SWIFT-BAT observations of the recently discovered magnetar SGR 0501+4516

We present results on the Soft Gamma Repeater (SGR) 0501+4516, discovered by the SWIFT Burst Alert Telescope (BAT) on 2008 August 22. More than 50 bursts were identified from this source, out of which 18 bursts had enough counts to carry out spectral analysis. We performed time-averaged spectral analysis on these 18 bursts using 8 models, among which the cut-off powerlaw and the two-blackbody models provided the best fit in the 15-150 keV energy range. The cut-off powerlaw model fit yields a mean photon index Gamma_{CPL} = 0.54+/-0.11 and a cut-off energy E_C = 19.1+/-1.8 keV for the bursts. The mean hard and soft blackbody temperatures are found to be kT_{BB_h} = 12.8+/-0.7 keV and kT_{BB_s} = 4.6+/-0.5 keV, respectively, and are anti-correlated with the square of the radii of the hard and soft emitting regions (R_{BB_h} and R_{BB_s}) as R_{BB_h}^2 $\propto$ kT^{-5.8} and R_{BB_s}^2 $\propto$ kT^{-2.7}, respectively. The soft and hard component temperatures with different indices support the idea of two distinct emitting regions with the hard component corresponding to a smaller radius and the soft component corresponding to a larger radius, which further corroborate the idea of the propagation of extraordinary (E) and ordinary (O) mode photons across the photosphere, as predicted in the magnetar model. We notice strong burst fluence-duration correlation as well as hardness ratio-duration and hardness ratio-fluence anti-correlations for the SGR 0501+4516 bursts. The burst fluences range from ~ 4.4\times10^{-9} ergs/cm^-2 to ~ 2.7\times10^{-6} ergs/cm^{-2}, consistent with those observed for typical short SGR bursts.

💡 Research Summary

This paper presents a comprehensive analysis of the soft gamma-ray burst activity from the newly discovered magnetar SGR 0501+4516, using data from the Burst Alert Telescope (BAT) aboard the Swift satellite. The source was initially detected on August 22, 2008, and exhibited intense bursting activity over the following days.

The authors identified more than 50 bursts, from which 18 with sufficient counts were selected for detailed temporal and spectral analysis. Data reduction followed standard procedures using HEASoft tools, including background-subtracted light curve extraction in multiple energy bands, manual determination of burst durations (T100, ranging from ~0.03 to 0.5 s), and generation of response matrices for spectral fitting restricted to the 15–150 keV band.

The core of the study involved time-averaged spectral fitting of the 18 bursts using eight different models: four single-component models (Power Law, Cut-off Power Law - CPL, Thermal Bremsstrahlung, Black Body - BB) and four double-component models that added a black body to each of the single models (BB+PL, BB+CPL, BB+Bremss, BB+BB). The statistical goodness-of-fit was evaluated by comparing the average reduced chi-squared values and performing F-tests.

The results indicated that among the single-component models, the Cut-off Power Law (CPL) provided the best fit, yielding a mean photon index Γ = 0.54 ± 0.11 and a mean cut-off energy E_c = 19.1 ± 1.8 keV. However, among all tested models, the two-blackbody model (BB+BB) emerged as statistically superior, providing a comparably good fit with greater physical interpretability. The two blackbody components had distinctly different mean temperatures: a hard component with kT_h = 12.8 ± 0.7 keV and a soft component with kT_s = 4.6 ± 0.5 keV.

A key finding was the strong anti-correlation between the square of the emitting region’s radius (R^2) and its temperature (kT), described by power-laws: R_BB_h^2 ∝ kT_h^(-5.8) for the hard component and R_BB_s^2 ∝ kT_s^(-2.7) for the soft component. The derived radii were approximately 0.9 D10 km for the hot component and 7.2 D10 km for the cool component (where D10 is the distance in units of 10 kpc). The different power-law indices for the two components strongly suggest they originate from two physically distinct emission regions.

The authors interpret these results within the framework of the magnetar model. In the extreme magnetic field (~10^14-15 G), photons propagate in two modes: ordinary (O-mode) and extraordinary (E-mode), which have different scattering cross-sections and photospheric depths. The analysis suggests the larger, cooler emitting region corresponds to O-mode photons, while the smaller, hotter region corresponds to E-mode photons. This observationally corroborates the theoretical “trapped fireball” scenario, where radiation is processed through a magnetically dominated photosphere.

Furthermore, the study investigated correlations between burst properties. A strong positive correlation was found between burst fluence and duration (T100). Conversely, the spectral hardness ratio showed anti-correlations with both burst duration and fluence, implying that longer, more energetic bursts tend to have softer spectra. The measured burst fluences ranged from ~4.4 × 10^-9 to ~2.7 × 10^-6 erg cm^-2, consistent with the typical range for short SGR bursts.

In conclusion, this work provides the first detailed spectral analysis of the early burst activity from SGR 0501+4516 using Swift-BAT. It demonstrates the efficacy of the two-blackbody model in describing the spectra and offers strong observational support for magnetar theory, specifically the predicted behavior of photon propagation in an ultra-strong magnetic field. The study deepens our understanding of the emission mechanisms and physical processes at work during magnetar outbursts.