The 2021 outburst of the symbiotic recurrent nova RS Oph was monitored with the Neutron Star Interior Composition Explorer Mission (NICER) in the 0.2-12 keV range from day one after the optical ...maximum, until day 88, producing an unprecedented, detailed view of the outburst development. The X-ray flux preceding the supersoft X-ray phase peaked almost 5 days after optical maximum and originated only in shocked ejecta for 21 to 25 days. The emission was thermal; in the first 5 days only a non-collisional-ionization equilibrium model fits the spectrum, and a transition to equilibrium occurred between days 6 and 12. The ratio of peak X-rays flux measured in the NICER range to that measured with Fermi in the 60 MeV-500 GeV range was about 0.1, and the ratio to the peak flux measured with H.E.S.S. in the 250 GeV-2.5 TeV range was about 100. The central supersoft X-ray source (SSS), namely the shell hydrogen burning white dwarf (WD), became visible in the fourth week, initially with short flares. A huge increase in flux occurred on day 41, but the SSS flux remained variable. A quasi-periodic oscillation every ~35 s was always observed during the SSS phase, with variations in amplitude and a period drift that appeared to decrease in the end. The SSS has characteristics of a WD of mass >1 M(solar). Thermonuclear burning switched off shortly after day 75, earlier than in 2006 outburst. We discuss implications for the nova physics.
The nuclear transient AT2019cuk/Tick Tock/SDSS J1430+2303 has been suggested to harbor a supermassive black hole (SMBH) binary near coalescence. We report results from high-cadence NICER X-ray ...monitoring with multiple visits per day from January-August 2022, as well as continued optical monitoring during the same time period. We find no evidence of periodic/quasi-periodic modulation in the X-ray, UV, or optical bands, however we do observe exotic hard X-ray variability that is unusual for a typical AGN. The most striking feature of the NICER light curve is repetitive hard (2-4 keV) X-ray flares that result in distinctly harder X-ray spectra compared to the non-flaring data. In its non-flaring state, AT2019cuk looks like a relatively standard AGN, but it presents the first case of day-long, hard X-ray flares in a changing-look AGN. We consider a few different models for the driving mechanism of these hard X-ray flares, including: (1) corona/jet variability driven by increased magnetic activity, (2) variable obscuration, and (3) self-lensing from the potential secondary SMBH. We prefer the variable corona model, as the obscuration model requires rather contrived timescales and the self-lensing model is difficult to reconcile with a lack of clear periodicity in the flares. These findings illustrate how important high-cadence X-ray monitoring is to our understanding of the rapid variability of the X-ray corona and necessitate further high-cadence, multi-wavelength monitoring of changing-look AGN like AT2019cuk to probe the corona-jet connection.
Approximately one-third of the gamma-ray sources in the third Fermi-LAT catalog are unidentified or unassociated with objects at other wavelengths. Observations with Swift-XRT have yielded possible ...counterparts in \(\sim\)30% of these source regions. The objective of this work is to identify the nature of these possible counterparts, utilizing their gamma ray properties coupled with the Swift derived X-ray properties. The majority of the known sources in the Fermi catalogs are blazars, which constitute the bulk of the extragalactic gamma-ray source population. The galactic population on the other hand is dominated by pulsars. Blazars and pulsars occupy different parameter space when X-ray fluxes are compared with various gamma-ray properties. In this work, we utilize the X-ray observations performed with the Swift-XRT for the unknown Fermi sources and compare their X-ray and gamma-ray properties to differentiate between the two source classes. We employ two machine learning algorithms, decision tree and random forest classifier, to our high signal-to-noise ratio sample of 217 sources, each of which correspond to Fermi unassociated regions. The accuracy score for both methods were found to be 97% and 99%, respectively. The random forest classifier, which is based on the application of a multitude of decision trees, associated a probability value (P\(_{bzr}\)) for each source to be a blazar. This yielded 173 blazar candidates with P\(_{bzr}\) \(\geq\) 90% for each of these sources, and 134 of these possible blazar source associations had P\(_{bzr}\) \(\geq\) 99%. The results yielded 13 sources with P\(_{bzr}\) \(\leq\) 10%, which we deemed as reasonable candidates for pulsars, 7 of which result with P\(_{bzr}\) \(\leq\) 1%. There were 31 sources that exhibited intermediate probabilities and were termed ambiguous due to their unclear characterization as a pulsar or a blazar.
Based on the rate of change of its orbital period, PSR J2043+1711 has a substantial peculiar acceleration of 3.5 \(\pm\) 0.8 mm/s/yr, which deviates from the acceleration predicted by equilibrium ...Milky Way models at a \(4\sigma\) level. The magnitude of the peculiar acceleration is too large to be explained by disequilibrium effects of the Milky Way interacting with orbiting dwarf galaxies (\(\sim\)1 mm/s/yr), and too small to be caused by period variations due to the pulsar being a redback. We identify and examine two plausible causes for the anomalous acceleration: a stellar flyby, and a long-period orbital companion. We identify a main-sequence star in \textit{Gaia} DR3 and Pan-STARRS DR2 with the correct mass, distance, and on-sky position to potentially explain the observed peculiar acceleration. However, the star and the pulsar system have substantially different proper motions, indicating that they are not gravitationally bound. However, it is possible that this is an unrelated star that just happens to be located near J2043+1711 along our line of sight (chance probability of 1.6\%). Therefore, we also constrain possible orbital parameters for a circumbinary companion in a hierarchical triple system with J2043+1711; the changes in the spindown rate of the pulsar are consistent with an outer object that has an orbital period of 80 kyr, a companion mass of 0.3 \(M_\odot\) (indicative of a white dwarf or low-mass star), and a semi-major axis of 2000 AU. Continued timing and/or future faint optical observations of J2043+1711 may eventually allow us to differentiate between these scenarios.
Pulsar timing arrays (PTAs) are designed to detect low-frequency
gravitational waves (GWs). GWs induce achromatic signals in PTA data, meaning
that the timing delays do not depend on radio-frequency. ...However, pulse arrival
times are also affected by radio-frequency dependent "chromatic" noise from
sources such as dispersion measure (DM) and scattering delay variations.
Furthermore, the characterization of GW signals may be influenced by the choice
of chromatic noise model for each pulsar. To better understand this effect, we
assess if and how different chromatic noise models affect achromatic noise
properties in each pulsar. The models we compare include existing DM models
used by NANOGrav and noise models used for the European PTA Data Release 2
(EPTA DR2). We perform this comparison using a subsample of six pulsars from
the NANOGrav 15 yr data set, selecting the same six pulsars as from the EPTA
DR2 six-pulsar dataset. We find that the choice of chromatic noise model
noticeably affects the achromatic noise properties of several pulsars. This is
most dramatic for PSR J1713+0747, where the amplitude of its achromatic red
noise lowers from $\log_{10}A_{\text{RN}} = -14.1^{+0.1}_{-0.1}$ to
$-14.7^{+0.3}_{-0.5}$, and the spectral index broadens from $\gamma_{\text{RN}}
= 2.6^{+0.5}_{-0.4}$ to $\gamma_{\text{RN}} = 3.5^{+1.2}_{-0.9}$. We also
compare each pulsar's noise properties with those inferred from the EPTA DR2,
using the same models. From the discrepancies, we identify potential areas
where the noise models could be improved. These results highlight the potential
for custom chromatic noise models to improve PTA sensitivity to GWs.
Adeno-associated viruses (AAVs) are typically single-stranded deoxyribonucleic acid (ssDNA) encapsulated within 25-nm protein capsids. Recently, tissue-specific AAV capsids (e.g. PHP.eB) have been ...shown to enhance brain delivery in rodents via the LY6A receptor on brain endothelial cells. Here, we create a non-invasive positron emission tomography (PET) methodology to track viruses. To provide the sensitivity required to track AAVs injected at picomolar levels, a unique multichelator construct labeled with a positron emitter (Cu-64, t
= 12.7 h) is coupled to the viral capsid. We find that brain accumulation of the PHP.eB capsid 1) exceeds that reported in any previous PET study of brain uptake of targeted therapies and 2) is correlated with optical reporter gene transduction of the brain. The PHP.eB capsid brain endothelial receptor affinity is nearly 20-fold greater than that of AAV9. The results suggest that novel PET imaging techniques can be applied to inform and optimize capsid design.
We present the discovery of 528.6 Hz pulsations in the new X-ray transient MAXI J1816-195. Using NICER, we observed the first recorded transient outburst from the neutron star low-mass X-ray binary ...MAXI J1816-195 over a period of 28 days. From a timing analysis of the 528.6 Hz pulsations, we find that the binary system is well described as a circular orbit with an orbital period of 4.8 hours and a projected semi-major axis of 0.26 light-seconds for the pulsar, which constrains the mass of the donor star to \(0.10-0.55 M_\odot\). Additionally, we observed 15 thermonuclear X-ray bursts showing a gradual evolution in morphology over time, and a recurrence time as short as 1.4 hours. We did not detect evidence for photospheric radius expansion, placing an upper limit on the source distance of 8.6 kpc.
The millisecond pulsar J1713+0747 underwent a sudden and significant pulse shape change between April 16 and 17, 2021 (MJDs 59320 and 59321). Subsequently, the pulse shape gradually recovered over ...the course of several months. We report the results of continued multi-frequency radio observations of the pulsar made using the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the 100-meter Green Bank Telescope (GBT) in a three-year period encompassing the shape change event, between February 2020 and February 2023. As of February 2023, the pulse shape had returned to a state similar to that seen before the event, but with measurable changes remaining. The amplitude of the shape change and the accompanying TOA residuals display a strong non-monotonic dependence on radio frequency, demonstrating that the event is neither a glitch (the effects of which should be independent of radio frequency, \(\nu\)) nor a change in dispersion measure (DM) alone (which would produce a delay proportional to \(\nu^{-2}\)). However, it does bear some resemblance to the two previous "chromatic timing events" observed in J1713+0747 (Demorest et al. 2013; Lam et al. 2016), as well as to a similar event observed in PSR J1643-1224 in 2015 (Shannon et al. 2016).
Noise characterization for pulsar-timing applications accounts for interstellar dispersion by assuming a known frequency-dependence of the delay it introduces in the times of arrival (TOAs). However, ...calculations of this delay suffer from mis-estimations due to other chromatic effects in the observations. The precision in modeling dispersion is dependent on the observed bandwidth. In this work, we calculate the offsets in infinite-frequency TOAs due to mis-estimations in the modeling of dispersion when using varying bandwidths at the Green Bank Telescope. We use a set of broadband observations of PSR J1643-1224, a pulsar with an excess of chromatic noise in its timing residuals. We artificially restricted these observations to a narrowband frequency range, then used both data sets to calculate residuals with a timing model that does not include short-scale dispersion variations. By fitting the resulting residuals to a dispersion model, and comparing the ensuing fitted parameters, we quantify the dispersion mis-estimations. Moreover, by calculating the autocovariance function of the parameters we obtained a characteristic timescale over which the dispersion mis-estimations are correlated. For PSR J1643-1224, which has one of the highest dispersion measures (DM) in the NANOGrav pulsar timing array, we find that the infinite-frequency TOAs suffer from a systematic offset of ~22 microseconds due to DM mis-estimations, with correlations over ~1 month. For lower-DM pulsars, the offset is ~7 microseconds. This error quantification can be used to provide more robust noise modeling in NANOGrav's data, thereby increasing sensitivity and improving parameter estimation in gravitational wave searches.
The cosmic merger history of supermassive black hole binaries (SMBHBs) is expected to produce a low-frequency gravitational wave background (GWB). Here we investigate how signs of the discrete nature ...of this GWB can manifest in pulsar timing arrays through excursions from, and breaks in, the expected \(f_{\mathrm{GW}}^{-2/3}\) power-law of the GWB strain spectrum. To do this, we create a semi-analytic SMBHB population model, fit to NANOGrav's 15 yr GWB amplitude, and with 1,000 realizations we study the populations' characteristic strain and residual spectra. Comparing our models to the NANOGrav 15 yr spectrum, we find two interesting excursions from the power-law. The first, at \(2 \; \mathrm{nHz}\), is below our GWB realizations with \(p\)-value significance \(p = 0.05\) to \(0.06\) (\(\approx 1.8 \sigma - 1.9 \sigma\)). The second, at \(16 \; \mathrm{nHz}\), is above our GWB realizations with \(p = 0.04\) to \(0.15\) (\(\approx 1.4 \sigma - 2.1 \sigma\)). We explore the properties of a loud SMBHB which could cause such an excursion. Our simulations also show that the expected number of SMBHBs decreases by three orders of magnitude, from \(\sim 10^6\) to \(\sim 10^3\), between \(2\; \mathrm{nHz}\) and \(20 \; \mathrm{nHz}\). This causes a break in the strain spectrum as the stochasticity of the background breaks down at \(26^{+28}_{-19} \; \mathrm{nHz}\), consistent with predictions pre-dating GWB measurements. The diminished GWB signal from SMBHBs at frequencies above the \(26\)~nHz break opens a window for PTAs to detect continuous GWs from individual SMBHBs or GWs from the early universe.
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