arXiv subjects

Run-Chao Chen

Publications and source records attributed to Run-Chao Chen.

10 recordsLinked to original sources

arXiv2026

Short Spike, Long Story: Episode-Dependent Shifts of Long-Duration Type-I GRBs on $E_{\rm p,z}$--$E_{\rm iso}$ Plane

Observations of peculiar GRBs have challenged the traditional $T_{90}$-based classification, demonstrating that duration does not map uniquely onto progenitor type. A striking class is long-duration Type~I GRBs -- merger-origin events whose prompt emission lasts far longer than the canonical 2 s boundary, typically comprising an initial short hard spike followed by softer extended emission. Identifying the physical origin of such bursts requires diagnostics beyond duration alone, among which the Amati relation, linking rest-frame spectral peak energy $E_{\rm p,z}$ and isotropic-equivalent energy $E_{\rm iso}$, is widely used as a complementary classification tool. We analyze a sample of eight long-duration Type~I GRBs and merger candidates by separating the initial spike from the extended emission and examining their episode-dependent locations on the $E_{\rm p,z}$--$E_{\rm iso}$ plane. We find that the initial spike generally lies within, or close to, the empirical Type~I region, consistent with a compact-merger-like prompt-emission component. In contrast, the extended-emission episode systematically occupies a region closer to Type~II GRBs, and could be misidentified as collapsar-like if analyzed in isolation. This episode-dependent Type~I-to-Type~II transition is further supported by time-resolved spectral analysis, although its magnitude and trajectory vary among bursts, suggesting diversity in central-engine evolution or outflow properties between the two phases. Our results caution that the Amati relation alone can lead to misleading empirical classification when the initial hard spike is weak, outside the instrumental bandpass, or missed entirely, leaving only the extended emission to be analyzed. Broad temporal and spectral coverage, and independent multi-wavelength diagnostics, is therefore essential for identifying the physical origin of these events.

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arXiv2026

A 4.5-s Quasiperiodic Spectral Oscillation in GRB 230307A: Evidence for Free Precession of a Post-Merger Magnetar?

Millisecond magnetars, rapidly rotating neutron stars with ultra-strong magnetic fields, have long been proposed as central engines of gamma-ray bursts (GRBs). For GRBs produced by neutron star mergers, the survival of a long-lived magnetar remnant remains uncertain, as the merger remnant may rapidly collapse into a black hole. In GRB 230307A, multiwavelength observations together with a previously reported 909-Hz periodic signal consistent with millisecond spin in its prompt emission provide strong evidence that such a post-merger magnetar may power the burst. Here we report the discovery of a quasiperiodic modulation with a characteristic period of 4.5 s in the spectral evolution of GRB 230307A, detected consistently across multiple gamma-ray instruments. The modulation is manifested as a coherent, energy-dependent variation of the spectral shape, with the strongest signature in the evolution of the peak energy. Within the magnetar-engine framework, such a low-frequency modulation can be interpreted as a manifestation of large-scale periodic variations associated with the central engine. If interpreted in terms of free precession, the observed timescale implies a stellar ellipticity of $\epsilon \gtrsim 2.4 \times 10^{-4}$, corresponding to an internal magnetic field strength of $B_t \gtrsim 1.6 \times 10^{16}$ G, alongside a dipole field of $B_p \approx 5.6 \times 10^{15}$ G inferred from the early X-ray emission. These results suggest that such systems may provide potential sources of post-merger gravitational waves (GWs), motivating targeted searches following GRB triggers.

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arXiv2026

Detection of a Millisecond Periodicity in BATSE Short Gamma-Ray Bursts

Coherent oscillations at kilohertz frequencies have recently been detected in a small number of gamma-ray bursts (GRBs), suggesting quasi-periodic dynamics in their central engines. A prominent example is GRB~230307A, which exhibited a brief, highly coherent, energy-dependent periodic signal interpreted as the possible spin signature of a nascent millisecond magnetar formed after a compact binary merger. Motivated by these developments, we conducted a comprehensive search for similar signals, accounting for both temporal and spectral dependencies, in 532 short GRBs with time-tagged event data recorded by the Burst and Transient Source Experiment (BATSE) onboard the \textit{Compton Gamma-Ray Observatory}. Within this sample, we identify a single statistically significant case: GRB~960616 (BATSE trigger~5502), in which the $\sim$30~ms main emission episode is coherently modulated at 1100~Hz, with the strongest modulation above 320~keV and a fractional amplitude of $\sim$47\%. Assuming the presence of a coherent periodic modulation, we use data-driven Monte Carlo simulations to place an upper limit of $\sim$8\% on the fraction of the total radiated energy that can be modulated by the QPO. This event, exhibiting a periodicity at $\sim$0.91~ms, further supports the possibility that millisecond periodicities can arise during GRBs in merger-driven scenarios.

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arXiv2025

Fast-Cooling Synchrotron Prompt Emission from Internal Shocks in GRB 241030A

We present a time-resolved, joint Swift-Fermi spectral study of GRB 241030A (z=1.411) that cleanly isolates the synchrotron origin of its prompt emission and favors a matter-dominated, internal-shock scenario. The light curve shows two episodes separated by a quiescent gap. Episode I (0-45 s) is well described by a single power law with photon index $\simeq -3/2$, consistent with the fast-cooling synchrotron slope below the peak. Episode II (100-200 s), exhibits two robust spectral breaks: a low-energy break at $E_{b}$$\sim$$2-3$ keV that remains nearly constant in time, and a spectral peak $E_{p}$ that tracks the flux within pulses but steps down between them. The photon indices below and above $E_{b}$ cluster around -2/3 and -3/2, respectively, as expected for fast-cooling synchrotron emission. The burst displays an unusually small (consistent with zero) spectral lag across GBM bands. At later times ($\geq 230$ s), the spectrum softens toward $\sim-2.7$, as expected when the observing band lies above both $\nu_m$ and $\nu_c$. These behaviors are difficult to reconcile with a globally magnetized outflow with a decaying field, which naturally produces hard-to-soft Ep evolution, growing $\nu_c$, and appreciable lags. By contrast, internal shocks with a roughly steady effective magnetic field and a time-variable minimum electron Lorentz factor (equivalently, e.g., a varying fraction of accelerated electrons simultaneously account for (i) the stable $E_{b}$, (ii) the intensity-tracking yet step-down $E_{p}$, (iii) the canonical -2/3 and -3/2 slopes, and (iv) the near-zero lag.

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arXiv2025

Evidence for a brief appearance of gamma-ray periodicity after a compact star merger

The product of a compact star merger is usually hypothesized to be a hyperaccreting black hole, typically resulting in a gamma-ray burst (GRB) with a duration shorter than 2~s. However, recent observations of GRB~211211A and GRB~230307A, both arising from compact star mergers, challenge this model due to their minute-long durations. The data from both events are consistent with having a nascent, rapidly spinning highly magnetized neutron star (a millisecond magnetar) as the merger product and GRB engine, but a smoking gun signature is still missing. Here we report strong but not yet conclusive evidence for the detection of a 909-Hz gamma-ray periodic signal during a brief time window of GRB~230307A, which is consistent with the rotation frequency of such a millisecond magnetar. Notably, the periodic signal appeared for only 160~ms at an epoch coinciding with the transition epoch when the jet emission from the GRB central engine ceased and when the delayed emission from high latitudes started. If this signal is real, the temporal and spectral features of this gamma-ray periodicity can be consistently interpreted as asymmetric mini-jet emission from a dissipating Poynting-flux-dominated jet, as revealed by the energy-dependent light curve data of this burst.

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arXiv2025

Spectral Hardening Reveals Afterglow Emergence in Long-Duration Fast X-ray Transients: A Case Study of GRB 250404A/EP250404a

The prompt emission and afterglow phases of gamma-ray bursts (GRBs) have been extensively studied, yet the transition between these two phases remains inadequately characterized due to limited multiwavelength observational coverage. Among the recent growing samples of fast X-ray transients observed by Einstein Probe (EP), a subgroup of GRBs are captured with long-duration X-ray emission, potentially containing featured evolution from prompt emission to the afterglow phase. In this Letter, we present a detailed analysis of GRB 250404A/EP250404a, a bright fast X-ray transient detected simultaneously by EP and the Fermi Gamma-ray Burst Monitor in X-rays and gamma rays. Its continuous X-ray emission reveals a long-duration tail, accompanied by distinct spectral evolution manifested by the spectral index $\alpha_{\rm X}$ with an initial softening, followed by an evident hardening, eventually reaching a plateau at the value of $\sim$ -2. Early optical and near-infrared observations enable broadband modeling with forward- and reverse-shock components, confirming that the X-ray hardening signals the emergence of the external-shock afterglow. From this spectral hardening we infer that the prompt phase in soft X-rays lasted $\sim300\;\mathrm{s}$, which is more than 3 times longer than the gamma-ray $T_{90}$. This well-tracked soft-hard-flat spectral pattern provides a clear indication of afterglow emergence from the fading prompt emission and offers a practical criterion for identifying a distinct population of GRBs among fast X-ray transients, even when the detection of the gamma-ray counterpart or obvious temporal break is absent.

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arXiv2024

Repeated partial disruptions in a WD-NS or WD-BH merger modulate the prompt emission of long-duration merger-type GRBs

The progenitors of gamma-ray bursts (GRBs) have long been an unresolved issue. GRB 230307A stands out as an exceptionally bright event, belonging to the long-duration GRBs but also exhibiting a late emission component reminiscent of a kilonova. Together with the similar events GRBs 060614 and 211211A, they make up a new sub-group of GRBs with intriguing progenitors. If such long-duration merger-type GRBs originated from the coalescence of a white dwarf (WD) with a neutron star (NS) or a black hole (BH), as proposed in the recent literature, then the larger tidal disruption radius of the WD, together with a non-negligible residual orbital eccentricity, would make repeated partial tidal disruptions inevitable. This may modulate the mass accretion and jet launching process at the NS or BH, resulting in a quasi-periodic modulation (QPM) in the light curve of the GRB, on the orbital period. The detection of potential QPMs during the early episode of prompt emission of these three GRBs supports this scenario, and the relatively slow QPM ($>$ 1 s) suggests that the lighter object can not be a NS. We propose that the progenitor system of GRBs 230307A, 060614, and 211211A consist of a WD of mass 1.3 $M_\odot$, 0.9 $M_\odot$ and 1.4 $M_\odot$, respectively, and a NS (or BH). After several cycles of modulations, the WD is completely destructed, and the accretion of the remaining debris dominates the extended emission episode.

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arXiv2024

Triggering the Untriggered: The First Einstein Probe-Detected Gamma-Ray Burst 240219A and Its Implications

The Einstein Probe (EP) achieved its first detection and localization of a bright X-ray flare, EP240219a, on 2024 February 19, during its commissioning phase. Subsequent targeted searches triggered by the EP240219a alert identified a faint, untriggered gamma-ray burst (GRB) in the archived data of Fermi Gamma-ray Burst Monitor (GBM), Swift Burst Alert Telescope (BAT), and Insight-HXMT/HE. The EP Wide-field X-ray Telescope (WXT) light curve reveals a long duration of approximately 160 s with a slow decay, whereas the Fermi/GBM light curve shows a total duration of approximately 70 s. The peak in the Fermi/GBM light curve occurs slightly later with respect to the peak seen in the EP/WXT light curve. Our spectral analysis shows that a single cutoff power-law (PL) model effectively describes the joint EP/WXT--Fermi/GBM spectra in general, indicating coherent broad emission typical of GRBs. The model yielded a photon index of $\sim -1.70 \pm 0.05$ and a peak energy of $\sim 257 \pm 134$ keV. After detection of GRB 240219A, long-term observations identified several candidates in optical and radio wavelengths, none of which was confirmed as the afterglow counterpart during subsequent optical and near-infrared follow-ups. The analysis of GRB 240219A classifies it as an X-ray rich GRB (XRR) with a high peak energy, presenting both challenges and opportunities for studying the physical origins of X-ray flashes, XRRs, and classical GRBs. Furthermore, linking the cutoff PL component to nonthermal synchrotron radiation suggests that the burst is driven by a Poynting flux-dominated outflow.

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arXiv2023

A Comptonized Fireball Bubble Fits the Second Extragalactic Magnetar Giant Flare GRB 231115A

Magnetar giant flares (MGFs), originating from noncatastrophic magnetars, share noteworthy similarities with some short gamma-ray bursts (GRBs). However, understanding their detailed origin and radiation mechanisms remains challenging due to limited observations. The discovery of MGF GRB 231115A, the second extragalactic MGF located in the Cigar galaxy at a luminosity distance of $\sim 3.5$ Mpc, offers yet another significant opportunity for gaining insights into the aforementioned topics. This Letter explores its temporal properties and conducts a comprehensive analysis of both the time-integrated and time-resolved spectra through empirical and physical model fitting. Our results reveal certain properties of GRB 231115A that bear resemblances to GRB 200415A. We employ a Comptonized fireball bubble model, in which the Compton cloud, formed by the magnetar wind with high density $e^{\pm}$, undergoes Compton scattering and inverse Compton scattering, resulting in reshaped thermal spectra from the expanding fireball at the photosphere radius. This leads to dynamic shifts in dominant emission features over time. Our model successfully fits the observed data, providing a constrained physical picture, such as a trapped fireball with a radius of $\sim 1.95 \times 10^{5}$ cm and a high local magnetic field of $2.5\times 10^{16}$ G. The derived peak energy and isotropic energy of the event further confirm the burst's MGF origin and its contribution to the MGF-GRB sample. We also discuss prospects for further gravitational wave detection associated with MGFs, given their high-event-rate density ($\sim 8\times 10^5\ \rm Gpc^{-3}\ yr^{-1}$) and ultrahigh local magnetic field.

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arXiv2023

Detection of Quasi-periodic Oscillations in SGR 150228213

The detection of quasi-periodic oscillations (QPOs) in magnetar giant flares (GFs) has brought a new perspective to study the mechanism of magnetar bursts. Due to the scarcity of GFs, searching QPOs from magnetar short bursts is reasonable. Here we report the detection of a high frequency QPO at approximately 110 Hz and a wide QPO at approximately 60 Hz in a short magnetar burst SGR 150228213, with a confidence level of 3.35$\sigma$. This burst was initially attributed to 4U 0142+61 by $Fermi$/GBM on location, but we haven't detected such QPOs in other bursts from this magnetar. We also found that there was a repeating fast radio burst associated with SGR 150228213 on location. Finally, we discuss the possible origins of SGR 150228213.

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