[Paper Review] The Luminosity Phase Space of Galactic and Extragalactic X-ray Transients Out to Intermediate Redshifts

Ava Polzin, R. Margutti+9 more|arXiv (Cornell University)|Oct 2, 2022

Astrophysical Phenomena and ObservationsPhysics and Astronomy201 references2 citations

TL;DR

This paper compiles and analyzes the X-ray luminosity-phase space (DLPS) for low- to intermediate-redshift (z ≤ 1) transients across 0.3–10 keV, integrating light curves from 14 transient classes including XRBs, SNe, GRB afterglows, and ULXs. It identifies a critical observational gap at Lx = 10³⁴–10⁴² erg s⁻¹ and t = 10⁻⁴–0.1 days, defining this as a prime discovery phase space for future X-ray surveys and transient classification.

ABSTRACT

We present a detailed compilation and analysis of the X-ray phase space of low- to intermediateredshift transients that consolidates observed light curves (and theory where necessary) for a large variety of classes of transient/variable phenomena in the 0.3–10 keV band including gamma-ray burst afterglows, supernovae, supernova shocks interacting with the environment, tidal disruption events and active galactic nuclei, fast blue optical transients, cataclysmic variables, magnetar flares/outbursts and fast radio bursts, cool stellar flares, X-ray binary outbursts, and ultraluminous X-ray sources. Our over-arching goal is to offer a comprehensive resource for the examination of these ephemeral events, extending the X-ray duration-luminosity phase space (DLPS) to show luminosity evolution. We use existing observations (both targeted and serendipitous) to characterize the behavior of various transient/variable populations. Contextualizing transient signals in the larger DLPS serves two primary purposes: to identify areas of interest (i.e., regions in the parameter space where one would expect detections, but in which observations have historically been lacking) and to provide initial qualitative guidance in classifying newly discovered transient signals. We find that while the most luminous (largely extragalactic) and least luminous (largely Galactic) part of the phase space is well-populated at t > 0.1 days, intermediate luminosity phenomena (Lx = 1034 − 1042 erg s−1 ) represent a gap in the phase space. We thus identify Lx = 1034 − 1042 erg s−1 and t = 10−4 − 0.1 days as a key discovery phase space in transient X-ray astronomy.

Motivation & Objective

• To consolidate and analyze the X-ray duration-luminosity phase space (DLPS) for low- to intermediate-redshift (z ≤ 1) transients.
• To identify underpopulated regions in the DLPS where future observations could yield new discoveries.
• To provide a resource for classifying newly detected transients based on their phase space location.
• To extend the DLPS framework to include luminosity and time evolution, not just duration and luminosity.
• To guide future X-ray surveys by highlighting regions with high detection potential but sparse observational coverage.

Proposed method

• Compiled 1,062 X-ray light curves from 14 transient classes, including GRB afterglows, SNe, TDEs, XRBs, ULXs, and flares.
• Used both targeted and serendipitous observations to populate the DLPS in the 0.3–10 keV band.
• Mapped luminosity (Lx) and duration (t) for each transient, with redshifts ≤1 to ensure demographic representativeness.
• Classified sources into Galactic (low-luminosity) and extragalactic (high-luminosity) populations to assess phase space coverage.
• Identified gaps in the DLPS using visual and statistical analysis of light curve density across luminosity and timescale bins.
• Provided a public GitHub repository with light curve data and metadata for community use and follow-up.

Experimental results

Research questions

• RQ1 Which regions of the X-ray duration-luminosity phase space (DLPS) remain underpopulated despite high theoretical detection potential?
• RQ2 How does the distribution of X-ray transients vary across luminosity and timescale, and what does this reveal about underlying physical mechanisms?
• RQ3 Can the DLPS be used to guide future transient surveys by identifying optimal discovery windows for new transient classes?
• RQ4 What is the extent of observational coverage for intermediate-luminosity transients (Lx = 10³⁴–10⁴² erg s⁻¹) at early times (t = 10⁻⁴–0.1 days)?
• RQ5 To what extent can the DLPS support retroactive classification of transients from archival data?

Key findings

• The most luminous (extragalactic) and least luminous (Galactic) regions of the DLPS are well-populated for t > 0.1 days.
• A significant gap exists in the DLPS at intermediate luminosities (Lx = 10³⁴–10⁴² erg s⁻¹) and short timescales (t = 10⁻⁴–0.1 days).
• This intermediate-luminosity, early-time phase space is identified as a key discovery frontier for transient X-ray astronomy.
• The DLPS framework enables rapid initial classification of unknown transients based on their position in luminosity and timescale.
• The study provides a publicly available dataset of 1,062 light curves across 14 transient classes, supporting future follow-up and archival analysis.
• The phase space reveals that recurrent outbursts (e.g., in XRBs) and flares are underrepresented in current surveys, especially at high luminosities and short durations.