signal also detected by the Fermi Gamma-ray Burst Monitor (GBM). Optical spectroscopy of the afterglow, obtained by Gran Telescopio Canarias and Keck, identified the redshift of z = 1.6734. EP240801a exhibits a burst duration of 148 s in X-rays and 22.3 s in gamma rays, with X-rays leading by 80.61 s. Spectral lag analysis indicates that the gamma-ray signal arrived 8.3 s earlier than the X-rays. Joint spectral fitting of EP Wide-field X-ray Telescope and Fermi/GBM data yields an isotropic energy ${E}_{\gamma ,{\rm{iso}}}=(5.5{7}_{-0.50}^{+0.54})\times 1{0}^{51}\,{\rm{erg}}$, a peak energy ${E}_{{\rm{peak}}}=14.9{0}_{-4.71}^{+7.08}\,{\rm{keV}}$, and a fluence ratio S(25-50 keV)/S$(50\,\unicode{x02013}\,100\,{\rm{keV}})=1.6{7}_{-0.46}^{+0.74}$, classifying EP240801a as an X-ray flash (XRF). The host-galaxy continuum spectrum, inferred using Prospector, was used to correct its contribution for the observed outburst optical data. Unusual early R-band behavior and EP Follow-up X-ray Telescope observations suggest multiple components in the afterglow. Three models are considered: a two-component jet model, a forward-reverse shock model, and a forward shock model with energy injection. All three provide reasonable explanations. The two-component jet model and the energy injection model imply a relatively small initial energy and velocity of the jet in the line of sight, while the forward-reverse shock model remains typical. Under the two-component jet model, EP240801a may resemble GRB 221009A (BOAT) if the bright narrow beam is viewed on-axis. Therefore, EP240801a can be interpreted as an off-beam (narrow) jet or an intrinsically weak GRB jet. Our findings provide crucial clues for uncovering the origin of XRFs.

Abstract Copyright: © 2025. The Author(s). Published by the American Astronomical Society.

Journal keyword(s): Gamma-ray bursts - X-ray transient sources

Simbad objects: 10

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