Optical brightening of MAXI J1348-630

ATel #12439; David M. Russell, Cristina Maria Baglio (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 28 Jan 2019; 05:43 UT
Credential Certification: David M. Russell (dave.russell5@gmail.com)

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On 2019 January 26 at 03:16 UT, MAXI/GSC detected a new X-ray transient, named MAXI J1348-630 (ATel #12425). The source brightened and triggered the Swift/BAT instrument on three occasions within 24 hours of the MAXI discovery (GCN #23795, #23796, #23797) and triggered INTEGRAL on numerous occasions on January 27 (GCN #23799). 13 hours after the MAXI discovery, the optical counterpart was identified by a wide-field search by iTelescope.Net T31 0.51-m telescope at Siding Spring, Australia (ATel #12430). This counterpart was confirmed by a localization by Swift/XRT and detection by Swift/UVOT (ATel #12434). Here we report on optical observations with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes, starting on January 26 at 06:15 UT, just 3 hours after the initial MAXI discovery.

On January 26 at 06:15-06:20 UT, two 60 sec exposure images in SDSS i' filter were acquired with a 1-m LCO telescope at Cerro Tololo, Chile. On January 26 and 27, further images were taken (200 sec exposures in SDSS g', i' and PanSTARRS y-bands) with the same 1-m at Cerro Tololo, and with the 2-m Faulkes Telescope South (FTS) and a 1-m LCO telescope at Siding Spring Observatory, Australia. We used four isolated, unsaturated nearby stars in the APASS catalogue to calibrate the magnitudes in g' and i', shown below.

MJD, i', +-(relative) +-(all:systematic), g', +-(relative) +-(all:systematic)
58509.260 15.385 0.006 0.059
58509.264 15.382 0.006 0.059
58509.582 15.289 0.002 0.059 16.769 0.004 0.028
58509.641 15.251 0.005 0.059 16.743 0.008 0.029
58510.333 15.043 0.003 0.059 16.525 0.006 0.027
58510.622 14.910 0.002 0.059 16.435 0.004 0.027

From the six i'-band images we measure a brightening of the source by 0.48 mag in 1.4 days. We checked 7 other nearby isolated stars (of i' mags within +- 1 mag of the transient) and none of them are variable to < 3 sigma. There is no pre-outburst detection of the source in USNO or NOMAD, to at least I > 17.7 (PanSTARRS and VPHAS do not cover this region), implying the transient has brightened by at least ~ 2.8 mag. This optical rise is typical of a low-mass X-ray binary (LMXB), less typical of a high-mass X-ray binary (HMXB). The tentative X-ray periodicity of 9.8s or 4.9s (ATel #12434) would be unusual for a LMXB.

The Galactic neutral hydrogen column density in this direction is n_H ~ (1.6-1.9)E22 cm^-2 (Kalberla et al. 2005, A&A, 440, 775; Dickey & Lockman, 1990, ARAA, 28, 215). The Swift/XRT spectrum was fitted (ATel #12434) with a value lower than Galactic, 7E21 cm^-2 (J. Kennea, private communication), suggesting the source is within the galaxy. This n_H implies an optical extinction of Av ~ 2.4 mag (Foight et al. 2016, ApJ, 826, 66). To help constrain the nature of the compact object, we de-reddened our optical magnitudes using Av = 2.4 and constructed the optical - X-ray luminosity diagram for LMXBs (see Russell et al. 2006, MNRAS, 371, 1334) (X-ray flux from ATel #12434). The distance to the source is unknown so we used several trial distances: 0.5, 1, 3, 5, 8 and 20 kpc. We found that for distances less than ~ 2-3 kpc, MAXI J1348-630 is in a region of the optical - X-ray diagram occupied only by neutron star LMXBs (see link below). For distances greater than 3 kpc it is consistent with black hole LMXBs. A HMXB nature is ruled out for distances less than ~ 8 kpc.

Radio, IR and multi-wavelength fast timing observations are encouraged to uncover the nature of the source. We thank Jamie Kennea, Klaas Wiersema and Daniele Malesani for helpful information regarding the Swift observations. 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 - X-ray diagram for MAXI J1348-630