Insight-HXMT long exposure hard X-ray observations of SRGA J144459.2-604207 enable accurate measurements of the spin and orbital parameters and the detection of pulsed emission up to 60 keV

ATel #16548; Zhaosheng Li (Xiangtan University), L. Kuiper (SRON), M. Falanga (ISSI, UniBE), R. X. Xu (PKU), Y. Y. Pan (Xiangtan University), S. N. Zhang (IHEP), S. Zhang (IHEP), J. L. Qu (IHEP), L. M. Song (IHEP), Y. P. Chen (IHEP), S. M. Jia (IHEP), X. B. Li (IHEP), S. J. Zheng (IHEP), Y. Huang (IHEP), J. Poutanen (University of Turku)
on 21 Mar 2024; 13:36 UT
Credential Certification: Zhaosheng Li (lizhaosheng@xtu.edu.cn)

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SRGA J144459.2-604207 is a new X-ray transient discovered by the SRG/ART-XC (ATel #16464), and confirmed as an accreting millisecond X-ray pulsar by NICER (ATel #16474, ATel #16480). Insight-HXMT carried out six target of opportunity (ToO) observations of SRGA J144459.2-604207 between 2024-02-23 UTC 05:20:21 and 2024-03-15 UTC 19:19:02. We performed a timing analysis of the HXMT data from the first four observations taken between MJD 60363.294-60371.705. The arrival times of the (cleaned) events from the Insight-HXMT Medium-Energy (ME; 5-35 keV; 150 ks exposure) and High-Energy (HE; 20-350 keV; 112.4 ks exposure) X-ray telescopes were solar system barycenter corrected using the JPL DE405 solar system ephemeris and adopting the ATCA (radio) position RA=221.245833 and Dec=-60.698917 (ATel #16511). Starting with the orbital parameters and constant spin frequency as given in Ray et al. (ATEL #16480), we applied a 4d-optimization scheme based on a SIMPLEX algorithm (see De Falco et al. 2017; Li et al. 2021 for the 3d-version for more details) by maximizing the Z^2-test statistics for 4 parameters. The resulting accurate orbital solution (baseline 8.41 days) is described by an orbital period Porb of 18803.6557(41) s, a projected semi-major axis ax*sin(i) of 0.650827(12) lt-s, a time of the ascending node Tasc of 60361.6412232(11) MJD (TDB), and a spin frequency of 447.87172074(4) Hz. Uncertainties are all given at 1 sigma confidence level. Next, we folded the barycentered event arrival times, taking into account the binary orbital motion using the improved orbital parameters, upon the accurate spin frequency to obtain pulse-phase distributions for different energy bands. We found the pulsations very prominently present in ME and HE. For the HE, we detected a significant pulsed signal (approximately sinusoidal) up to ~60 keV, with 15-30, 30-45 and 45-60 keV pulsed signal Z_1^2 significances of 26.7, 14.1 and 5.0 sigma, respectively. For the ME, significant pulsations are detected across the 5-28 keV band, with a 10-28 keV pulsed signal strength of 32.9 sigma. We also performed a timing analysis of the publicly available INTEGRAL Near Real Time (NRT) data from INTEGRAL revolutions 2747 (60 ks), 2748 (100.8 ks) and 2749 (90 ks) covering the time interval MJD 60364.36-60370.465, nicely overlapping the HXMT observation period. We clearly detected the pulsations (with a HXMT-consistent shape) in the INTEGRAL ISGRI data with a 15-45 keV Z_1^2 significance of 5.3 sigma. From 1 s binned light curves from ME, we found more than 40 type I X-ray bursts, with the recurrence time increase from ~1.5 hr to ~2.5 hr as the persistent flux decreasing. Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://www.hxmt.org.