Discovery of burst oscillations in the newly discovered millisecond X-ray pulsar SRGA J144459.2$-$604207

Abstract

Burst oscillations during thermonuclear X-ray bursts are powered by thermonuclear energy on the neutron star (NS) surface and typically occur close to the spin frequency of the NS. We performed a comprehensive timing analysis of all thermonuclear bursts from the newly discovered millisecond X-ray pulsar SRGA J144459.2$-$604207, observed with NICER, XMM-Newton, and NuSTAR during the 2024 outburst. A total of 39 bursts were detected, allowing for a detailed search for burst oscillations, which had not been previously observed from this source. We report the discovery of burst oscillations at 447.7$-$448.0 Hz from SRGA J144459.2$-$604207 using XMM-Newton and NuSTAR data, consistent with the spin frequency of the NS. The strongest burst oscillation in the XMM-Newton data occurred with a single-trial significance of $5.1σ$ and maximum $Z^2$ power of $\sim31$. In the NuSTAR data, the strongest oscillation signal has a significance of $5.2σ$ and maximum $Z^2$ power of $\sim32$. The folded pulse profile corresponding to the strongest signal in the 0.5-10 keV band of the XMM-Newton data shows a sinusoidal shape with a fractional rms amplitude of $\sim8.5\%$, while the measurements of the NuSTAR data (3-40 keV range) yield $\sim21\%$. These results represent the first detection of burst oscillations in SRGA J144459.2$-$604207. Additionally, we report the detection of 447.6 Hz oscillations occurring just before a burst onset observed with XMM-Newton. This marks only the second instance in which burst oscillations have been observed before the burst onset.

Figures (6)

• Figure 1: The 1-s binned light curves of SRGA J144459.2-604207 from the NuSTAR and XMM-Newton observations, showing several thermonuclear X-ray bursts. Each thermonuclear bursts are numbered as given in both panels.
• Figure 2: Left panels: Dynamic power density spectra of SRGA J144459.2-604207, overlaid with the XMM-Newton EPIC-PN and NuSTAR burst light curves in the top and bottom panels, respectively. Dynamic PDS computed with a 4-s sliding $Z^2$ window (0.25-s step) over a 3 Hz band around the spin frequency at 448 Hz with a resolution of 0.01 Hz. Contours span $Z^2=14-32$ in steps of 4. Cyan dashed lines mark the spin frequency (horizontal) and the time of maximum power (vertical). Middle panels: Power density spectra from 8-s and 6-s light-curve segments of the XMM-Newton EPIC-PN and NuSTAR bursts, respectively, centered on the maximum power. Right panels: The pulse profiles of SRGA J144459.2-604207, generated by using the burst oscillation frequencies of 448 Hz for XMM-Newton and 447.7 Hz for NuSTAR data, are shown along with the best-fit sinusoidal functions.
• Figure 3: The dynamic power spectrum and the best-fit pulse profiles for the segment are shown with maximum power before the burst for XMM-Newton TNB-12.
• Figure 4: Distribution of maximum $Z_1^2$ values from 50,000 Monte Carlo simulations of the burst. The black curve shows the posterior log-normal fit, while the red vertical line marks the observed maximum $Z_1^2$ from the XMM-Newton EPIC-PN TNB-3 burst (left) and NuSTAR TNB-22 (right).
• Figure 5: The dynamic power spectra and the best-fit pulse profiles for the segments (4-6 s duration) with maximum power across six thermonuclear bursts observed with NuSTAR are shown. The dynamic power density spectra of SRGA J144459.2-604207 are overlaid with the corresponding NuSTAR burst light curves. The dynamic PDS is generated from a 4-s sliding window using the $Z^2$ statistic with a step size of 0.25 s over a 3 Hz band centered around the spin frequency. A frequency resolution of 0.01 Hz is employed. Contours are plotted at $Z^2$ levels from 14 to 32 in steps of 4. The horizontal dashed line (cyan) denotes the measured spin frequency, and the vertical dashed line (cyan) denotes the time corresponding to maximum power.