[Paper Review] STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales from Microseconds to Years

Paul S. Ray, Zaven Arzoumanian+157 more|arXiv (Cornell University)|Mar 7, 2019

Astrophysics and Cosmic PhenomenaPhysics and Astronomy37 citations

TL;DR

STROBE-X is a probe-class mission concept delivering high-throughput X-ray timing and broadband spectroscopy with three instruments (XRCA, LAD, WFM) to study black hole spins, neutron star equation of state, and counterparts to gravitational-wave sources across microsecond to year timescales.

ABSTRACT

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science.

Motivation & Objective

• Measure spins and map inner accretion flows around black holes across the mass spectrum (stellar to SMBHs).
• Constrain the dense-matter equation of state by surveying rotation-powered and accreting neutron stars.
• Identify and study electromagnetic counterparts to gravitational-wave sources and other transients in a time-domain, multi-messenger context.

Proposed method

• Employ three instruments with large collecting area and high timing resolution: XRCA (0.2–12 keV, ~100 ns timing, 85–175 eV spectral resolution), LAD (2–30 keV, 200–300 eV resolution), and WFM (2–50 keV, wide field for triggers).
• Use X-ray reverberation mapping and high-frequency QPO analysis to measure black hole spins and probe corona geometry.
• Leverage an all-sky monitor to detect transients and trigger pointed observations, enabling rapid response and cross-messenger coordination.
• Rely on mature technologies (NICER-inspired XRCA, silicon drift detectors for LAD, microchannel plate collimators for WFM) and a Phase A study timeline targeting a 2031 launch.
• Apply timing-spectral techniques to distinguish absorption, reflection, and continuum components in accretion flows.

Experimental results

Research questions

• RQ1 What are the spin distributions of accreting stellar-mass and supermassive black holes, and how do they inform formation and growth scenarios?
• RQ2 What is the dense-matter equation of state as constrained by neutron star timing and spectroscopy across a broad sample of sources?
• RQ3 How can X-ray timing and reverberation mapping reveal the geometry of the corona and inner accretion disks around black holes?
• RQ4 What are the electromagnetic counterparts and precursors to gravitational-wave sources, and how can wide-field X-ray monitoring aid their discovery and characterization?
• RQ5 How does STROBE-X enable time-domain, multi-wavelength, and multi-messenger astrophysics across microseconds to years?

Key findings

• STROBE-X provides over an order of magnitude increase in effective area compared to NICER in the soft band and RXTE in the hard band, enabling simultaneous spectral and variability studies.
• X-ray reverberation mapping with STROBE-X will measure iron K lag times and constrain the inner accretion flow geometry and corona properties across stellar-mass and supermassive black holes.
• HFQPO detections and multiple spin-measurement techniques (continuum fitting, reflection/reverberation, HFQPOs) will yield cross-calibrated black hole spin constraints and reduce systematic uncertainties.
• The Wide Field Monitor (WFM) will detect transients and provide triggers for pointed observations, expanding the discovery space for tidal disruption events and multi-messenger counterparts.
• STROBE-X aims to measure spins and mass-radius relations for neutron stars, aiding the mapping of the equation of state for ultradense matter, and to identify counterparts to gravitational-wave sources.