The X-ray Polarization Probe mission concept

The X-ray Polarization Probe (XPP) is a second generation X-ray polarimeter following up on the Imaging X-ray Polarimetry Explorer (IXPE). The XPP will offer true broadband polarimetery over the wide 0.2-60 keV bandpass in addition to imaging polarimetry from 2-8 keV. The extended energy bandpass and improvements in sensitivity will enable the simultaneous measurement of the polarization of several emission components. These measurements will give qualitatively new information about how compact objects work, and will probe fundamental physics, i.e. strong-field quantum electrodynamics and strong gravity.Comment: submitted to Astrophysics Decadal Survey as a State of the Profession white pape

Investigation of Nonequilibrium Ionization Plasma during a Giant Flare of UX Arietis Triggered with MAXI and Observed with NICER

We detected a giant X-ray flare from the RS CVn–type binary star UX Ari using the Monitor of All-sky X-ray Image on 2020 August 17 and started a series of Neutron star Interior Composition Explorer observations 89 minutes later. For a week, the entire duration of the flare was covered with 32 snapshot observations including the rising phase. The X-ray luminosity reached 2 × 10 ^33 erg s ^−1 , and the entire energy release was ∼10 ^38 erg in the 0.5–8.0 keV band. X-ray spectra characterized by continuum emission with lines of Fe xxv He α and Fe xxvi Ly α were obtained. We found that the temperature peaks before the flux does, which suggests that the period of plasma formation in the magnetic flare loop was captured. Using the continuum information (temperature, flux, and their delay time), we estimated the flare loop size to be ∼3 × 10 ^11 cm and the peak electron density to be ∼4 × 10 ^10 cm ^−3 . Furthermore, using the line ratio of Fe xxv and Fe xxvi , we investigated any potential indications of deviation from collisional ionization equilibrium (CIE). The X-ray spectra were consistent with CIE plasma throughout the flare, but the possibility of an ionizing plasma away from CIE was not rejected in the flux rising phase

NICER observation of LS V+44 17 during the record high outburst of 2022-2023

The MAXI/GSC nova alert system detected X-ray brightening from the Be/X-ray binary pulsar LS V +44 17 (also known as BSD 24-491 and RX J0440.9+4431) on 2022 December 29 (ATel #15835). The X-ray flux from the source was seen to increase from December 24 (MJD 59937) and it is still increasing as detected in hard X-rays by Swift/BAT (15−50 keV), reaching a record high value of 572±43 mCrab on MJD 59948. MAXI/GSC (2−20 keV) flux has also been increasing since the trigger on December 29 and reached a maximum of nearly 230 mCrab on MJD 59948. LS V +44 17 was last seen in an outburst in April 2011.NICER observed the source on December 29 beginning at 19:01:00 UTC (MJD 59942.79) with 26.6 ks exposure and recorded an average (0.5−10 keV) count rate of ∼325 cnts/s. A strong pulsation with a pulse period of ∼204.185 s is evident in the NICER data, with a multi-peak shape including an absorption structure in the pulse profile. The fractional amplitude of the pulse profile is ~25 percent (0.5−10 keV).The NICER spectrum can be fitted with an absorbed cutoff power-law model together with a blackbody component. The best-fit results provide a photon index of −0.32±0.03 and a column density of (0.53±0.01) × 1022 cm-2 (chi-squared/d.o.f. = 1198/937). The blackbody temperature is found to be 0.33±0.04 keV. A strong iron emission line is also detected at 6.4 keV. There are some spectral residuals in the 1−2 keV range, which is accounted for by adding a Gaussian component near 1.5 keV. The absorbed source flux is 3.25 × 10-9 ergs/cm2/s in 0.5−10 keV. All reported uncertainties are 90% confidence limits.Further NICER observations of the source are underway. We encourage multi-wavelength observations of the source.NICER is a 0.2−12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.The latest X-ray light curves can be found on the following pagesSwift/BAT: https://swift.gsfc.nasa.gov/results/transients/weak/LSVp4417/MAXI: http://maxi.riken.jp/pubdata/v7.7l/J0440+445/index.htmlFermi/GBM: https://gammaray.nsstc.nasa.gov/gbm/science/pulsars/lightcurves/rxj0440.htmlBeXRB monitor page: http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_LSV+4417.htm

NICER detection of two X-ray bursts in the follow-up observations of Terzan 5 X-3

MAXI/GSC reported a new outburst from a source in Terzan 5 on 2023 February 27 at 17:26 UT (ATel #15917). NICER conducted observations scanning a grid of pointings centered on the MAXI coordinates, finding a source position R.A. = 267.0137 deg. and Dec. = -24.7658 deg. (J2000) consistent with being just 1 arcmin from the nominal center of the cluster. The position was later refined with Swift-XRT, suggesting the association with Terzan 5 X-3 (also known as Swift J174805.3-244637), a neutron star low-mass X-ray binary located in the globular cluster Terzan 5 (ATel #15919). Given the large number of X-ray sources in the field, Chandra observations could confirm the association of the current burster with Terzan 5 X-3.NICER collected ~7.3 ks of pointed observations starting on 2023 February 28 18:00:40 UT. The source lightcurve shows a gradually increasing count-rate, rising from ~160 cts/s up to ~230 cts/s in the 0.5 - 10 keV energy range; we note that the low starting count rate may partially result from the initially offset NICER pointing before the source coordinates were firmly established. The power spectrum in this band does not present significant periodic signals in the frequency range 0.005-2500 Hz. However, we detected a broad-band (0.1-20 Hz) noise component with fractional rms of ~20%.Two Type-I X-ray bursts from this source were detected on 2023 February 28 18:04:04 and 2023 March 1 06:46:26 UT. The tail of a likely third Type-I X-ray burst is partially detected starting from 2023 March 1 08:07:56 UT, suggesting a burst rate of one every ~40 minutes. Both Type-I bursts displayed a fast rise over a few seconds and exponential decay over ~100 seconds. The burst decay is significantly longer than the only previously observed burst from this source (Bahramian et al. 2014), likely implying different accretion rates or different elemental abundances in the accreted fuel. We searched for burst oscillations using a moving window of duration 2, 4, and 8 seconds and steps of 0.5 seconds in the frequency range 10-1000 Hz. We found no significant signals.We performed spectral analysis of the persistent emission with ~2.4 ks exposure (corresponding to the last four orbits of the dataset where no Type-I bursts are present) using an absorbed disk blackbody plus blackbody model in the 1-10 keV range. The inferred hydrogen column density is (2.2\pm0.1) \times 10^{22} cm^{-2} using the tbabs model and assuming ISM abundances (Wilms et al. 2001), a value similar to that reported by Bahramian et al. (2014). The best-fit temperatures of the disk blackbody and blackbody components are 0.95\pm0.05 and 1.94\pm0.04 keV, respectively. The derived unabsorbed flux in the 0.5-10 keV range is 2.61\times10^{-9} erg/s/cm^{-2}.Time-resolved spectroscopy of two Type-I bursts did not show any evidence of photospheric radius expansion. The bolometric peak fluxes of these bursts are estimated to be 1.27\pm0.09 and 2.00\pm0.37 \times 10^{-8} erg/s/cm^{-2}, corresponding to ~30% and ~50% Eddington luminosity (for a NS mass of 1.4 Msun and assuming a distance of 5.9 kpc for Terzan 5), respectively. The maximum blackbody temperatures are 2.05\pm0.08 and 2.17\pm0.22 keV, respectively, while the apparent emitting radius of the blackbody settles around 6 and 7 km for about 30 seconds in the cooling tail of both bursts.Further NICER observations are planned, and we encourage additional observations of this source with other facilities. NICER is a 0.2–12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA