Optical fade in MAXI J1348-630 during transition towards the soft state

ATel #12491; Maria Cristina Baglio, David M. Russell, Daniel Bramich (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 10 Feb 2019; 17:52 UT
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The black hole candidate MAXI J1348-630 was discovered in outburst on 2019 January 26 using the MAXI satellite (ATel#12425). The optical counterpart was first detected thanks to a wide-field search made with the iTelescope.Net T31 0.51-m telescope located in Siding Spring, Australia (see ATel #12430), and then confirmed by the Swift/UVOT instrument (ATel#12434) and by observations taken with the Las Cumbres Observatory (LCO) network 2-m and 1-m telescopes (ATel#12439). The detection of a brightening radio counterpart was reported from observations performed with the Australia Telescope Compact Array (ATCA); the radio spectrum was consistent with a flat spectrum from a compact jet (ATel #12456). The radio detection, and strong X-ray variability detected by NICER (ATel #12447) support its identification of a new black hole XRB.

A softening of the X-ray spectrum has been reported over the last week, from observations performed with MAXI, INTEGRAL and Swift (ATel #12469, #12471, #12477), and a 0.7 Hz QPO was detected by Insight-HXMT (ATel #12470). This X-ray behaviour indicates that the source is undergoing a hard-to-soft state transition.

Here we report on the ongoing LCO network monitoring (1-m network and 2-m Faulkes Telescope) observations of the outburst of MAXI J1348-630 at optical frequencies (g',r',i',y bands), starting from January 26th (MJD 58509), three hours after the discovery. The details of the calibration are reported in ATel #12439.

The optical flux brightened from g' = 16.8, i' = 15.4 on January 26th to g' = 16.1, i' = 14.4 on February 5th (MJD 58519). The first sign of an optical fading during the ongoing hard/soft transition detected in the X-rays was observed on February 7th (MJD 58521), with a significant delay of the optical fade with respect to the X-ray softening that was reported to start on February 3rd (MJD 58517; ATel#12469). The fading is more significant at the lower frequencies (i.e. the y-band), but visible in all bands. In particular, the change in magnitude from February 5th to 9th is (0.63 +/- 0.08) mag in the y-band, against a fade of (0.46 +/- 0.08) mag and (0.31 +/- 0.04) mag in the i'- and g'-bands, respectively. The most recent magnitudes, on February 9th are g' = 16.4, i' = 14.9. A link to our light curve is below. This frequency-dependent fading of the optical fluxes indicates that there is likely a jet contribution in the optical to infrared which is decreasing while the X-ray spectrum evolves from hard to soft over the transition, as is often seen in black hole X-ray binaries.

According to the accretion-ejection coupling scenario for X-ray binaries, we are therefore expecting the emission of a radio flare in the next few days or weeks, before the source enters the soft state. Radio and IR observations are strongly encouraged to confirm this.

The LCO observations are part of an on-going monitoring campaign of ~ 40 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO).

Optical LCO light curves of MAXI J1348-630