EP241113a: dissipative photospheric emission from a dirty fireball

Abstract

EP241113a, a soft X-ray transient detected by the Einstein Probe (EP), has an isotropic-equivalent energy ($E_{\gamma ,\mathrm{iso}}\sim 10^{51}\mathrm{erg}$) comparable to classical gamma-ray bursts (GRBs) but an exceptionally low peak energy ($E_{\mathrm{p}}\lesssim 1\mathrm{keV}$), placing it off the canonical $E_{\mathrm{p}}$--$E_{\gamma ,\mathrm{iso}}$ (Amati) relation at $>3\sigma$ confidence. The afterglow of EP241113a is also unusual, with an extremely low plateau luminosity and no jet break up to the latest observation. Such properties have been interpreted as arising from an energetic ``dirty fireball,'' i.e., a relativistic jet with energy comparable to classical GRBs but much higher baryon loading, leading to a bulk Lorentz factor of $\Gamma \sim 20$. In this {\it Letter}, we propose that low $E_{\mathrm{p}}$ arises from dissipative photospheric emission in such a low-$\Gamma$ jet. As $\Gamma$ decreases, the photosphere shifts outward while the internal shock radius moves inward, causing dissipation to occur well below the photosphere and making the prompt emission photosphere-dominated. Meanwhile, the thermalization radius also moves outward, reducing the comoving radiation temperature where the spectral peak is established. Combined with weaker Lorentz boosting, these effects naturally shift the observed $E_{\mathrm{p}}$ into the soft X-ray band. We further suggest that such a low-$\Gamma$ jet may be powered by neutrino--antineutrino ($\nu \bar{\nu }$) annihilation in a hyperaccreting disk formed during the core collapse of a massive star. Compared with the Blandford--Znajek process, neutrino annihilation can produce a dirtier jet through baryon entrainment from a neutrino-driven wind. EP241113a-like events therefore offer new insight into the dissipation physics and launching mechanisms of jets from collapsing massive stars.

arXiv comment12 pages, 5 figures; published in ApJL

References in corpus (52)

• Collapsars - Gamma-Ray Bursts and Explosions in "Failed Supernovae"
• Gamma-Ray Bursts and the Fireball Model
• Gamma-Ray Bursts as the Death Throes of Massive Binary Stars
• Intrinsic spectra and energetics of BeppoSAX Gamma-Ray Bursts with known redshifts
• Efficient Generation of Jets from Magnetically Arrested Accretion on a Rapidly Spinning Black Hole
• Gamma-Ray Bursts
• Are Gamma-Ray Bursts in Star Forming Regions?
• The Physics of Gamma-Ray Bursts and Relativistic Jets
• The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-Ray Bursts
• Hyper-Accreting Black Holes and Gamma-Ray Bursts
• Nucleosynthesis in neutrino-driven winds: I. The physical conditions
• Dissipative Photosphere Models of Gamma-ray Bursts and X-ray Flashes
• Steep Slopes and Preferred Breaks in GRB Spectra: the Role of Photospheres and Comptonization
• The Ep,i - Eiso correlation in GRBs: updated observational status, re-analysis and main implications
• Relativistic Jets in Collapsars
• Efficient acceleration and radiation in Poynting flux powered GRB outflows
• Accretion Models of Gamma-Ray Bursts
• Collisional mechanism for GRB emission
• Neutrino trapping and accretion models for Gamma-Ray Bursts
• Neutrino-Cooled Accretion Disks around Spinning Black Hole
• The observable effects of a photospheric component on GRB's and XRF's prompt emission spectrum
• Total and Jet Blandford-Znajek Power in Presence of Accretion Disk
• The Blandford-Znajek Process as a Centeral Engine for a Gamma Ray Burst
• An analysis of gamma-ray burst spectral break models
• Neutrino heating near hyper-accreting black holes
• Hyper-accreting black hole as GRB central engine. I: Baryon loading in GRB jets
• Failed gamma-ray bursts and orphan afterglows
• Neutrino-dominated accretion flows as the central engine of gamma-ray bursts
• Prompt emission spectra from the photosphere of a GRB
• Fireballs Loading and the Blast Wave Model of Gamma Ray Bursts
• Spectral analysis of 35 GRBs/XRFs observed with HETE-2/FREGATE
• Structure and Luminosity of Neutrino-cooled Accretion Disks
• On the Conditions for Neutron-Rich Gamma-Ray Burst Outflows
• Photosphere emission from a hybrid relativistic outflow with arbitrary dimensionless entropy and magnetization in GRBs
• Regulation of the spectral peak in gamma-ray bursts
• The peak energy of dissipative GRB photospheres
• Instabilities in the time-dependent neutrino disc in Gamma-Ray Bursts
• Peak energy clustering and efficiency in compact objects
• Discovery of a Cosmological, Relativistic Outburst via its Rapidly Fading Optical Emission
• Global Properties of X-Ray Flashes and X-Ray-Rich Gamma-Ray Bursts Observed by Swift
• On thermalization in gamma-ray burst jets and the peak energies of photospheric spectra
• Radiative Transfer Models for Gamma-Ray Bursts
• Dirty Fireballs and Orphan Afterglows: A Tale of Two Transients
• A Toy Model for Blandford-Znajek Mechanism
• Photospheric Emission of Gamma-Ray Bursts
• Observational signatures of sub-photospheric radiation mediated shocks in the prompt phase of GRBs
• Cosmological Fast Optical Transients with the Zwicky Transient Facility: A Search for Dirty Fireballs
• Sub-Photospheric Emission from Relativistic Radiation Mediated Shocks in GRBs
• Prompt optical emission from residual collisions in GRB outflows
• A fast X-ray transient from a weak relativistic jet associated with a type Ic-BL supernova
• Radiation mediated shocks in gamma-ray bursts: Subshock photon production
• Observational characteristics of radiation-mediated shocks in photospheric gamma-ray burst emission