4U 1630-47 in an unusually radio-bright state

ATel #17164; I. Mariani, S. E. Motta, M. C. Baglio, S. Campana, M. M. Messa (INAF-OAB), M. Del Santo (INAF-IASF Palermo), R. P. Fender (Oxford University), P. Casella (INAF-OAR), on behalf of the X-KAT collaboration
on 29 Apr 2025; 16:42 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)

Tweet

The black hole transient 4U 1630-47 started a new outburst on MJD 60768. Data from the Swift (XRT+BAT) instrument taken between the 6th and the 7th of April showed that the source was in the hard state, which has been rarely observed in this source (ATel #17139).

Following the X-ray detections, we initiated follow-up radio observations with the MeerKAT radio telescope as part of the X-KAT programme. We observed 4U1630-47 on April 12th, 2025 and between April 20th and 21st, 2025, with each observation lasting 15 minutes. Observations were carried out at a central frequency of 1.28 GHz (L-band), with a total bandwidth of 0.85 GHz. J1939-6342 was used for flux and bandpass calibration, J1726-5529 for complex gain calibration. We marginally detected the source on April 12th, with a flux density of 0.63 +/- 0.28 mJy at position RA, DEC (J2000) = 16:34:01.523 +/- 0.05, -47:23:35.2 +/- 0.7, i.e. consistent with the nominal position of 4U1630-47. On April 20th we detected 4U1630-47 at a significantly higher flux density of 26.4+/-0.5 mJy, the highest ever measured in radio for this system. The in-band radio spectral index is approximately flat, suggesting the presence of a compact rather than a transient jet.

As part of the SwiftKAT program, we triggered a weekly monitoring of the source based on Swift/XRT to obtain observations (quasi-)simultaneous with the MeerKAT ones. Further observations have been taken as part of other dedicated monitoring programmes. Preliminary analysis shows that the observed (absorbed) fluxes in the 2-10 keV energy range on April 12th and April 20th are approximately 9E-10 and 6E-09 erg/cm^2/s, respectively. With an N_H of the order of 1E+23 cm^(-2), these result in unabsorbed fluxes of 1.3E-09 and 1E-08 erg/cm^2/s, respectively. Detailed broadband (with XRT+BAT) spectral analysis is ongoing.

We obtained near-infrared (NIR; K-band, lambda~2.2 um) observations with the 60 cm REM telescope at La Silla Observatory on April 14th, 2025. Three sets of five 15-second exposures were averaged to subtract the sky background and increase the signal-to-noise. A source was detected at a position consistent with the MeerKAT coordinates of 4U1630-47, with a 2MASS-calibrated magnitude of K~13.6, matching the 2MASS catalogue value of K~13.5. The lack of variability suggests this is not the NIR counterpart of 4U1630-47, but likely the same source previously identified by Augusteijn et al. (2001), with a magnitude of K ~ 13.6 and located approximately 1.5 arcseconds from the X-ray binary.
We therefore estimate a 3-sigma upper limit of K>13.8 on the NIR counterpart of 4U 1630-47.

We will continue a weekly monitoring of 4U 1630-47 with the MeerKAT radio telescope and with Swift. We thank the Swift team and the MeerKAT operation team for their assistance and the quick planning of the observations.

X-KAT is a large MeerKAT open-time programme to observe X-ray binaries in the radio band, performing weekly monitoring of bright, active systems, with capacity for higher cadence observations, and in coordination with large X-ray and optical monitoring programmes. For further information on this programme, contact Rob Fender. The Swift follow-up of X-ray binaries is largely performed as part of the SwiftKAT program, which provides quasi-simultaneous X-ray coverage of the X-KAT targets.