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Register a new userThe Dynamic X-ray Sky of the Local Universe
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Alicia M. Soderberg
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Over the next decade, we can expect time domain astronomy to flourish at optical and radio wavelengths. In parallel with these efforts, a dedicated transient machine operating at higher energies (X-ray band through soft gamma-rays) is required to reveal the unique subset of events with variable emission predominantly visible above 100 eV. Here we focus on the transient phase space never yet sampled due to the lack of a sensitive, wide-field and triggering facility dedicated exclusively to catching high energy transients and enabling rapid coordinated multi-wavelength follow-up. We first describe the advancements in our understanding of known X-ray transients that can only be enabled through such a facility and then focus on the classes of transients theoretically predicted to be out of reach of current detection capabilities. Finally there is the exciting opportunity of revealing new classes of X-ray transients and unveiling their nature through coordinated follow-up observations at longer wavelengths.
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While the exceptional sensitivity of Chandra and XMM-Newton has resulted in revolutionary studies of the Galactic neighborhood in the soft (<10 keV) X-ray band, there are many open questions. We discuss these issues and how they would be addressed by very wide-area (> 100 sq. deg.) X-ray surveys.
This white paper is a summarising report of the Forum on monitoring the transient X-ray Universe in the multi-messenger era organized by the International Space Science Institute in Beijing (ISSI-BJ) on May 6-7, 2014. Time-domain astronomy will enter a golden era towards the end of this decade with the advent of major facilities across the electromagnetic spectrum and in the multi-messenger realms of gravitational wave and neutrino. In the soft X-ray regime, the novel micro-pore lobster-eye optics provides a promising technology to realise, for the first time, focusing X-ray optics for wide-angle monitors to achieve a good combination of sensitivity and wide field of view. In this context, Einstein Probe - a soft X-ray all-sky monitor - has been proposed and selected as a candidate mission of priority in the space science programme of the Chinese Academy of Sciences. This report summarises the most important science developments in this field towards 2020 and beyond and how to achieve them technologically, which were discussed at this brainstorming forum. It also introduces briefly the Einstein Probe mission, including its key science goals and mission definition, as well as some of the key technological issues.
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Opening up the dynamic infrared sky for systematic time-domain exploration would yield many scientific advances. Multi-messenger pursuits such as localizing gravitational waves from neutron star mergers and quantifying the nucleosynthetic yields require the infrared. Another multi-messenger endeavor that needs infrared surveyors is the study of the much-awaited supernova in our own Milky Way. Understanding shocks in novae, true rates of supernovae and stellar mergers are some other examples of stellar evolution and high energy physics wherein the answers are buried in the infrared. We discuss some of the challenges in the infrared and pathfinders to overcome them. We conclude with recommendations on both infrared discovery engines and infrared follow-up machines that would enable this field to flourish in the next decade.
Using the Chandra Source Catalog 2.0 and a newly compiled catalogue of galaxies in the local Universe, we deliver a census of ultraluminous X-ray source (ULX) populations in nearby galaxies. We find 629 ULX candidates in 309 galaxies with distance smaller than 40,Mpc. The foreground/background contamination is ${sim}20%$. The ULX populations in bona-fide star-forming galaxies scale on average with star-formation rate (SFR) and stellar mass ($M_star$) such that the number of ULXs per galaxy is $0.45^{+0.06}_{-0.09}timesfrac{rm SFR}{rm M_odot,yr^{-1}}{+}3.3^{+3.8}_{-3.2}timesfrac{M_star}{rm M_odot}$. The scaling depends strongly on the morphological type. This analysis shows that early spiral galaxies contain an additional population of ULXs that scales with $M_star$. We also confirm the strong anti-correlation of the ULX rate with the host galaxys metallicity. In the case of early-type galaxies we find that there is a non-linear dependence of the number of ULXs with $M_star$, which is interpreted as the result of star-formation history differences. Taking into account age and metallicity effects, we find that the predictions from X-ray binary population synthesis models are consistent with the observed ULX rates in early-type galaxies, as well as, spiral/irregular galaxies.
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