The Case for a Low Mass Black Hole in the Low Mass X-ray Binary V1408 Aquilae (= 4U 1957+115)

Abstract

There are very few confirmed black holes with a mass that could be $\sim 4M_{\odot }$ and no neutron stars with masses greater than $\sim 2M_{\odot }$, creating a gap in the observed distribution of compact star masses. Some black holes with masses between 2 and $4M_{\odot }$ might be hiding among other X-ray sources, whose masses are difficult to measure. We present new high-speed optical photometry of the low-mass X-ray binary V1408 Aql (= 4U 1957+115), which is a persistent X-ray source thought to contain a black hole. The optical light curve of V1408~Aql shows a nearly sinusoidal modulation at the orbital period of the system superimposed on large night-to-night variations in mean intensity. We combined the new photometry with previously-published photometry to derive a more precise orbital period, $P=0.388893(3)$\ d, and to better define the orbital light curve and night-to-night variations. The orbital light curve agrees well with a model in which the modulation is caused entirely by the changing aspect of the heated face of the secondary star. The lack of eclipses rules out orbital inclinations greater than $65^{\circ }$. Our best models for the orbital light curve favor inclinations near $13^{\circ }$ and black hole masses near $3M_{\odot }$ with a 90\% upper bound of $6.2M_{\odot }$, and a lower bound of $2.0M_{\odot }$ imposed solely by the maximum mass of neutron stars. We favor a black hole primary over a neutron star primary based on evidence from the X-ray spectra, the high spin of the compact object, and the fact that a type I X-ray burst has not been observed for this system. Although uncertainties in the data and the models allow higher masses, possibly much higher masses, the compact star in V1408~Aql is a viable candidate for a black hole lying in the mass gap.

Accepted to ApJ

References in corpus (5)

• Shapiro delay measurement of a two solar mass neutron star
• A Massive Pulsar in a Compact Relativistic Binary
• Testing Accretion Disk Theory in Black Hole X-ray Binaries
• Disk Dominated States of 4U 1957+11: Chandra, XMM, and RXTE Observations of Ostensibly the Most Rapidly Spinning Galactic Black Hole
• Simultaneous optical and near-IR photometry of 4U 1957+115 - a missing secondary star

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• Demonstrating the likely neutron star nature of five M31 globular cluster sources with Swift-NuSTAR spectroscopy
• Variational order for forced Lagrangian systems
• Black Holes and Neutron Stars in Nearby Galaxies: Insights from NuSTAR
• The stringent upper limit on jet power in the persistent soft state source 4U~1957+11
• Selection Effects on the Orbital Period Distribution of Low Mass Black Hole X-ray Binaries
• NICER/NuSTAR Characterization of 4U 1957+11: A Near Maximally Spinning Black Hole Potentially in the Mass Gap
• Nitrogen X-ray absorption in the local ISM
• A Dependable Distance Estimator to Black Hole Low-Mass X-ray Binaries
• A Phenomenological Model for the Light Curve of three Quiescent Low-inclination Dwarf Novae and one Pre-Cataclysmic Variable
• Spectral characteristics of the black hole binary 4U 1957+115: A multi-mission perspective
• The orbital period and inclination of the neutron star X-ray transient MAXI J1807+132
• Broad-band spectral study of LMXB black hole candidate 4U 1957+11 with NuSTAR