NuSTAR follow-up of 4U 1630-47 in the hard intermediate state

ATel #18034; Maxime Parra (Ehime University, Japan), Pierre-Olivier Petrucci (IPAG, France), Stefano Bianchi (Univ. Roma Tre, Italy), Floriane Cangemi (APC, France), Alessio Marino (ICE-CSIC, Spain), Poshak Gandhi (U. Southampton, UK), Megumi Shidatsu (Ehime University, Japan), et al.
on 7 Sep 2026; 18:27 UT
Credential Certification: Maxime Parra (maxime.parra@univ-grenoble-alpes.fr)

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We report on a follow-up NuSTAR exposure on the Transient Black Hole LMXB candidate 4U 1630-47, obtained as part of an XMM-NuSTAR large program, following the detection of the source by SVOM (Atel #18030). This is a frequently outbursting transient, with recent outbursts in 2020 (Atel #13564), 2021, 2022 to 2024 (Atel #15575), and 2025 (Atel #17139).

Following the trigger of a ToO XMM-NuSTAR large program, the source was observed by NuSTAR for 16ks on September 06 between 8h and 18h UTC. The source was relatively stable during the observation.

The time-averaged spectra show that the source is well detected in the entire NuSTAR band, with a count rate of ~105cts/s/FPM.
In the 3-79keV band, both spectra can be modeled at first order using a simple comptonized disk model with interstellar absorption fixed at 10^23 cm^-2: we derive a comptonization fraction cf=0.7(+/-0.03) at a 90% c.i., a photon index of Gamma=2.05(+/-0.01), and a weakly constrained kTe=15(+/-1) keV.

The spectra of the two FPMs present large residuals around 6.4 keV, which can be well reproduced at first order with a single, resolved gaussian emission line from iron fluorescence, with velocity (~1000km/s) comparable with NuSTAR's absolute energy calibration, a resolved width of 0.3(+/-0.05) keV, and an EW of 44(+-7) eV.

Following this addition, the fit is still imperfect (Chi~/d.o.f.~1.5), largely due to the lack of a consideration for the weak but clearly detectable Compton hump between 20 and 40 keV.

We note that while the disk seed, with kTin=1.2(+/-0.1), clearly improves the fit compared to a pure powerlaw spectrum, the high interstellar absorption makes it highly degenerate when fitting with more complex reflection models such as relxill.

Using the empirical thermal componization + line model, we obtain absorbed flux values of 3.10(+/-0.1) e-9 erg/s/cm^2 in 3-10keV, and 7.4 (+/-0.1) e-9 erg/s/cm^2 in the full 3-79keV band.

Finally, a visual inspection of the cross-spectrum between the two FPM modules confirms a strong type-C QPO feature around 3.5Hz.


We extracted additional unabsorbed flux measurements in 3-6, 6-10, and 15-50 keV, to compare them with the evolution of the source in long-term Hardness Luminosity Diagrams from Parra et al. 2025, computed in deabsorbed Eddington units assuming d=11.5kpc and Mbh=8 Msun.

The 2026 NuSTAR observation, shown with a black cross in both the soft HLD and hard HLD, is typical of a rising hard (intermediate) state, similar to those observed during previous outbursts in 2021 (Parra+25) and 2025 (Atel #17139) alongside compact radio jets.

Although the source was significantly fainter in this NuSTAR observation than during the short flare seen on September 05 with MAXI, this source only has one recorded failed outburst out of more than 10 outbursts in the last 25 years - this is thus likely to be simple stochastic variability. At current flux levels, the source is expected to transit to the SPL/VHS (Steep Powerlaw/Very High State), potentially through a Quasi-Regular Modulation dominated state, within few days to a week.

XMM and NuSTAR will monitor the source for more than 300/200ks during the next few weeks. Follow-up multi-wavelength observations are encouraged, particularly to constrain the jet evolution along the transition, and the evolution of line features beyond CCD resolution.

We thank the XMM and NuSTAR science teams for their prompt follow-up and scheduling efforts.