Multi-wavelength monitoring of MAXI J1957+032 from outburst to quiescence
ATel #17187; Giulia Illiano (INAF-OAB), Alessandro Papitto (INAF-OAR), Alessio Marino (ICE-CSIC), Arianna Miraval Zanon (ASI), Maria Cristina Baglio (INAF-OAB), Christian Malacaria (INAF-OAR), Kevin Alabarta, David M. Russell (NYU Abu Dhabi), Boris Sbarufatti, Chiara Salvaggio (INAF-OAB), Caterina Ballocco (INAF-OAR), D. A. H. Buckley (South African Astronomical Observatory/University of Cape Town)
on 16 May 2025; 15:35 UT
Credential Certification: Giulia Illiano (giulia.illiano@inaf.it)
Subjects: Optical, X-ray, Binary, Neutron Star, Transient, Pulsar
On 2025 May 6, the EP telescope detected a new X-ray outburst from the accreting millisecond pulsar MAXI J1957+032 (GCN #40375). The event was quickly confirmed in X-rays by MAXI/GCS and Swift/XRT (ATel #17170, #17172), and in the optical band by ATLAS (ATel #17171).
Following the initial Swift/XRT observation on May 8 (ATel #17172), high-cadence monitoring (~1 ks every ~6 hrs) was carried out in Windowed Timing mode. The source flux declined by ~2 orders of magnitude over three days. Consistent with previous outbursts (e.g., Sanna et al. 2022), MAXI J1957+032 returned to quiescence within a week, as confirmed by Swift/XRT observations in Photon Counting (PC) mode on May 15.
Swift/XRT spectra from May 8-11 were fitted with an absorbed power-law, yielding the unabsorbed 0.3-10 keV fluxes listed below. Due to limited statistics, May 10-11 data were combined. For the final observation in PC mode on May 15, we report a 3σ upper limit on the unabsorbed 0.3-10 keV flux of 9✕ 10-13erg cm-2 s-1 (luminosity of 3✕ 1033erg s-1 for 5 kpc; Ravi et al. 2017).
| Start Time (UTC) |
Exp. (ks) |
Unabs. Flux [erg cm-2 s-1] |
| 2025-05-08T03:50 | 1.2 | (4.0+/-0.1)✕10-11 |
| 2025-05-09T08:09 | 0.9 | (6+/-1)✕10-11 |
| 2025-05-09T13:55 | 1.9 | (5.3+0.8-0.6)✕10-11 |
| 2025-05-10T02:52 | 1.0 | (5+2-1)✕10-11 |
| 2025-05-10T09:07 | 0.9 | (1.9+1.1-0.4)✕10-11 |
| 2025-05-10T14:53 | 1.0 | (2+3-1)✕10-11 |
| 2025-05-10/11 | 2.1 | (6+32-3)✕10-12 |
| 2025-05-15T00:59 | 1.8 | <9✕10-13 (3σ) |
Note: At the lowest flux levels, the source count rate in WT mode is comparable to the background, which may explain the large uncertainties.
The system was also monitored in the optical band with the 1m telescopes of the Las Cumbres Observatory (LCO) network as part of ongoing monitoring of ~50 LMXBs (Lewis et al. 2008). All images are analyzed with the real-time data analysis pipeline 'X-ray Binary New Early Warning System' (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020 and ATel #13451 for details). Observations were performed with the g', r', and i' SDSS filters, starting on MJD 60803.10, ~1 day after the first X-ray detection.
The source was first detected with magnitudes: g'=19.75+/-0.02, r'=19.77+/-0.03, i'=19.48+/-0.05; it then reached maximum brightness on May 10 (MJD 60805): g'=19.43+/-0.03, r'=19.36+/-0.04, i'=19.26+/-0.07. After that date, the optical brightness decreased. The latest LCO observations on May 13 (MJD 60308.03) showed an optical magnitude of g'=20.30+/-0.19. At its peak, this outburst reached a brightness level consistent with the secondary peak of the previous outburst that occurred on July 2, 2022 (MJD 59762), when MAXI J1957+032 reached optical magnitudes of r'=19.50+/-0.08 and i'=19.09+/-0.23, but not as bright as the first peak (ATel #15448).
We thank the Swift team for promptly scheduling and performing observations. This material is based on work supported by Tamkeen under the NYU Abu Dhabi Research Institute grant CASS (Center for Astrophysics and Space Science).
