NuSTAR follow-up of MAXI J1752-457

ATel #16910; Sean Pike (UCSD), Hitoshi Negoro (Nihon U.), Tatehiro Mihara (RIKEN), John Tomsick (SSL, UCB), Benjamin Coughenour (UVU), Amruta Jaodand (CfA), Joel Coley (Howard U.), Javier Garcia (GSFC), Brian Grefenstette (Caltech), Kristin Madsen (GSFC), Hiromasa Miyasaka (Caltech), John Paice (Durham U.), Aarran Shaw (Butler U.), Mutsumi Sugizaki (Kanazawa U.)
on 16 Nov 2024; 01:59 UT
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Credential Certification: Sean Pike (seannpike.astro@gmail.com)

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We present NuSTAR follow-up observations of MAXI J1752-457, which was detected by MAXI at 18:23 on November 9 2024 UT (ATel #16898). The source was first observed by NuSTAR on November 12, 2024, at 06:30 UTC with an exposure time of 7ks (OBSID 81010302001), and again on November 13, 2024, at 00:50 UTC for an exposure of about 26ks (OBSID 81001347001). Due to the small (<45 degrees) solar aspect angle of the source, about 80% of the exposure time was spent in Mode 6. Using the available Mode 1 data, we determined a source position of (R.A., Dec) = 268.269, -45.866. This position is offset from the Swift XRT position of EP240809a reported in ATel #16765 by about 9 arcsec. Given that this falls well within NuSTAR's PSF of 18 arcsec (FWHM), we conclude that EP240809a and MAXI J1752-457 are the same source. We also note that the Galactic coordinates of the source are (l, b) = (346.09, -9.85), placing it at an offset of 17 degrees from the Galactic Center. We performed a preliminary spectral analysis which included both the Mode 6 and Mode 1 data. We found that for both NuSTAR observations, the spectrum is best described by a combination of a power law and a single-temperature spherical blackbody (Cstat/d.o.f. = 359/310 for the second NuSTAR observation) or multi-temperature disk blackbody component (Cstat/d.o.f. = 358/310 for the second NuSTAR observation). We attempted to fit the spectra to a power law or a blackbody component alone, but neither of these models achieved an acceptable fit statistic compared to the combined blackbody + power law model. The spectral fits are not sensitive to absorption, so we froze the column density at NH = 10^21 cm^-2 based on HI observations. In the case of a spherical blackbody, we observe a temperature of kT=0.60 +/- 0.02 keV and a radius roughly consistent with the expected radius of a neutron star, and the power law component has a photon index of 3.8 +/- 0.4. In the case of a disk blackbody, we find an inner disk temperature of kT=0.75 +/- 0.02 keV and a photon index of 3.4 +/- 0.3. The blackbody temperature and power law index did not change significantly between the two observations. During the first NuSTAR observation, MAXI J1752-457 had a flux of (1.2 +/- 0.1)x10^-10 erg/cm^2/s (2-10 keV). The source therefore declined in flux by two orders of magnitude over the course of about 2.5 days between the first MAXI detection and the first NuSTAR observation. During the second NuSTAR observation, about 18 hours later, the flux had further decreased to a value of (6.5 +/- 0.3)x10^-11 erg/cm^2/s. The preferred spectral models and parameters remain consistent with the accreting compact object scenario proposed in ATel #16902, but further observations are encouraged to confirm the nature of the source.