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NICER and NUSTAR discovery of an extremely deep cyclotron resonant scattering feature in the Be/X-ray binary GRO J1750-27

ATel #15241; C. Malacaria (USRA), J. B. Coley (Howard University/CRESST/GSFC), L. Ducci (IAAT), F. Fuerst (Quasar SR for ESA/ESAC), G. K . Jaisawal (DTU Space), P. Kretschmar (ESA), K. Pottschmidt (CRESST/UMBC/GSFC), P. Pradhan (MIT Kavli Institute for Astrophysics and Space Research), J. Wilms (Remeis-Observatory, FAU Erlangen-Nuernberg)
on 25 Feb 2022; 00:34 UT
Credential Certification: Christian Malacaria (cmalacaria@usra.edu)

Subjects: X-ray, Neutron Star, Transient

Based on the outburst activity of the accreting pulsar in the Be/X-ray binary system GRO J1750-27 in 2021 (ATel #14930), we report the results of our initiated monitoring campaign with NICER (performed from September 24 until November 4, 2021) and from our 30 ks Discretionary Director's Time observation with NuSTAR (performed on September 27, 2021).
The source was clearly detected with both instruments, with an average count rate of about 42 c/s for FPMA and FPMB combined (4-60 keV), and about 50 c/s in the almost contemporaneous NICER (0.5-10 keV) observation.

The combined NICER and NuSTAR data show a hard spectrum with multiple features.
We have tested several phenomenological, semi-phenomenological and physical spectral models. The spectrum presents a combination of hard components and absorption features that require careful handling in order to be modeled consistently. Moreover, given that the NuSTAR observation is highly contaminated by ghost rays and stray light, we also coordinated with the NuSTAR calibration team to ensure a proper background and source events extraction.
The best-fit continuum spectrum results from an absorbed power-law with high-energy cutoff (highecut in XSPEC) plus a broad Gaussian emission component (gauss in XSPEC) to smooth the steep high-energy cutoff. On top of the continuum, we added an Iron Kα line at around 6.4 keV and a soft blackbody component (bbodyrad in XSPEC). A deep absorption feature (gabs in XSPEC) is present at about 43 keV, that we identify with a Cyclotron Resonant Scattering Feature, observed for the first time in this source. The presence of this feature proved to be independent of the underlying continuum mode used. The best-fit parameters are reported in the following (all reported errors are at 1σ c.l.).

For the continuum high-energy cutoff power-law: Column density NH = (7.7 +/- 0.4)x1022 cm-2, spectral photon index Γ = 1.51 +/- 0.05, HighECut = 19.8 +/- 0.4 keV, foldE = 15+/- 1 keV,
For the broad Gaussian emission component: Egauss = 6.5 +/- 0.4 keV, σgauss = 9.8 +/- 0.3 keV, normgauss = (9.7 +/- 0.7)x10-2.
For the cyclotron line: Ecyc = 43.4 +/- 0.5 keV, σcyc = 6.2 +/- 0.4 keV, Strengthcyc = 18 +/- 3.
For the soft blackbody component: kTBB = 0.129 +/- 0.007 keV, normBB = (4.1 +/- 2.0)x106.
The resulting χ2/d.o.f. is 2045/2030. The broadband (0.5-60 keV) energy flux is 2.25x10-9 erg/cm2/s. At a source distance of 18 kpc (Lutovinov et al. 2019, MNRAS, 485, 770L) this translates into a luminosity of about 8.7x1037 erg/s, i.e., almost at the edge of the Eddington luminosity.

We notice that the strength of the Gaussian absorption feature is one of the deepest ever observed in a cyclotron line and might reflect a multiple cyclotron line-forming region (similar to what observed in GX 301-2, see Fuerst et al. 2018, A&A, 20, A153). Further data analysis in this respect is ongoing.

We thank the NuSTAR and NICER teams for approving the DDT requests and for rapidly carrying out the observation.