Rm Erg Research Articles

Ultraluminous X-ray sources as neutrino pulsars

The classical limit on the accretion luminosity of a neutron star is given by the Eddington luminosity. The advanced models of accretion onto magnetized neutron stars account for the appearance of magnetically confined accretion columns and allow the accretion luminosity to be higher than the Eddington value by a factor of tens. However, the recent discovery of pulsations from ultraluminous X-ray source (ULX) in NGC 5907 demonstrates that the accretion luminosity can exceed the Eddington value up to by a factor of 500. We propose a model explaining observational properties of ULX-1 in NGC 5907 without any ad hoc assumptions. We show that the accretion column at extreme luminosity becomes advective. Enormous energy release within a small geometrical volume and advection result in very high temperatures at the bottom of accretion column, which demand to account for the energy losses due to neutrino emission which can be even more effective than the radiation energy losses. We show that the total luminosity at the mass accretion rates above $10^{21}\,{\rm g\,s^{-1}}$ is dominated by the neutrino emission similarly to the case of core-collapse supernovae. We argue that the accretion rate measurements based on detected photon luminosity in case of bright ULXs powered by neutron stars can be largely underestimated due to intense neutrino emission. The recently discovered pulsating ULX-1 in galaxy NGC 5907 with photon luminosity of $\sim 10^{41}\,{\rm erg\,s^{-1}}$ is expected to be even brighter in neutrinos and is thus the first known Neutrino Pulsar.

Mapping jet–ISM interactions in X-ray binaries with ALMA: a GRS 1915+105 case study

We present Atacama Large Millimetre/Sub-Millimetre Array (ALMA) observations of IRAS 19132+1035, a candidate jet-ISM interaction zone near the black hole X-ray binary (BHXB) GRS 1915+105. With these ALMA observations (combining data from the 12 m array and the Atacama Compact Array), we map the molecular line emission across the IRAS 19132+1035 region. We detect emission from the $^{12}$CO [$J=2-1$], $^{13}$CO [$\nu=0$, $J=2-1$], C$^{18}$O [$J=2-1$], ${\rm H}_{2}{\rm CO}$ [$J=3_{0,3}-2_{0,2}$], ${\rm H}_{2}{\rm CO}$ [$J=3_{2,2}-2_{2,1}$], ${\rm H}_{2}{\rm CO}$ [$J=3_{2,1}-2_{2,0}$], SiO [$\nu=0$, $J=5-4$], CH$_3$OH [$J=4_{2,2}-3_{1,2}$], and CS [$\nu=0$, $J=5-4$] transitions. Given the morphological, spectral, and kinematic properties of this molecular emission, we present several lines of evidence that support the presence of a jet-ISM interaction at this site, including a jet-blown cavity in the molecular gas. This compelling new evidence identifies this site as a jet-ISM interaction zone, making GRS 1915$+$105 the third Galactic BHXB with at least one conclusive jet-ISM interaction zone. However, we find that this interaction occurs on much smaller scales than was postulated by previous work, where the BHXB jet does not appear to be dominantly powering the entire IRAS 19132+1035 region. Using estimates of the ISM conditions in the region, we utilize the detected cavity as a calorimeter to estimate the time-averaged power carried in the GRS 1915+105 jets of $(8.4^{+7.7}_{-8.1})\times10^{32}\,{\rm erg\,s}^{-1}$. Overall, our analysis demonstrates that molecular lines are excellent diagnostic tools to identify and probe jet-ISM interaction zones near Galactic BHXBs.

Orbital Dynamics of Candidate Transitional Millisecond Pulsar 3FGL J1544.6-1125: An Unusually Face-on System

Abstract We present the orbital solution for the donor star of the candidate transitional millisecond pulsar (tMSP) 3FGL J1544.6-1125, currently observed as an accreting low-mass X-ray binary. The orbital period is days, entirely consistent with the spectral classification of the donor star as a mid-to-late K dwarf. The semi-amplitude of the radial velocity curve is exceptionally low at km s−1, implying a remarkably face-on inclination in the range 5°–8°, depending on the neutron star and donor masses. After determining the veiling of the secondary, we derive a distance to the binary of 3.8 ± 0.7 kpc, yielding a 0.3–10 keV X-ray luminosity of , similar to confirmed tMSPs. As face-on binaries rarely occur by chance, we discuss the possibility that Fermi-selected samples of transitional millisecond pulsars in the sub-luminous disk state are affected by beaming. By phasing emission line strength on the spectroscopic ephemeris, we find coherent variations, and argue that these variations are most consistent with emission from an asymmetric shock originating near the inner disk.

Predicting Hα emission-line galaxy counts for future galaxy redshift surveys

Knowledge of the number density of H$\alpha$ emitting galaxies is vital for assessing the scientific impact of the Euclid and WFIRST missions. In this work we present predictions from a galaxy formation model, Galacticus, for the cumulative number counts of H$\alpha$-emitting galaxies. We couple Galacticus to three different dust attenuation methods and examine the counts using each method. A $\chi^2$ minimisation approach is used to compare the model predictions to observed galaxy counts and calibrate the dust parameters. We find that weak dust attenuation is required for the Galacticus counts to be broadly consistent with the observations, though the optimum dust parameters return large values for $\chi^2$, suggesting that further calibration of Galacticus is necessary. The model predictions are also consistent with observed estimates for the optical depth and the H$\alpha$ luminosity function. Finally we present forecasts for the redshift distributions and number counts for two Euclid-like and one WFIRST-like survey. For a Euclid-like survey with redshift range $0.9\leqslant z\leqslant 1.8$ and H$\alpha+{\rm [NII]}$ blended flux limit of $2\times 10^{-16}{\rm erg}\,{\rm s}^{-1}\,{\rm cm}^{-2}$ we predict a number density between 3900--4800 galaxies per square degree. For a WFIRST-like survey with redshift range $1\leqslant z\leqslant 2$ and blended flux limit of $1\times 10^{-16}{\rm erg}\,{\rm s}^{-1}\,{\rm cm}^{-2}$ we predict a number density between 10400--15200 galaxies per square degree.

Warm dark matter constraints from high-z direct collapse black holes using the JWST

We use a semi-analytic model, ${\it Delphi}$, that jointly tracks the dark matter and baryonic assembly of high-redshift ($z \simeq 4-20$) galaxies to gain insight on the number density of Direct Collapse Black Hole (DCBH) hosts in three different cosmologies: the standard Cold Dark Matter (CDM) model and two Warm Dark Matter (WDM) models with particle masses of 3.5 and 1.5 keV. Obtaining the Lyman-Werner (LW) luminosity of each galaxy from ${\it Delphi}$, we use a clustering bias analysis to identify all, pristine halos with a virial temperature $T_{vir}>=10^4$ K that are irradiated by a LW background above a critical value as, DCBH hosts. In good agreement with previous studies, we find the DCBH number density rises from $\sim10^{-6.1}$ to $\sim 10^{-3.5}\, \mathrm{cMpc^{-3}}$ from $z\simeq 17.5$ to $8$ in the CDM model using a critical LW background value of $30 J_{21}$ (where $J_{21}= 10^{-21} \, {\rm erg\, s^{-1}\, Hz^{-1} \, cm^{-2} \, sr^{-1}}$). We find that a combination of delayed structure formation and an accelerated assembly of galaxies results in a later metal-enrichment and an accelerated build-up of the LW background in the 1.5 keV WDM model, resulting in DCBH hosts persisting down to much lower redshifts ($z \simeq 5$) as compared to CDM where DCBH hosts only exist down to $z \simeq 8$. We end by showing how the expected colours in three different bands of the Near Infrared Camera (NIRCam) onboard the forthcoming James Webb Space Telescope (${\it JWST}$) can be used to hunt for potential $z \simeq 5-9$ DCBHs, allowing hints on the WDM particle mass.

A Tale of Two Transients: GW 170104 and GRB 170105A

Abstract We present multi-wavelength follow-up campaigns by the AstroSat CZTI and GROWTH collaborations in search of an electromagnetic counterpart to the gravitational wave event GW 170104. At the time of the GW 170104 trigger, the AstroSat CZTI field of view covered 50.3% of the sky localization. We do not detect any hard X-ray (>100 keV) signal at this time, and place an upper limit of , for a 1 s timescale. Separately, the ATLAS survey reported a rapidly fading optical source dubbed ATLAS17aeu in the error circle of GW 170104. Our panchromatic investigation of ATLAS17aeu shows that it is the afterglow of an unrelated long, soft GRB 170105A, with only a fortuitous spatial coincidence with GW 170104. We then discuss the properties of this transient in the context of standard long GRB afterglow models.

Black holes, disks, and jets following binary mergers and stellar collapse: The narrow range of electromagnetic luminosities and accretion rates.

We have performed magnetohydrodynamic simulations in general relativity of binary neutron star and binary black hole-neutron star mergers, as well as the magnetorotational collapse of supermassive stars. In many cases the outcome is a spinnng black hole (BH) immersed in a magnetized disk, with a jet emanating from the poles of the BH. While their formation scenarios differ and their BH masses, as well as their disk masses, densities, and magnetic field strengths, vary by orders of magnitude, these features conspire to generate jet Poynting luminosities that all lie in the same, narrow range of ~1052±1 erg s-1. A similar result applies to their BH accretion rates upon jet launch, which is ~0.1-10 M⊙ s-1. We provide a simple model that explains these unanticipated findings. Interestingly, these luminosities reside in the same narrow range characterizing the observed luminosity distributions of over 400 short and long GRBs with distances inferred from spectroscopic redshifts or host galaxies. This result, together with the GRB lifetimes predicted by the model, supports the belief that a compact binary merger is the progenitor of an SGRB, while a massive, stellar magnetorotational collapse is the progenitor of an LGRB.

Spectra of black hole accretion models of ultraluminous X-ray sources

We present general relativistic radiation MHD simulations of super-Eddington accretion on a $10M_\odot$ black hole. We consider a range of mass accretion rates, black hole spins, and magnetic field configurations. We compute the spectra and images of the models as a function of viewing angle, and compare them with the observed properties of ultraluminous X-ray sources (ULXs). The models easily produce apparent luminosities in excess of $10^{40}~{\rm erg\,s^{-1}}$ for pole-on observers. However, the angle-integrated radiative luminosities rarely exceed $2.5\times10^{39}~{\rm erg\,s^{-1}}$ even for mass accretion rates of tens of Eddington. The systems are thus radiatively inefficient, though they are energetically efficient when the energy output in winds and jets is also counted. The simulated models reproduce the main empirical types of spectra --- disk-like, supersoft, soft, hard --- observed in ULXs. The magnetic field configuration, whether MAD ("magnetically arrested disk") or SANE ("standard and normal evolution"), has a strong effect on the results. In SANE models, the X-ray spectral hardness is almost independent of accretion rate, but decreases steeply with increasing inclination. MAD models with non-spinning black holes produce significantly softer spectra at higher values of $\dot{M}$, even at low inclinations. MAD models with rapidly spinning black holes are quite different from all other models. They are radiatively efficient (efficiency factor $\sim10-20\%$), super-efficient when the mechanical energy output is also included ($70\%$), and produce hard blazar-like spectra. In all models, the emission shows strong geometrical beaming, which disagrees with the more isotropic illumination favored by observations of ULX bubbles.

Diffuseγ-ray emission near the young massive cluster NGC 3603

We report the Fermi Large Area Telescope's detection of extended gamma-ray emission towards the direction of the young massive star cluster NGC 3603. The emission shows a hard spectrum with a photon index of 2.3 from 1 GeV to 250 GeV. The large size and high luminosity of this structure make it unlikely a pulsar wind nebular. On the other hand the spatial correlation with the ionised gas indicate a hadronic origin. The total cosmic ray (CR) protons energy are estimated to be of the order $10^{50} ~\rm erg$ assuming the gamma-ray are produced in the interaction of CRs with ambient gas . The environment and spectral features show significant similarity with the Cygnus cocoon. It reveals that the young star clusters may be a gamma-ray source population and they can potentially accelerate a significant fraction of the Galactic cosmic rays.

X-ray spectroscopy of the z = 6.4 quasar SDSS J1148+5251

We present the 78-ks Chandra observations of the $z=6.4$ quasar SDSS J1148+5251. The source is clearly detected in the energy range 0.3-7 keV with 42 counts (with a significance $\gtrsim9\sigma$). The X-ray spectrum is best-fitted by a power-law with photon index $\Gamma=1.9$ absorbed by a gas column density of $\rm N_{\rm H}=2.0^{+2.0}_{-1.5}\times10^{23}\,\rm cm^{-2}$. We measure an intrinsic luminosity at 2-10 keV and 10-40 keV equal to $\sim 1.5\times 10^{45}~\rm erg~s^{-1}$, comparable with luminous local and intermediate-redshift quasar properties. Moreover, the X-ray to optical power-law slope value ($\alpha_{\rm OX}=-1.76\pm 0.14$) of J1148 is consistent with the one found in quasars with similar rest-frame 2500 \AA ~luminosity ($L_{\rm 2500}\sim 10^{32}~\rm erg~s^{-1}$\AA$^{-1}$). Then we use Chandra data to test a physically motivated model that computes the intrinsic X-ray flux emitted by a quasar starting from the properties of the powering black hole and assuming that X-ray emission is attenuated by intervening, metal-rich ($Z\geq \rm Z_{\odot}$) molecular clouds distributed on $\sim$kpc scales in the host galaxy. Our analysis favors a black hole mass $M_{\rm BH} \sim 3\times 10^9 \rm M_\odot$ and a molecular hydrogen mass $M_{\rm H_2}\sim 2\times 10^{10} \rm M_\odot$, in good agreement with estimates obtained from previous studies. We finally discuss strengths and limits of our analysis.

Giant Lyα Nebulae in the Illustris Simulation

Abstract Several “giant” Lyα nebulae with an extent ≳300 kpc and observed Lyα luminosity of ≳1044 erg s−1 cm−2 arcsec−2 have recently been detected, and it has been speculated that their presence hints at a substantial cold gas reservoir in small cool clumps not resolved in modern hydrodynamical simulations. We use the Illustris simulation to predict the Lyα emission emerging from large halos (M > 1011.5 M ⊙) at z ∼ 2 and thus test this model. We consider both active galactic nucleus (AGN) and star driven ionization, and compare the simulated surface brightness maps, profiles, and Lyα spectra to a model where most gas is clumped below the simulation resolution scale. We find that with Illustris, no additional clumping is necessary to explain the extents, luminosities, and surface brightness profiles of the “giant Lyα nebulae” observed. Furthermore, the maximal extents of the objects show a wide spread for a given luminosity and do not correlate significantly with any halo properties. We also show how the detected size depends strongly on the employed surface brightness cutoff, and predict that further examples of such objects will be found in the near future.

Constraining high-energy cosmic neutrino sources: Implications and prospects

We consider limits on the local ($z=0$) density ($n_0$) of extragalactic neutrino sources set by the nondetection of steady high-energy neutrino sources producing $\gtrsim50$ TeV muon multiplets in the present IceCube data, taking into account the redshift evolution, luminosity function and neutrino spectrum of the sources. We show that the lower limit depends moderately on source spectra and strongly on redshift evolution. We find $n_0\gtrsim{10}^{-8}-{10}^{-7}~{\rm Mpc}^{-3}$ for standard candle sources evolving rapidly, $n_s\propto{(1+z)}^3$, and $n_0\gtrsim{10}^{-6}-{10}^{-5}~{\rm Mpc}^{-3}$ for nonevolving sources. The corresponding upper limits on their neutrino luminosity are $L_{{\nu_\mu}}^{\rm eff}\lesssim10^{42}-10^{43}~{\rm erg}~{\rm s}^{-1}$ and $L_{{\nu_\mu}}^{\rm eff}\lesssim10^{41}-10^{42}~{\rm erg}~{\rm s}^{-1}$, respectively. Applying these results to a wide range of classes of potential sources, we show that powerful blazar jets associated with active galactic nuclei are unlikely to be the dominant sources. For almost all other steady candidate source classes (including starbursts, radio galaxies, and galaxy clusters and groups), an order of magnitude increase in the detector sensitivity at $\sim0.1-1$ PeV will enable a detection (as point sources) of the few brightest objects. Such an increase, which may be provided by next-generation detectors like IceCube-Gen2 and an upgraded KM3NET, can improve the limit on $n_0$ by more than two orders of magnitude. Future gamma-ray observations (by Fermi, HAWC and CTA) will play a key role in confirming the association of the neutrinos with their sources.

Coronal properties of the luminous radio-quiet quasar QSO B2202–209

We present an analysis of the joint $XMM$-$Newton$ and $NuSTAR$ observations of the radio-quiet quasar QSO B2202$-$209. Using an optical observation from the Hale Telescope at the Palomar Observatory, we revise the redshift of the source from the previously reported $z=1.77$ to $z=0.532$, and we estimate the mass of the central black hole, $\log (M_{\rm BH}/M_{\odot}) = 9.08 \pm 0.18$. The X-ray spectrum of this source can be well described by a power-law of photon index $\Gamma = 1.82 \pm 0.05$ with $E_{\rm cut} = 152_{-54}^{+103}\,{\rm keV}$, in the rest frame of the source. Assuming a Comptonisation model, we estimate the coronal temperature to be $kT_{\rm e}=42\pm 3 \,{\rm keV}$ and $kT_{\rm e}= 56 \pm 3\,{\rm keV}$ for a spherical and a slab geometry, respectively. The coronal properties are comparable to the ones derived for local AGN, despite a difference of around one order of magnitude in black hole mass and X-ray luminosity ($L_{2-10} = 1.93\times 10^{45}\,{\rm erg\,s^{-1}}$). The quasar is X-ray loud, with an unusually flat observed optical-to-X-ray spectral slope $\alpha_{\rm OX} = 1.00 \pm 0.02$, and has an exceptionally strong optical [O III] line. Assuming that both the X-ray emission and the [O III] line are isotropic, these two extreme properties can be explained by a nearly edge-on disk, leading to a reduction in the observed UV continuum light.

The Lyα emission from high-zgalaxies hosting strong damped Lyα systems

We study the average Ly$\alpha$ emission associated with high-$z$ strong (log $N$(H I) $\ge$ 21) damped Ly$\alpha$ systems (DLAs). We report Ly$\alpha$ luminosities ($L_{\rm Ly\alpha}$) for the full as well as various sub-samples based on $N$(H I), $z$, $(r-i)$ colours of QSOs and rest equivalent width of Si II$\lambda$1526 line (i.e., $W_{1526}$). For the full sample, we find $L_{\rm Ly\alpha}$$< 10^{41} (3\sigma)\ \rm erg\ s^{-1}$ with a $2.8\sigma$ level detection of Ly$\alpha$ emission in the red part of the DLA trough. The $L_{\rm Ly\alpha}$ is found to be higher for systems with higher $W_{1526}$ with its peak, detected at $\geq 3\sigma$, redshifted by about 300-400 $\rm km\ s^{-1}$ with respect to the systemic absorption redshift, as seen in Lyman Break Galaxies (LBGs) and Ly$\alpha$ emitters. A clear signature of a double-hump Ly$\alpha$ profile is seen when we consider $W_{1526} \ge 0.4$ \AA\ and $(r-i) < 0.05$. Based on the known correlation between metallicity and $W_{1526}$, we interpret our results in terms of star formation rate (SFR) being higher in high metallicity (mass) galaxies with high velocity fields that facilitates easy Ly$\alpha$ escape. The measured Ly$\alpha$ surface brightness requires local ionizing radiation that is 4 to 10 times stronger than the metagalactic UV background at these redshifts. The relationship between the SFR and surface mass density of atomic gas seen in DLAs is similar to that of local dwarf and metal poor galaxies. We show that the low luminosity galaxies will contribute appreciably to the stacked spectrum if the size-luminosity relation seen for H I at low-$z$ is also present at high-$z$. Alternatively, large Ly$\alpha$ halos seen around LBGs could also explain our measurements.

Compton-thick AGN in the 70-monthSwift-BAT All-Sky Hard X-ray Survey: A Bayesian approach

The 70-month Swift/BAT catalogue provides a sensitive view of the extragalactic X-ray sky at hard energies (>10 keV) containing about 800 Active Galactic Nuclei. We explore its content in heavily obscured, Compton-thick AGN by combining the BAT (14-195 keV) with the lower energy XRT (0.3-10 keV) data. We apply a Bayesian methodology using Markov chains to estimate the exact probability distribution of the column density for each source. We find 53 possible Compton-thick sources (with probability 3 to 100%) translating to a ~7% fraction of the AGN in our sample. We derive the first parametric luminosity function of Compton-thick AGN. The unabsorbed luminosity function can be represented by a double power-law with a break at $L_{\star} 2 \times 10^{42}$ $\rm ergs~s^{-1}$ in the 20-40 keV band.

Kband SINFONI spectra of twoz~ 5 submillimeter galaxy systems: upper limits to the unobscured star formation from [O II] optical emission line searches

We present deep SINFONI K band integral field spectra of two submillimeter (SMG) galaxy systems: BR 1202-0725 and J1000+0234, at $z=4.69$ and $4.55$ respectively. Spectra extracted for each object in the two systems do not show any signature of the [O II]$\lambda\lambda$3726,29\AA$\,$ emission-lines, placing upper flux limits of $3.9$ and $2.5 \times 10^{-18}\,$${\rm erg\,s^{-1}\,\,cm^{-2} \,}$ for BR 1202-0725 and J1000+0234, respectively. Using the relation between the star formation rate (SFR) and the luminosity of the [O II] doublet from Kennicutt (1998), we estimate unobscured SFR upper limits of $\sim$ $10-15\,\rm M_\odot\,yr^{-1} \,$ and $\sim$ $30-40\,\rm M_\odot\,yr^{-1} \,$ for the objects of the two systems, respectively. For the SMGs, these values are at least two orders of magnitude lower than those derived from SED and IR luminosities. The differences on the SFR values would correspond to internal extinction of, at least, $3.4 - 4.9$ and $2.1 - 3.6$ mag in the visual for BR 1202-0725 and J1000+0234 SMGs, respectively. The upper limit for the [O II]-derived SFR in one of the LAEs (Ly$\alpha2$) in the BR1202-0725 system is at least one order of magnitude lower than the previous SFR derived from infrared tracers, while both estimates are in good agreement for Ly$\alpha$1. The lower limits to the internal extinction in these two Lyman-alpha emitters (LAEs) are $0.6$ mag and $1.3$ mag, respectively. No evidence for the previously claimed (Ohta et al. 2000) [O II] emission associated with Ly$\alpha$1 is identified in our data, implying that residuals of the K-band sky emission lines after subtraction in medium-band imaging data could provide the adequate flux.

A luminous hot accretion flow in the low-luminosity active galactic nucleus NGC 7213

The active galactic nucleus (AGN) NGC 7213 shows a complex correlation between the monochromatic radio luminosity $L_R$ and the 2--10 keV X-ray luminosity $L_X$, i.e. the correlation is unusually weak with $p\sim 0$ (in the form $L_R\propto L_X^p$) when $L_X$ is below a critical luminosity, and steep with $p>1$ when $L_X$ is above that luminosity. Such a hybrid correlation in individual AGNs is unexpected as it deviates from the Fundamental Plane of AGN activity. Interestingly, a similar correlation pattern is observed in the black hole X-ray binary H1743--322, where it has been modelled by switching between different modes of accretion. We propose that the flat $L_R$--$L_X$ correlation of NGC 7213 is due to the presence of a luminous hot accretion flow, an accretion model whose radiative efficiency is sensitive to the accretion rate. Given the low luminosity of the source, $L_X\sim 10^{-4}$ of the Eddington luminosity, the viscosity parameter is determined to be small, $\alpha\approx 0.01$. We also modelled the broad-band spectrum from radio to $\gamma$-rays, the time lag between the radio and X-ray light curves, and the implied size and the Lorentz factor of the radio jet. We predict that NGC 7213 will enter into a two-phase accretion regime when $L_X > 1.5 \times 10^{42}\, {\rm erg\,s^{-1}}$. When this happens, we predict a softening of the X-ray spectrum with the increasing flux and a steep radio/X-ray correlation.

Time-dependent models of accretion discs with nuclear burning following the tidal disruption of a white dwarf by a neutron star

We construct time-dependent one-dimensional (vertically averaged) models of accretion disks produced by the tidal disruption of a white dwarf (WD) by a binary neutron star (NS) companion. Nuclear reactions in the disk midplane burn the WD matter to increasingly heavier elements at sequentially smaller radii, releasing substantial energy which can impact the disk dynamics. A model for disk outflows is employed, by which cooling from the outflow balances other sources of heating (viscous, nuclear) in regulating the Bernoulli parameter of the midplane to a fixed value $\lesssim 0$. We perform a comprehensive parameter study of the compositional yields and velocity distributions of the disk outflows for WDs of different initial compositions. For C/O WDs, the radial composition profile of the disk evolves self-similarly in a quasi-steady-state manner, and is remarkably robust to model parameters. The nucleosynthesis in helium WD disks does not exhibit this behavior, which instead depends sensitively on factors controlling the disk midplane density (e.g. the strength of the viscosity, $\alpha$). By the end of the simulation, a substantial fraction of the WD mass is unbound in outflows at characteristic velocities of $\sim 10^9~{\rm cm~s}^{-1}$. The outflows from WD-NS merger disks contain $10^{-4}-3 \times 10^{-3} M_\odot$ of radioactive $^{56}$Ni, resulting in fast ($\sim$ week long) dim ($\sim10^{40}~{\rm erg~s}^{-1}$) optical transients; shock heating of the ejecta by late time outflows may increase the peak luminosity to $\sim 10^{43}~{\rm erg~s}^{-1}$. The accreted mass onto the neutron star is probably not sufficient to induce gravitational collapse, but may be capable of spinning up the NS to periods of $\sim 10~{\rm ms}$. This is a new possible channel for forming isolated recycled pulsars.

Detectable MeV neutrinos from black hole neutrino-dominated accretion flows

Neutrino-dominated accretion flows (NDAFs) around rotating stellar-mass black holes (BHs) have been theorized as the central engine of relativistic jets launched in massive star core collapse events or compact star mergers. In this work, we calculate the electron neutrino/anti-neutrino spectra of NDAFs by fully taking into account the general relativistic effects, and investigate the effects of viewing angle, BH spin, and mass accretion rate on the results. We show that even though a typical NDAF has a neutrino luminosity lower than that of a typical supernova (SN), it can reach $10^{50}-10^{51}~{\rm erg~s^{-1}}$ peaking at $\sim 10$ MeV, making them potentially detectable with the upcoming sensitive MeV neutrino detectors if they are close enough to Earth. Based on the observed GRB event rate in the local universe and requiring that at least 3 neutrinos are detected to claim a detection, we estimate a detection rate up to $\sim$ (0.10-0.25) per century for GRB-related NDAFs by the Hyper-Kamiokande (Hyper-K) detector if one neglects neutrino oscillation. If one assumes that all Type Ib/c SNe have an engine-driven NDAF, the Hyper-K detection rate would be $\sim$ (1-3) per century. By considering neutrino oscillations, the detection rate may decrease by a factor of 2-3. Detecting one such event would establish the observational evidence of NDAFs in the universe.

TRANSIENT X-RAY SOURCE POPULATION IN THE MAGELLANIC-TYPE GALAXY NGC 55

ABSTRACT We present the spectral and temporal properties of 15 candidate transient X-ray sources detected in archival XMM-Newton and Chandra observations of the nearby Magellanic-type, SB(s)m galaxy NGC 55. Based on an X-ray color classification scheme, the majority of the sources may be identified as X-ray binaries (XRBs), and six sources are soft, including a likely supernova remnant. We perform a detailed spectral and variability analysis of the data for two bright candidate XRBs. Both sources displayed strong short-term X-ray variability, and their X-ray spectra and hardness ratios are consistent with those of XRBs. These results, combined with their high X-ray luminosities (∼1038 erg s−1), strongly suggest that they are black hole (BH) binaries. Seven less luminous sources have spectral properties consistent with those of neutron star or BH XRBs in both normal and high-rate accretion modes, but one of them is the likely counterpart to a background galaxy (because of positional coincidence). From our spectral analysis, we find that the six soft sources are candidate super soft sources (SSSs) with dominant emission in the soft (0.3–2 keV) X-ray band. Archival Hubble Space Telescope optical images for seven sources are available, and the data suggest that most of them are likely to be high-mass XRBs. Our analysis has revealed the heterogeneous nature of the transient population in NGC 55 (six high-mass XRBs, one low-mass XRBs, six SSSs, one active galactic nucleus), helping establish the similarity of the X-ray properties of this galaxy to those of other Magellanic-type galaxies.