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In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.

In this paper we present the enhanced X-ray Timing and Polarimetry mission—eXTP. eXTP is a space science mission designed to study fundamental physics under extreme conditions of density, gravity and magnetism. The mission aims at determining the equation of state of matter at supra-nuclear density, measuring effects of QED, and understanding the dynamics of matter in strong-field gravity. In addition to investigating fundamental physics, eXTP will be a very powerful observatory for astrophysics that will provide observations of unprecedented quality on a variety of galactic and extragalactic objects. In particular, its wide field monitoring capabilities will be highly instrumental to detect the electro-magnetic counterparts of gravitational wave sources. The paper provides a detailed description of: (1) the technological and technical aspects, and the expected performance of the instruments of the scientific payload; (2) the elements and functions of the mission, from the spacecraft to the ground segment.

In this paper we describe the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to accretion flows in the strong field gravity regime around both stellar-mass and supermassive black-holes. eXTP has the unique capability of using advanced “spectral-timing-polarimetry” techniques to analyze the rapid variations with three orthogonal diagnostics of the flow and its geometry, yielding unprecedented insight into the inner accreting regions, the effects of strong field gravity on the material within them and the powerful outflows which are driven by the accretion process. X-spinmeasurements

Microquasars are stellar-mass black holes accreting matter from a companion star and ejecting plasma jets at almost the speed of light. They are analogues of quasars that contain supermassive black holes of 106to 1010solar masses. Accretion in microquasars varies on much shorter timescales than in quasars and occasionally produces exceptionally bright X-ray flares. How the flares are produced is unclear, as is the mechanism for launching the relativistic jets and their composition. An emission line near 511 kiloelectronvolts has long been sought in the emission spectrum of microquasars as evidence for the expected electron–positron plasma. Transient high-energy spectral features have been reported in two objects, but their positron interpretation remains contentious. Here we report observations of γ-ray emission from the microquasar V404 Cygni during a recent period of strong flaring activity. The emission spectrum around 511 kiloelectronvolts shows clear signatures of variable positron annihilation, which implies a high rate of positron production. This supports the earlier conjecture that microquasars may be the main sources of the electron–positron plasma responsible for the bright diffuse emission of annihilation γ-rays in the bulge region of our Galaxy. Additionally, microquasars could be the origin of the observed megaelectronvolt continuum excess in the inner Galaxy.

We report on 0.3-10 keV X-ray observations by the Chandra X-ray Observatory of the fields of 22 sources that were discovered as hard X-ray (20-100 keV) sources by the International Gamma-Ray Astrophysics Satellite ("IGR" sources). The observations were made during Chandra's 9th observing cycle, and their purpose is to localize the sources and to measure their soft X-ray spectra in order to determine the nature of the sources. We find very likely Chandra counterparts for 18 of the 22 sources. We discuss the implications for each source, considering previous results and new optical or infrared identifications, and we identify or suggest identifications for the nature of 16 of the sources. Two of the sources, IGR J14003-6326 and IGR J17448-3232, are extended on arcminute scales. We identify the former as a pulsar wind nebula with a surrounding supernova remnant (SNR) and the latter as an SNR. In the group of 242 IGR sources, there is only one other source that has previously been identified as an SNR. Seven of the sources are definite or candidate high-mass X-ray binaries (HMXBs). We confirm a previous identification of IGR J14331-6112 as an HMXB, and based on combinations of hard X-ray spectra, inferred distances and X-ray luminosities, and/or column density variations, we suggest that IGR J17404-3655, IGR J16287-5021, IGR J17354-3255, IGR J17507-2647, IGR J17586-2129, and IGR J13186-6257 are candidate HMXBs. Our results indicate or confirm that IGR J19267+1325, IGR J18173-2509, and IGR J18308-1232 are cataclysmic variables (CVs), and we suggest that IGR J15529-5029 may also be a CV. We confirm that IGR J14471-6414 is an active galactic nucleus (AGN), and we also suggest that IGR J19443+2117 and IGR J18485-0047 may be AGNs. Finally, we found Chandra counterparts for IGR J11098-6457 and IGR J18134-1636, but more information is required to determine the nature of these two sources.

The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is discovering hundreds of new hard X-ray sources, many of which remain unidentified. We report on optical spectroscopy of five such sources for which X-ray observations at lower energies ([?]0.5-10 keV) and higher angular resolutions than INTEGRAL have allowed for unique optical counterparts to be located. We find that INTEGRAL Gamma-Ray (IGR) J16426+6536 and IGR J22292+6647 are Type 1 Seyfert active galactic nuclei (with IGR J16426+6536 further classified as a Seyfert 1.5) which have redshifts of z = 0.323 and z = 0.113, respectively. IGR J18308-1232 is identified as a cataclysmic variable (CV), and we confirm a previous identification of IGR J19267+1325 as a magnetic CV. IGR J18214-1318 is identified as an obscured high-mass X-ray binary (HMXB), which are systems thought to have a compact object embedded in the stellar wind of a massive star. We combine Chandra fluxes with distances based on the optical observations to calculate X-ray luminosities of the HMXB and CVs, finding L 0.3-10 keV = 5 x 1036 erg s-1 for IGR J18214-1318, L 0.3-10 keV = 1.3 x 1032 erg s-1 for IGR J18308-1232, and L 0.3-10 keV = 6.7 x 1032 erg s-1 for IGR J19267+1325.

We report on a [?]12 hr XMM-Newton observation of the supergiant high-mass X-ray binary (HMXB) IGR J16207 - 5129. This is only the second soft X-ray (0.4-15 keV, in this case) study of the source since it was discovered by the International Gamma-Ray Astrophysics Laboratory satellite. The average energy spectrum is very similar to those of neutron star HMXBs, being dominated by a highly absorbed power-law component with a photon index of G = 1.15+0.07 -0.05. The spectrum also exhibits a soft excess below [?]2 keV and an iron Ka emission line at 6.39 +- 0.03 keV. For the primary power-law component, the column density is (1.19+0.06 -0.05) x 1023 cm-2, indicating local absorption, likely from the stellar wind, and placing IGR J16207 - 5129 in the category of obscured IGR HMXBs. The source exhibits a very high level of variability with an rms noise level of 64% +- 21% in the 10-4-0.05 Hz frequency range. Although the energy spectrum suggests that the system may harbor a neutron star, no pulsations are detected with a 90% confidence upper limit of [?]2% in a frequency range from [?]10-4 to 88 Hz. We discuss similarities between IGR J16207 - 5129 and other apparently nonpulsating HMXBs, including other IGR HMXBs as well as 4U 2206+54 and 4U 1700 - 377.

We report on the results of observations of hard X-ray sources in the Galactic plane with the Chandra X-Ray Observatory. The hard X-ray "IGR" sources were discovered by the INTEGRAL satellite, and the goals of the Chandra observations are to provide subarcsecond localizations to obtain optical and infrared counterparts and to provide constraints on their 0.3-10 keV spectra. We obtained relatively short, ~5 ks, observations for 20 IGR sources and find a bright Chandra source in INTEGRAL error circles in 12 cases. In 11 of these cases, a cross-correlation with optical and/or infrared source catalogs yields a counterpart, and the range of J-band magnitudes is 8.1-16.4. Also, in four cases, the Chandra X-ray spectra show evidence for absorbing material surrounding the compact object with a column density of local material in excess of 5 x 1022 cm[?]2. We confirm that IGR J00234+6141 is a cataclysmic variable and IGR J14515-5542 is an active galactic nucleus (AGN). We also confirm that IGR J06074+2205, IGR J10101-5654, IGR J11305-6256, and IGR J17200-3116 are high-mass X-ray binaries (HMXBs). Our results (along with follow-up optical spectroscopy reported elsewhere) indicate that IGR J11435-6109 is an HMXB and IGR J18259-0706 is an AGN. We find that IGR J09026-4812, IGR J18214-1318, and IGR J18325-0756 may be HMXBs. In cases where we do not find a Chandra counterpart, the flux upper limits place interesting constraints on the luminosities of black hole and neutron star X-ray transients in quiescence.

Hard X-ray imaging of the Galactic plane by the INTEGRAL satellite is uncovering large numbers of 20-100 keV "IGR" sources. We present results from Chandra, INTEGRAL, optical, and IR observations of four IGR sources: three sources in the Norma region of the Galaxy (IGR J16195-4945, IGR J16207-5129, and IGR J16167-4957) and one that is closer to the Galactic center (IGR J17195-4100). In all four cases, one relatively bright Chandra source is seen in the INTEGRAL error circle, and these are likely to be the soft X-ray counterparts of the IGR sources. They have hard 0.3-10 keV spectra with power-law photon indices of G = 0.5-1.1. While many previously studied IGR sources show high column densities (NH ~ 1023-1024 cm-2), only IGR J16195-4945 has a column density that could be as high as 1023 cm-2. Using optical and IR sky survey catalogs and our own photometry, we have obtained identifications for all four sources. The J-band magnitudes are in the range 14.9-10.4, and we have used the optical/IR spectral energy distributions (SEDs) to constrain the nature of the sources. Blackbody components with temperature lower limits of >9400 K for IGR J16195-4945 and >18,000 K for IGR J16207-5129 indicate that these are very likely high-mass X-ray binaries (HMXBs). However, for IGR J16167-4957 and IGR J17195-4100, low extinction and the SEDs indicate later spectral types for the putative companions, suggesting that these are not HMXBs.

The field containing the candidate High Mass X-ray Binary IGR J01363+6610 was observed by XMM-Newton on 2009 July 31 for 28 ks. A Be star was previously suggested as the possible counterpart of the INTEGRAL source, and although Chandra, during a 2007 observation, did not detect an X-ray source at the position of the Be star, we find a variable source (XMMU J013549.5+661243) with an average X-ray flux of 2 x 10-13 erg cm-2 s-1 (0.2-12 keV, unabsorbed) at this position with XMM-Newton. The spectrum of this source is consistent with a hard power law with a photon index of G = 1.4 +- 0.3 and a column density of N H = (1.5+0.7 -0.5) x 1022 cm-2 (90% confidence errors). These results, along with our optical investigation of other X-ray sources in the field, make the association with the Be star very likely, and the 2 kpc distance estimate for the Be star indicates an X-ray luminosity of 9.1 x 1031 erg s-1. This is lower than typical for a Be X-ray binary, and the upper limit on the luminosity was even lower (<1.4 x 1031 erg s-1 assuming the same spectral model) during the Chandra observation. We discuss possible implications of the very low quiescent luminosity for the physical properties of IGR J01363+6610.