GRB 240715A: Revealing Novel Intrinsic Mechanism by Different Individual Pulse

TL;DR

GRB 240715A presents a rare case of a short GRB with three prompt pulses analyzed via joint SVOM/GRM and Fermi/GBM data. The authors perform pulse-resolved temporal and spectral analyses, finding a large negative spectral lag in the first pulse and a positive lag in the third, with the two showing opposite energy evolution. Spectral modeling indicates the first-pulse lag arises from the evolution of the low-energy index $\alpha$ rather than $E_{\rm peak}$, consistent with electron cooling in a decaying magnetic field, while the third pulse is well described by a quasi-thermal multicolor blackbody compatible with photospheric emission. These results imply distinct emission mechanisms and central-engine states within a single GRB and demonstrate the power of joint, time-resolved analyses for constraining GRB radiation processes.

Abstract

The Space-based multiband astronomical Variable Objects Monitor (SVOM), detected its first short gamma-ray burst, GRB 240715A, in-flight, which was jointly observed by Fermi. Based on observational data of SVOM/GRM and Fermi/GBM, we perform a comprehensive temporal and spectral analysis for individual pulse in the prompt emission of this burst, and novel characteristics are revealed. Firstly, opposite evolutions of spectral lag are found in the first and third pulse of this burst. Second, the large negative lag of the first pulse is an outlier in short GRB sample, especially when the pulse duration is considered. Spectral analysis shows that the negative lag of the first pulse is caused by the evolution of spectrum index, and is irrelevant to Epeak, which is inconsistent with the previous study. The intrinsic mechanism is probably attributed to electron cooling in the decaying magnetic field, which leads to the continuous hardening of the spectrum index and results in negative lag. Furthermore, spectral analysis also shows that the third pulse is more likely to be described by a quasi-thermal spectrum, indicating the existence of photospheric emission. It is difficult to explain how the synchrotron radiation appears before photospheric emission in a single GRB and some assumptions are discussed.

Figures (5)

• Figure 1: Light curves and pulse fitting. a, Light curves of SVOM/GRM and FERMI/GBM with 5 ms bin width, and time-energy diagram of SVOM/GRM within the energy range from 10 keV to 1000 keV. b, Multi-wavelength light curves of SVOM/GRM with 10 ms bin width. c, The triple-pulse fitting of the light curves in each engergy band with triple-gaussian function. d, The time delay of the peak of each pulse. e, The width of each pulse in each energy band .
• Figure 2: a, Spectral lag of the each pulse of GRB 240715A. b, Spectral lag of the whole light curve of GRB 240715A. c, Spectral lags of each pulse for short GRBs from the GBM sample. The bars in different colors represent the lag of different pulses. Left panel shows the negative lags of pulses in GRBs, right panel shows the positive lags of pulses in GRBs. d, The burst duration versus burst lag (in absolute value). The GRB sample is from Bernardinilag2015GehrelsLAG2006Goldstein0817Xiaolag2022. e, The histogram of lags of pulse in short GRBs, and the lag of the first pulse of GRB 240715A is labeled. f, The pulse duration versus the ratio of pulse lag and pulse duration (in absolute value). The Type II GRB sample is from 2018LuLag
• Figure 3: Spectrum fitting resluts.a, Top panel: Light curve of GRM and the seven slices for the fine time-resolved spectrum were marked with different colors. The following three panels are respectively the photon index, Epeak and flux of each slice fitted with GRM plus GBM data. b, The $E_{p,z}$ and $E_{\rm iso}$ correlation diagram. The best-fit for Type II (gray points), Type I (blue points) GRBs and MGFs (yellow points) are plotted (solid lines) with the 1$\sigma$ boundary (dashed line) marked. The red dashed line represent the evolution of the $E_{p,z}$ and $E_{\rm iso}$ of GRB 240715A as the redshift changes from 0.001 to 3. The GRB sample is from Lan23, and the MGF sample is from 2020zhangMGF2024MereghettiMGFc, E$_{peak}$ distribution of short GRBs observed by Fermi/GBM and GRB 240715A. The red bar represents $E_{peak}$ and error of GRB 240715A. The $E_{peak}$ data reference to 2017luEp.
• Figure 4: Spectral fitting results and "death line" of photonsphere of the third pulse.a-d, The observed photon count spectrum and $F\nu$ energy spectrum of BAND model, BAND-Cut model, mBB model and synchrotron model, respectively. e, The corner plot of the posterior probability distributions of the parameters of mBB model. f, The so-called "photosphere death line” constraints. The red line is for a strict blackbody, and the blue line is for a relativistic multicolor blackbody outflow. The red dashed line represents the rest-frame peak energy $E_{p,z}$ and the observed isotropic $\gamma$-ray luminosity $L_{iso}$ of the third pulse as the redshift varies.
• Figure B.1: Simulation of the lag generation.a Simulated Gaussian light curves in defferent enrgy bands along with the $\alpha$ evolves from soft to hard. The parameters of $A = 60, \mu =0.25,\sigma =0.001, \alpha_{0} = -1.5, k_{\alpha} = 7.3 \,\ and\,\ t_{0} = 0.19s$ are adopted to be consistent with the obsevations. b, The lag versus energy of the simulated Gaussian light curves. c, Simulated FRED light curves in defferent energy bands along with the $\alpha$ evolves from soft to hard. The parameters of $A = 60, \tau_{\rm{r}}=0.2,\tau_{\rm{d}}=0.5, \alpha_{0} = -1.5, k_{\alpha} = 0.2 \,\ and\,\ t_{0} = 0$ are adopted. d, The lag versus energy of the simulated Gaussian light curves.