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From: atel@astronomerstelegram.org
Subject: ATel  17164
Message-Id: <20250430070532.028E228804D4@server.astronomerstelegram.org>
Date: Wed, 30 Apr 2025 03:05:32 -0400 (EDT)

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Posted: Within the last 24 hours
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ATEL #17164							     ATEL #17164

Title:	4U 1630-47 in an unusually radio-bright state
Author:	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
		
Queries:	isabella.mariani@inaf.it
Posted:	29 Apr 2025; 16:42 UT
Subjects:Radio, Infra-Red, X-ray, Binary, Nova, Transient

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 <a href="https://ui.adsabs.harvard.edu/abs/2001A%26A...375..447A/abstract">Augusteijn
et al. (2001)</a>, 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.

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Password Certification: Sara Elisa Motta (sara.motta@physics.ox.ac.uk)
https://www.astronomerstelegram.org/?read=17164
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