Detection of thermonuclear bursts and X-ray reflection in Aql X-1 with the NuSTAR

ATel #16829; Manoj Mandal (MCC), Sabyasachi Pal (MCC)
on 24 Sep 2024; 12:36 UT
Credential Certification: Manoj Mandal (manojmandal@mcconline.org.in)

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The neutron star low mass-X-ray binary Aquila X-1 (Aql X-1) went through an outburst as reported in September 2024 in multi-wavelength (ATel #16821, #16822, #16823, #16826). We proposed observations with Neil Gehrels Swift Observatory and NuSTAR for Aql X-1 to probe the broadband spectro-timing properties of the source during this outburst.

NuSTAR observed Aql X-1 on September 23, 2022, starting at 14:00:07 (MJD 60576.5) with a total exposure time of nearly 22 ks. We report the results from the preliminary analysis of the latest available NuSTAR data 91001338002V4. The NuSTAR net count rate was nearly 45 cts/s for each module. We detected two thermonuclear bursts, where the peak count rates of the bursts were ~990 cts/s and ~695 cts/s respectively (average from two modules). Both of the burst profiles show a sharp rise followed by a slow exponential decay. The duration of each burst was nearly 150 s and the peak-to-persistent ratios were ~14 and ~10 respectively for the two bursts.

The preliminary spectral analysis indicates that the continuum spectrum can be fitted by the thermal Comptonization model (nthcomp), or a cutoff power law (cutoffpl) along with the absorption component, tbabs. Independent of the choice of continuum, the spectra show a broad iron line nearly at 6.4 keV and a Compton hump above 20 keV, indicating the X-ray reflection feature. We further adopted the reflection model to probe the feature in detail and the spectra can be modelled with a combination of tbabs × relxillCp. The best-fit results provide the hydrogen absorption column density N_H = (2.0 ± 0.3) × 10²² cm^-2, a photon index of 1.86 ± 0.02, electron temperature (kT_e) of 22.6 ± 3.0 keV, disk inclination of 34.7 ± 10 deg and the inner disk radius of nearly 1.5 times of innermost stable circular orbit (ISCO), indicating a moderately truncated disk. The best-fit result provides the reduced chi-square (dof) = 1.01 (1605). The unabsorbed X-ray flux is estimated to be (4.4 ± 0.9) × 10^-9 erg/s/cm², in the energy band 3−79 keV. The reported errors in the spectral parameters represent 90% confidence intervals.

We thank the NuSTAR SOC team for making these observations possible.