NuSTAR observation of state transition and Quasi-Regular Modulations in 4U 1630-47

ATel #18038; 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 12 Sep 2026; 14:32 UT
Credential Certification: Maxime Parra (maxime.parra@univ-grenoble-alpes.fr)

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We report on additional NuSTAR exposures 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) and its rise in the Hard Intermediate State (HIMS, Atel #18034).

In parallel, we note that since September ~9, the MAXI count rate has become comparable to the brightest periods in the 2012-2013 and 2022-2024 super-outbursts, a factor >2 above the peaks of normal outbursts. There is thus a significant probability that the source is starting a new 2 year long super-outburst.

The source was observed by NuSTAR for ~20ks on September 07 (05-15h UTC) and September 10 (13-01H UTC). The source was variable during both observations. We report statistical errors at a 90% c.i. but stress that these values are indicative, due to the variability and choice of spectral models.

The time-averaged spectrum of September 07 was a slightly brighter, slightly softer version of the HIMS observation of Atel #18034, with a significantly stronger iron line, slightly higher frequency type-C QPO, and a progressive ~30% flux variation over the course of the observation.

The time-averaged spectrum of September 10, on the other hand, is brighter by a factor ~4, and the source shows vastly different spectral-timing properties:
1. The spectrum is significantly softer, and now dominated by a gamma=2.6(+/-0.01) thermal comptonization component, with a covering fraction of 1 (>0.98), for a disk temperature of Tin=1.6 (+/-0.02) keV
2. The reflection component has almost completely disappeared, leaving a marginally significant iron line with an EW of 6 (+/-4) eV
3. The broad-band noise has decreased significantly, and the type-C QPO has disappeared

All these properties indicate that the source has transited to the Steep Powerlaw State / Very High State (SPL/VHS), as expected for this source following the rise in the HIMS. This observation matches the recent increase beyond 2.5 cts/s in MAXI.

With our empirical modeling of a comptonized disk and emission line, we derive an (averaged) absorbed flux of 1.5 (+/-0.1) e-8 erg/s/cm^2 in 3-10keV, and 2.7 (+/-0.1) e-8 ergs/cm^2/s in 3-79 keV.

Following Atel #18034, we derived 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, in deabsorbed Eddington units, assuming d=11.5kpc and Mbh=8 Msun.

The 2026 NuSTAR observations are shown with black crosses in both the soft HLD and hard HLD - the last observation matches the HLD position of previous SPL states.

While the fit of both observations remains poor (reduced C-stat/d.o.f.=1.3-1.6), this is likely due to intra-observation variability.

This is most notable for the observation of September 10, during which there is no significant long-term variation of the count rate, but a very pronounced, erratic intra-orbit variability. In particular, in two separate instances during a single NuSTAR orbit, the count rate drops by ~30%, and visibly starts oscillating with a period of ~20s. This lasts a few hundred seconds, before the source returns to the SPL. These are clearly indicative of strong Quasi-Regular Modulations (QRMs) / mHz QPOs, rarely seen across previous outbursts of this source (see e.g. Yang et al. 2022, Zhao et al. 2023).


Follow-up X-ray and multi-wavelength observations are encouraged.

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