Characterising the short-orbital period X-ray transient Swift J1910.2-0546

TL;DR

This study analyzes Swift J1910.2-0546 with time-series optical photometry and spectroscopy taken during its 2012 outburst and subsequent quiescence, complemented by late-epoch quiescent data. A coherent outburst modulation at $P\approx0.0941$ d ($2.26$ h) is identified and interpreted as an early superhump, implying the true orbital period may be near $P_{ m orb}\approx4.5$ h. Spectroscopic analyses yield a donor of spectral type M3–M3.5, a reddening of $E(B-V)=0.60\pm0.05$, a distance of $2.8$–$4.0$ kpc, and a donor velocity $K_2\approx230\pm17$ km s$^{-1}$ with a mass ratio $q\approx0.032\pm0.010$, enabling Monte Carlo constraints on the compact-object mass. Under two period scenarios, the inferred primary mass is $M_1\approx8$–$11\,M_\odot$ with a low inclination $i\approx13$–$18$°, favouring a black hole accretor, though a neutron-star alternative cannot be entirely excluded. The results highlight the value of phase-resolved spectroscopy in crowded fields and motivate further observations to refine the dynamical parameters.

Abstract

SwiftJ1910.2-0546 is a Galactic X-ray transient discovered during a bright outburst in 2012. We use time-series optical photometry and spectroscopy to estimate the orbital period, characterise the donor star, determine the interstellar extinction, distance, and system geometry, and constrain the component masses. Multi-site r-band and clear-filter light curves and WHT/ACAM spectra from the 2012 outburst are combined with time-series spectroscopy from GTC/OSIRIS and VLT/FORS2 in quiescence. Period searches are conducted using generalised Lomb-Scargle, phase-dispersion minimisation, and analysis-of-variance algorithms. Diffuse interstellar bands constrain E(B-V), while empirical correlations involving H$α$ yield estimates of K2, q, and i. We detect a double-humped modulation with a period of $0.0941\pm0.0007$d ($2.26\pm0.02$h) during the outburst. Its morphology is consistent with an early superhump, suggesting that the true orbital period may be slightly shorter than 4.52h. The H$α$ radial velocity curves do not yield a definitive orbital period. In quiescence, TiO bands indicate an M3-M3.5 donor contributing 70% of the red continuum. Diffuse interstellar bands give E(B-V)=$0.60\pm0.05$ and N_H=$(3.9\pm1.3)$x10$^{21}$cm$^{-2}$, placing the system at a distance of 2.8-4.0 kpc. The H$α$ line width in quiescence (FWHM_0 =$990\pm45$km/s), via a FWHM-K_2 calibration, provides an estimate of K_2, while its double peaked profile gives q and i. Adopting the resulting K_2=$230\pm17$km/s and q=$0.032\pm0.010$, and two orbital period scenarios (2.25 and 4.50h), Monte Carlo sampling returns a compact object mass M_1=8-11M_sun and an inclination i=13-18 deg for plausible donor masses (M_2=0.25-0.35M_sun). We favour an orbital period of 4.5h. Further phase-resolved spectroscopy and photometry during quiescence are needed to better determine its fundamental parameters.

Figures (10)

• Figure 1: VLT/FORS2 $I$-band acquisition image obtained during quiescence. J1910 is marked with a red cross at the centre of the 1.2 field. North is up, east is to the left.
• Figure 2: Evolution of the 2012 outburst. X-ray data are shown as a grey line, while the $R$-, $V$-, and $i'$-band magnitudes published by Saikia2023 are represented by red diamonds, green squares, and black circles, respectively. Time-series photometry obtained with the SMARTS 0.9-m and INT/WFC in the $r$-band is marked with blue triangles, whereas the time of the WHT/ACAM spectra are indicated by orange arrows. Clear-filter photometry from Fresno SRO is marked by vertical red lines.
• Figure 3: Average spectra of J1910 during outburst from the first (upper; blue) and second (lower; red) nights, obtained with ACAM after continuum normalisation. The Balmer series down to H$\delta$ is visible, exhibiting both emission and absorption components. Zoomed-in regions show the areas around H and H. He i and He ii lines are also present and identified in the spectra, along with several diffuse interstellar bands marked with orange bars.
• Figure 4: Top panel: $r$-band light curves from SMARTS 0.9-m and INT/WFC, expressed as flux ratios and magnitudes, relative to the comparison star. Middle panels: SMARTS light curve (top left) and INT/WFC light curves from the first three nights when J1910 maintained an almost constant flux level, after subtracting the nightly averages. The red dashed curve represents the best-fit sine wave with a period corresponding to the highest peak in the periodogram shown below. Bottom left panel: GLS periodogram of the four $r$-band light curves displayed in the middle panels (4, 9, 10, and 11 Jun 2012). The highest peak corresponds to a period of $0.0941$ d ($= 2.26$ h). Bottom middle and right panels: The four light curves folded on this period and twice that value. The pale blue points represent the unbinned data, while the purple points are binned data across 40 phase intervals. Zero phase corresponds to the HJD of the first data point, and a full cycle is repeated for continuity.
• Figure 5: Top panel: Clear-filter light curves from Fresno SRO on 27, 28 Jun, and 10, 20 Jul 2012. Middle panels: Zoomed-in view of the four light curves. The red dashed curve represents the best-fit sine wave with a period corresponding to the highest peak in the periodogram shown below. Bottom left panel: GLS periodogram of the four clear-filter light curves shown in the middle panels after subtraction of nightly averages. The highest peak corresponds to a period of $0.0948$ d ($2.275$ h). Bottom middle and right panels: The four light curves folded on this period and twice that value. The pale blue points represent the unbinned data, while the purple points are binned data across 40 phase intervals. Zero phase is the HJD of the first data point.