4U 1556-60 as a very faint neutron star X-ray binary at 700 pc with an undetected radio jet

Abstract

4U 1556-60 is a low-mass X-ray binary that was discovered more than 50 years ago as a persistent X-ray source; however, very little was known about it. Recently, Gaia obtained a parallax for the optical counterpart that places 4U 1556-60 at a distance of only about 700 pc, making it one of the closest X-ray binaries known to date. This close distance drastically alters what was previously assumed about the source. We revisit 4U 1556-60 in light of the newly determined distance of 700 pc, reinterpreting its literature and presenting new X-ray and radio observations to better understand various characteristics of the system. We conclude that a scenario in which 4U 1556-60 is a candidate ultracompact neutron star X-ray binary at a distance of ~700 pc is able to explain the observed properties of the source. It resides at a persistent X-ray luminosity of ~2x10^34 erg/s, an unusual value for a typical X-ray binary, but similar to several ultracompact systems. The ratio of the X-ray to optical luminosity is very high, also suggesting a physically small accretion disk. The radio jet is undetected with a very deep upper limit of 3x10^25 erg/s, which is about 10^3 times fainter than the expected black hole jet correlation, strongly indicating a neutron star accretor. The X-ray spectrum is dominated by a power law, and the X-ray timing properties are also consistent with observations of other very low accretion rate X-ray binaries. No spin or orbital periodicity are found in the X-ray data. Future observations, especially to determine its orbital period, will further aid in understanding 4U 1556-60.

Figures (7)

• Figure 1: NICER light curve of 4U 1556--60 from 0.5-8 keV with 8 s cadence after excluding residual background flaring features. Flux errors are not plotted and are typically about 5%. The stability of the flux observed in our NICER data justifies combining all observations to analyze spectral and timing properties. The light curve begins at MJD 60050.80902778.
• Figure 2: Lomb-Scargle of the full 75 ks of NICER data with a light curve time resolution of 1 s. There are no statistical features of the Lomb-Scargle periodogram that are not biased by the light curve sampling, as shown by the NICER window function Lomb-Scargle for comparison. Thus, the NICER data also do not show a sign of an orbital period.
• Figure 3: X-ray spectrum of 4U 1556--60 from NICER from all observations combined. The spectrum is fit with a powerlaw + bbodyrad, and a gaussian for the iron line. Background models are also shown. Full parameters are in Table \ref{['tab:NICERspect']}.
• Figure 4: ATCA radio image of 4U 1556--60 at the combined frequency of 7.25 GHz. The black circle is centered on the Gaia optical counterpart for visual reference. The contours plotted are $3\sigma$ and $10\sigma$, where $\sigma$ is the background RMS value of $3.5\,\mu$Jy. The bright source in the lower left corner has a peak flux density of 174$\mu$Jy ($\sim50\sigma)$.
• Figure 5: Plot of X-ray binary radio and X-ray luminosities from arash_bahramian_2022_7059313, with our radio nondetection of 4U 1556--60 indicated with a red star and a line of constant $\mathrm{L_R/L_X}$ plotted for reference at other source distances. Three correlation lines are plotted for visual reference: solid blue for BHXBs; dash-dot magenta for NSXBs with $\beta=0.7$; and dashed orange for NSXBs with $\beta=1.4$. AMXPs are accreting millisecond pulsars with stronger magnetic fields than general NSXBs, and tMSPs are transitional millisecond pulsars whose radio emission during their XRB-like state is thought to not arise from a jet alone. The very low upper limit of 4U 1556--60's radio emission for its X-ray luminosity is strongly indicative of a neutron star accretor, as it is about $1000$ times fainter than expected for a black hole accretor. The previous deepest upper limit in radio luminosity for a NSXB, Cen X--4 in quiescence at $\sim1.2$ kpc vandenEijnden2022MNRAS.516.2641V, is also shown as a blue plus sign.