Astrophysics

We analyse 55 ks of Chandra X-ray observations of the Galactic globular cluster M13. Using the latest radio timing positions of six known millisecond pulsars (MSPs) in M13 from Wang et al. (2020), we detect confident X-ray counterparts to five of the six MSPs at X-ray luminosities of L X (0.3-8 keV) ??? 10 30 ??10 31 erg s ?? , including the newly discovered PSR J1641+3627F. There are limited X-ray counts at the position of PSR J1641+3627A, for which we obtain an upper limit L X <1.3? 10 30 erg s ?? . We analyse X-ray spectra of all six MSPs, which are well-described by either a single blackbody or a single power-law model. We also incorporate optical/UV imaging observations from the Hubble Space Telescope (HST) and find optical counterparts to PSR J1641+3627D and J1641+3627F. Our colour-magnitude diagrams indicate the latter contains a white dwarf, consistent with the properties suggested by radio timing observations. The counterpart to J1641+3627D is only visible in the V band; however, we argue that the companion to J1641+3627D is also a white dwarf, since we see a blackbody-like X-ray spectrum, while MSPs with nondegenerate companions generally show non-thermal X-rays from shocks between the pulsar and companion winds. Our work increases the sample of known X-ray and optical counterparts of MSPs in globular clusters.

We present a uniform analysis of six examples of embedded wind shock (EWS) O star X-ray sources observed at high resolution with the Chandra grating spectrometers. By modeling both the hot plasma emission and the continuum absorption of the soft X-rays by the cool, partially ionized bulk of the wind we derive the temperature distribution of the shock-heated plasma and the wind mass-loss rate of each star. We find a similar temperature distribution for each star's hot wind plasma, consistent with a power-law differential emission measure, dlogEM dlogT , with a slope a little steeper than -2, up to temperatures of only about 10 7 K. The wind mass-loss rates, which are derived from the broadband X-ray absorption signatures in the spectra, are consistent with those found from other diagnostics. The most notable conclusion of this study is that wind absorption is a very important effect, especially at longer wavelengths. More than 90 per cent of the X-rays between 18 and 25 Angstrom units produced by shocks in the wind of ζ Puppis are absorbed, for example. It appears that the empirical trend of X-ray hardness with spectral subtype among O stars is primarily an absorption effect.

We report on the temporal properties of the ULX pulsar M51 ULX-7 inferred from the analysis of the 2018-2020 Swift/XRT monitoring data and archival Chandra data obtained over a period of 33 days in 2012. We find an extended low flux state, which might be indicative of propeller transition, lending further support to the interpretation that the NS is rotating near equilibrium. Alternatively, this off state could be related to a variable super-orbital period. Moreover, we report the discovery of periodic dips in the X-ray light curve that are associated with the binary orbital period. The presence of the dips implies a configuration where the orbital plane of the binary is closer to an edge on orientation, and thus demonstrates that favorable geometries are not necessary in order to observe ULX pulsars.These characteristics are similar to those seen in prototypical X-ray pulsars like Her X-1 and SMC X-1 or other ULX pulsars like NGC 5907 ULX1.

The leading explanation of the Fermi Galactic center γ -ray excess is the extended emission from a unresolved population of millisecond pulsars (MSPs) in the Galactic bulge. Such a population would, along with the prompt γ rays, also inject large quantities of electrons/positrons ( e ± ) into the interstellar medium. These e ± could potentially inverse-Compton (IC) scatter ambient photons into γ rays that fall within the sensitivity range of the upcoming Cherenkov Telescope Array (CTA). In this article, we examine the detection potential of CTA to this signature by making a realistic estimation of the systematic uncertainties on the Galactic diffuse emission model at TeV-scale γ -ray energies. We forecast that, in the event that e ± injection spectra are harder than E ?? , CTA has the potential to robustly discover the IC signature of a putative Galactic bulge MSP population sufficient to explain the GCE for e ± injection efficiencies in the range ??.9??4.1% , or higher, depending on the level of mismodeling of the Galactic diffuse emission components. On the other hand, for spectra softer than E ??.5 , a reliable CTA detection would require an unphysically large e ± injection efficiency of ??58% . However, even this pessimistic conclusion may be avoided in the plausible event that MSP observational and/or modeling uncertainties can be reduced. We further find that, in the event that an IC signal were detected, CTA can successfully discriminate between an MSP and a dark matter origin for the radiating e ± .

Primordial black holes created in the early Universe can constitute a substantial fraction of dark matter and serve as seeds for early galaxy formation. Binary primordial black holes with masses of the order of a few dozen solar masses can explain the observed LIGO/Virgo gravitational-wave events. In this Letter, we show that primordial black holes with log-normal mass spectrum centered at M 0 ??5??7 M ??simultaneously explain both the chirp mass distribution of the detected LIGO/Virgo binary black holes and the differential chirp mass distribution of merging binaries as inferred from the LIGO/Virgo observations. The obtained parameters of log-normal mass spectrum of primordial black holes also give the fraction of seeds with M??10 4 M ??required to explain the observed population of supermassive black holes at z=6?? .

We develop a model of the generation of coherent radio emission in the Crab pulsar, magnetars and Fast Radio Bursts (FRBs). Emission is produced by a reconnection-generated beam of particles via a variant of Free Electron Laser (FEL) mechanism, operating in a weakly-turbulent, guide-field dominated plasma. We first consider nonlinear Thomson scattering in a guide-field dominated regime, and apply to model to explain emission bands observed in Crab pulsar and in Fast Radio Bursts. We consider particle motion in a combined fields of the electromagnetic wave and the electromagnetic (Alfvenic) wiggler. Charge bunches, created via a ponderomotive force, Compton/Raman scatter the wiggler field coherently. The model is both robust to the underlying plasma parameters and succeeds in reproducing a number of subtle observed features: (i) emission frequencies depend mostly on the length λ of turbulence and the Lorentz factor of the reconnection generated beam, ???γ 2 b (c/λ) - it is independent of the absolute value of the underlying magnetic field. (ii) The model explains both broadband emission and the presence of emission stripes, including multiple stripes observed in the High Frequency Interpulse of the Crab pulsar. (iii) The model reproduces correlated polarization properties: presence of narrow emission bands in the spectrum favors linear polarization, while broadband emission can have arbitrary polarization.(iv) The mechanism is robust to the momentum spread of the particle beam. We also discuss a model of wigglers as non-linear force-free Alfven solitons (light darts).

We study high energy neutrino emission from relativistic jets launched by a black hole (BH) spiraling-in inside the envelope of a red supergiant (RSG), and find that such common envelope jets supernovae (CEJSNe) are a potential source for the ~10^15 eV neutrinos detected by IceCube. We first use the stellar evolution code MESA to mimic the effect of the jets on the RSG envelope, and find that the jets substantially inflate the envelope. We then study the propagation of jets inside the extended RSG envelope and find that in most cases the jets do not penetrate the envelope but are rather stalled. We show that such jets can accelerate cosmic rays to high enough energies to produce high energy neutrinos. While the neutrinos stream out freely, the gamma-rays that accompany the neutrino production remain trapped inside the optically thick envelope. This explains the lack of observational association between high energy neutrinos and gamma-rays.

We present a broadband spectral analysis of accreting neutron stars using data from XIS and PIN onboard \emph{Suzaku}. From spectral fits of these sources with a single continuum model including a powerlaw and high energy cut-off, cyclotron lines (where required), we studied the correlation between various spectral parameters. Among 39 sources we studied, 16 are those where the existence of a cyclotron line is known in literature, and 29 need a cutoff energy. Among these 29 sources, 18 have cutoff energy bunched in a range of 3-10 keV while for 11 sources, it spreads over 12-25 keV. This bi-modal behaviour is not based on the specific nature of the systems being a Be XRB or supergiant HMXB, nor on different beaming patterns characterizing their X-ray emission (as inferred from simultaneous study of their pulse profiles). The broadband coverage of \emph{Suzaku} also shows that the cutoff energies saturate for higher values of cyclotron line energies - consistent with previous works in literature - for both the groups and the width of the cyclotron line show a weak correlation with the cyclotron line energy. We also find an anticorrelation with luminosity for both spectral index and folding energy, respectively. Unlike previous works, we did not detect any anticorrelation between X-ray luminosity and EW of K α lines. Finally, we show that the EW and flux of the iron K α line are smaller in SFXTs than classical NS-HMXBs. We discuss these findings in terms of different properties of stellar winds and accretion mechanisms.

Multi-lepton anomalies at the Large Hadron Collider are reasonably well described by a two Higgs doublet model with an additional singlet scalar. Here, we demonstrate that using this model we are also able to describe the excesses in gamma-ray flux from the galactic centre and the cosmic-ray spectra from AMS-02. This is achieved through Dark Matter (DM) annihilation via the singlet scalar. Of great interest is the flux of synchrotron emissions which results from annihilation of DM in Milky-Way satellites. We make predictions for MeerKAT observations of the nearby dwarf galaxy Reticulum~II and we demonstrate the power of this instrument as a new frontier in indirect dark matter searches.

Observations of core-collapse supernovae (CCSNe) reveal a wealth of information about the dynamics of the supernova ejecta and its composition but reveal very little direct information about the progenitor star. Constraining properties of the progenitor and the explosion, such as explosion energy, requires coupling the observations with a theoretical model of the explosion. Here, we use a new model for driving turbulence-aided neutrino-driven core-collapse supernovae in 1D (STIR) which contains a non-parametric treatment of the neutrino transport while also accounting for turbulence and convection. We couple this with the SuperNova Explosion Code to produce bolometric light curves from a landscape of CCSNe driven from self-consistent CCSN simulations with robust neutrino transport. We compare our results to several well observed bolometric light curves of Type IIP CCSNe and find that our best fitting models differ from those found in previous studies using thermal bomb explosions, indicating ZAMS masses as much as about 10M ??greater than previous studies. Using our large sample of 136 self-consistent CCSN explosions, we explore correlations between observable features of the light curves and properties of the progenitor star. Among other significant correlations, we find a robust linear relationship between light curve plateau luminosity and the iron core mass of the progenitor. This relationship allows for properties of the core of the progenitor to be constrained for the first time from photometry alone.