A New, Faint Population of X-ray Transients

Abstract: We report on the detection of a remarkable new fast high-energy transient found in the Chandra Deep Field-South, robustly associated with a faint ($m_{\rm R}=27.5$ mag, $z_{\rm ph}$$\sim$2.2) host in the CANDELS survey. The X-ray event is comprised of 115$^{+12}_{-11}$ net 0.3-7.0 keV counts, with a light curve characterised by a $\approx$100 s rise time, a peak 0.3-10 keV flux of $\approx$5$\times$10$^{-12}$ erg s$^{-1}$ cm$^{-2}$, and a power-law decay time slope of $-1.53\pm0.27$. The average spectral slope is $\Gamma=1.43^{+0.23}_{-0.13}$, with no clear spectral variations. The \hbox{X-ray} and multi-wavelength properties effectively rule out the vast majority of previously observed high-energy transients. A few theoretical possibilities remain: an "orphan" X-ray afterglow from an off-axis short-duration Gamma-ray Burst (GRB) with weak optical emission; a low-luminosity GRB at high redshift with no prompt emission below $\sim$20 keV rest-frame; or a highly beamed Tidal Disruption Event (TDE) involving an intermediate-mass black hole and a white dwarf with little variability. However, none of the above scenarios can completely explain all observed properties. Although large uncertainties exist, the implied rate of such events is comparable to those of orphan and low-luminosity GRBs as well as rare TDEs, implying the discovery of an untapped regime for a known transient class, or a new type of variable phenomena whose nature remains to be determined.

• The paper describes the discovery of CDF-S XT1, a new ultra-faint and rapid X-ray transient event found in deep Chandra observations, suggesting the existence of a novel transient population.
• CDF-S XT1 shows a rapid ~100-second rise to peak brightness followed by a power-law decay with a slope of approximately -1.5, distinguishing it from typical X-ray transients.
• Potential explanations explored for this transient class include off-axis short GRB afterglows or low-luminosity high-redshift GRBs, emphasizing the need for future sensitive X-ray surveys to fully understand this new population.

A New, Faint Population of X-ray Transients

The research paper presented investigates an intriguing X-ray transient phenomenon detected in the Chandra Deep Field-South. The event, named CDF-S XT1, represents a category of ultra-faint and rapid X-ray transients that offer a new perspective into high-energy transient astrophysical events. This discovery was facilitated by deep observations using the Chandra X-ray Observatory, revealing a sudden and ephemeral increase in X-ray brightness followed by a power-law decay over several kiloseconds.

Key Observational Features

CDF-S XT1 was identified in the 0.3-7.0 keV range with a rapid 100-second rise to a peak flux, followed by a decay characterized by a power-law slope of approximately -1.5. The spectral analysis indicated an average photon index of Γ = 1.43. These properties differentiate CDF-S XT1 from the majority of previously known X-ray transients, which typically arise from accretion processes or other high-energy phenomena involving compact objects.

Potential Theoretical Explanations

The paper explores several possible mechanisms to explain the origin of CDF-S XT1:

1. Off-axis Short-Duration Gamma-ray Burst (GRB): This possibility involves observing the afterglow of a GRB that is not directly aligned with the observer's line of sight. Such scenarios predict weaker emissions at X-ray and other wavelengths, consistent with the observed faintness and lack of broader spectral features in CDF-S XT1.
2. Low-Luminosity GRB in High-Redshift: Another proposal is that CDF-S XT1 might be a GRB occurring at a substantial cosmological distance, with energies falling below typical GRB detection thresholds.
3. Tidal Disruption Event (TDE): The possibility of a TDE, where a star is disrupted by the gravitational pull of a black hole, particularly in scenarios involving less massive black holes and white dwarfs. However, the lack of significant variability and the hard X-ray spectrum present challenges to this model.

Implications and Future Directions

The study highlights the fact that CDF-S XT1 represents an untapped domain within the transient sky, either extending the parameter space of known phenomena or introducing a novel transient class. The potential rate of such events, based on the single observed incident, aligns with occurrences typical in orphan GRBs and rare TDEs but significantly rarer than core-collapse supernovae.

This discovery emphasizes the necessity for continued monitoring of deep fields using X-ray telescopes and aids in refining models of transient phenomena by providing new constraints and data points. Future X-ray observatories planned for the upcoming decade, with enhanced sensitivity and field-of-view, such as the Athena and Lynx missions, promise to capture more such events, facilitating a deeper understanding of this faint transient population. As observational capabilities improve, further study of such transients may also contribute to gravitational wave research, linking electromagnetic and gravitational wave observations in multi-messenger astrophysics.