Optical re-brightening of MAXI J1348-630

ATel #12829; David M. Russell, Aisha Al Yazeedi, D. M. Bramich, Maria Cristina Baglio (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 1 Jun 2019; 16:00 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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The black hole candidate MAXI J1348-630 was discovered by the MAXI X-ray telescope on 2019 Jan 26 (ATel #12425). We have been monitoring the source since the start of the outburst with Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes (ATel #12439, #12491) in g', r', i' and y-bands.

In early February (MJD ~ 58517-58521), the source made a transition from the hard state to the soft state (ATel #12469, #12471, #12477). Since then the X-ray and optical fluxes have been fading (from MAXI, Swift BAT and our optical LCO light curves). The rate of optical decay during the soft state is ~0.013 mag/day in both g' and i' bands). A state transition from the soft state back to the hard state is visible around April 27-30 (MJD ~58600-58603) as a rise in the Swift BAT hard X-ray flux and a small rise and then fade of the soft X-ray flux from MAXI.

During this transition to the hard state, we do not detect any prominent optical rise that commonly signifies the return of the compact jet (like in e.g. MAXI J1820+070; ATel #12128; or GX 339-4; Buxton et al. 2012). Instead we observe the optical flux to continue to decline, more rapidly (~0.06-0.07 mag/day) during and after the transition to the hard state, from g' = 18.0, i' = 16.4 on April 28 (MJD 58601.8) to g' = 19.1, i' = 17.3 on May 13 (MJD 58616.5).

Finally, we detect an optical re-brightening in the last week. Between 2019 May 21 and May 27 (MJD 58624.6 - 58630.4) the source brightened by 1.1 and 0.8 mag in g' and i'-bands, respectively. The latest magnitudes on May 31 (MJD 58634.8) are g' = 17.60 +- 0.03, i' = 16.02 +- 0.06. The rise is strongest in the bluer filters, which suggests it is due to the accretion disc and not the compact jet. It is not clear if it has reached the peak of this re-brightening yet. Further multi-wavelength observations are encouraged during this re-brightening period. Below is a link to our LCO light curves.

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). The research reported was supported by Mohammed Bin Rashid Space Centre (MBRSC), Dubai, UAE, under Grant ID number 201701.SS.NYUAD. We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical LCO light curves of MAXI J1348-630