Homogeneous Interstellar Medium Research Articles

Galactic cosmic-ray scattering due to intermittent structures

ABSTRACT Cosmic rays (CRs) with energies ≪ TeV comprise a significant component of the interstellar medium (ISM). Major uncertainties in CR behaviour on observable scales (much larger than CR gyroradii) stem from how magnetic fluctuations scatter CRs in pitch angle. Traditional first-principles models, which assume these magnetic fluctuations are weak and uniformly scatter CRs in a homogeneous ISM, struggle to reproduce basic observables such as the dependence of CR residence times and scattering rates on rigidity. We therefore explore a new category of ‘patchy’ CR scattering models, wherein CRs are pre-dominantly scattered by intermittent strong scattering structures with small volume-filling factors. These models produce the observed rigidity dependence with a simple size distribution constraint, such that larger scattering structures are rarer but can scatter a wider range of CR energies. To reproduce the empirically inferred CR scattering rates, the mean free path between scattering structures must be $\ell _{\rm mfp}\sim 10\, {\rm pc}$ at GeV energies. We derive constraints on the sizes, internal properties, mass/volume-filling factors, and the number density any such structures would need to be both physically and observationally consistent. We consider a range of candidate structures, both large scale (e.g. H ii regions) and small scale (e.g. intermittent turbulent structures, perhaps even associated with radio plasma scattering) and show that while many macroscopic candidates can be immediately ruled out as the primary CR scattering sites, many smaller structures remain viable and merit further theoretical study. We discuss future observational constraints that could test these models.

The surprising lack of effect from stellar feedback on the gas stripping rate from massive jellyfish galaxies

ABSTRACT We study the role of star formation and stellar feedback in a galaxy being ram pressure (RP) stripped on its infall into a cluster. We use hydrodynamical wind-tunnel simulations of a massive galaxy (Mstar = 1011 M⊙) moving into a massive cluster (Mcluster = 1015 M⊙). We have two types of simulations: with and without star formation and stellar feedback, SF, and radiative cooling (RC), respectively. For each type, we simulate four realizations of the same galaxy: a face-on wind, edge-on wind, 45° angled wind, and a control galaxy not subject to RP. We directly compare the stripping evolution of galaxies with and without star formation. We find that stellar feedback has no direct effect on the stripping process, i.e. there is no enhancement in stripping via a velocity kick to the interstellar medium (ISM) gas. The main difference between RC and SF galaxies is due to the indirect effect of stellar feedback, which produces a smoother and more homogeneous ISM. Hence, while the average gas surface density is comparable in both simulation types, the scatter is broader in the RC galaxies. As a result, at the galaxy outskirts overdense clumps survive in RC simulation, and the stripping proceeds more slowly. At the same time, in the inner disc, underdense gas in the RC holes is removed faster than the smoothly distributed gas in the SF simulation. For our massive galaxy, we therefore find that the effect of feedback on the stripping rate is almost negligible, independent of wind angle.

Star Formation Variability as a Probe for the Baryon Cycle within Galaxies

We investigate the connection between the regulation of star formation and the cycling of baryons both within and in and out of galaxies. We use idealized numerical simulations of Milky Way–mass galaxies, in which we vary the galaxy morphology and stellar feedback strength. By following individual gas parcels through the disk, spiral arms, and massive star-forming clumps, we quantify how gas moves through the different phases of the interstellar medium (ISM) and forms stars. We show that the residence time of gas in the dense ISM phase (τ SF), the nature of spiral arms, and the clump properties depend on both the galaxy morphology and stellar feedback. We quantify signatures of the baryon cycle within galaxies using the temporal and spatial power spectrum density (PSD) of the star formation rate (SFR). Stronger stellar feedback leads to more bursty star formation while the correlation timescale of the SFH is longer, because stronger feedback dissolves the dense ISM phase, leading to a more homogeneous ISM and a decrease in τ SF. The bulge strength has a similar effect: the deep gravitational potential in a bulge-dominant galaxy imposes a strong shear force that breaks apart gas clumps in the ISM; this subsequently inhibits the fragmentation of gas and therefore the star formation in the disk, leading to a decrease in the spatial power on scales of ∼1 kpc. We conclude that measurements of the temporal and spatial PSD of the SFR can provide constraints on the baryon cycle and the star formation process.

Afterglow Light Curves from Off-Axis GRB Jets in Stratified Circumburst Medium

We study the gamma-ray burst (GRB) afterglow light curves produced by an off-axis jet in a stratified circumburst medium and summarize the temporal indices of the coasting phase, the deceleration phase, the Newtonian phase, and the deep Newtonian phase for various viewing angles and power-law indices of medium density. Generally, the afterglow light curves of off-axis GRBs in the homogeneous interstellar medium have a steep rise arising due to jet deceleration. In the stratified medium, the flux rises is more shallow but peaks earlier for the same viewing angle due to faster deceleration of the jet running into the denser stratified medium, compared with the case of the interstellar medium (ISM). Observations of off-axis bursts will possibly increase over the coming years due to the arrival of the multi-messenger era and the forthcoming surveys in multiple bands. The temporal indices of off-axis afterglows derived in the paper will provide a reference for comparison with the observations and can diagnose the circumburst environment. The numerical code calculating the afterglow light curve also can be used to fit the multi-wavelength light curves.

Physical and numerical instabilities of radiatively cooling shocks in turbulent magnetized media

ABSTRACT We consider the influence of a fluctuating magnetic field on to the structure formation and instabilities of radiatively cooling blast waves. The study is based on an example of optically thin post-adiabatic supernova remnants (SNRs) in the homogeneous interstellar medium. By means of analytic estimations and full-scale multidimensional simulations, we investigate the roles of thermal, hydrodynamic (corrugation, pulsational, convective, Rayleigh–Taylor, linear and non-linear Vishniac) and numerical instabilities (‘carbuncle’ and grid-forced effects). It is found that of primary importance is the interplay of the thermal instability with quasi-regular and random components of the interstellar field. Bending fluctuations caused by the latter can be strongly amplified by non-linear Vishniac instability in the SNR regions where the regular component is almost normal to the shock. The instabilities driven by counter-directional pressure and density gradients are limited mostly to very narrow post-shock cooling layers, transient perturbations of the same short scales, and rather weakly magnetized environments. Some of these results can also be applied to radiative shocks separating optically thick media from thin or semitransparent ones. Several recommendations and requirements on numerical simulation techniques are formulated.

Simulating observable structures due to a perturbed interstellar medium in front of astrospheric bow shocks in 3D MHD

Context. While the shapes of many observed bow shocks can be reproduced by simple astrosphere models, more elaborate approaches have recently been used to explain differing observable structures. Aims. By placing perturbations of an otherwise homogeneous interstellar medium in front of the astrospheric bow shock of the runaway blue supergiant λ Cephei, the observable structure of the model astrosphere is significantly altered, providing insight into the origin of perturbed bow shock images. Methods. Three-dimensional single-fluid magnetohydrodynamic (MHD) models of stationary astrospheres were subjected to various types of perturbations and simulated until stationarity was reached again. As examples, simple perturbations of the available MHD parameters (number density, bulk velocity, temperature, and magnetic field) as well as a more complex perturbation were chosen. Synthetic observations were generated by line-of-sight integration of the model data, producing Hα, 70 μm dust emission, and bremsstrahlung maps of the perturbed astrosphere’s evolution. Results. The resulting shock structures and observational images differ strongly depending on the type of the injected perturbation and the viewing angles, forming arc-like protrusions or bifurcations of the bow shock structure, as well as rings, arcs, and irregular structures detached from the bow shock.

Parameterizing the Outflow from a Central Black Hole in Dwarf Spheroidal Galaxies: A 3D Hydrodynamic Simulation

Abstract Large galaxies harbor massive central black holes and their feedback exerts a substantial impact on their evolution. Recently, observations have suggested that dwarf galaxies might host black holes in their centers, but with lower masses (intermediate-mass black holes—IMBHs). The impact of IMBHs on the evolution of dwarf spheroidal galaxies (dSphs), however, has so far not been properly analyzed. In this work, we investigate the effects of an outflow from an IMBH on gas dynamics in dSphs by means of noncosmological, three-dimensional hydrodynamic simulations, letting the galactic gas distribution evolve over 3 Gyr under the influence of the IMBH’s outflow and supernova feedback. All simulations have a numerical resolution of 20.0 pc cell−1. Two scenarios are considered to infer differences in the propagation of the outflow, one with a homogeneous interstellar medium (ISM) and another one with inhomogeneities caused by supernova feedback. A minimal initial speed and a minimal initial density are required for the outflow to propagate, with the values depending on the conditions of the medium. In an unperturbed medium, the outflow propagates freely in both directions with the same velocity (lower than the initial one), removing a small fraction of gas from the galaxy (the exact fraction depends on the initial physical conditions of the outflow). However, in an inhomogeneous ISM, the impact of the outflow is substantially reduced, and its contribution to the removal of gas from the galaxy is almost negligible.

GRB 161017A, the circumburst environment is an intermediate regime between the homogeneous interstellar medium and wind-type medium

We investigate the multi-band properties of the GRB 161017A with [Formula: see text], which was detected by Swift and Fermi satellites, and other instruments. The optical and X-ray afterglows were all detected at early times after the prompt emission. The optical light curve shows a very bright onset peak at about 100 s for 13 mag of [Formula: see text]-band, while the X-ray light curve occurs several flares at the beginning. We investigate the origin of X-ray and optical afterglows by analyzing the optical and X-ray data. Considering the smooth onset bump in the early time of the optical band and the erratic pulses for the X-ray lightcurve, we suppose that the early optical afterglow may be produced by the external shock, while the early time of X-ray light curve is dominated by flares. Therefore, GRB optical afterglows with smooth onset bump features at early time are possibly produced by external — forward shock (FS). According to the fireball external-model, the temporal slopes of the onset bumps are determined by both the medium density profile and the electron spectral index. Therefore, the afterglow onset bumps would be an ideal probe to study the properties of the fireball and the circumburst medium. The density profile has a slope of [Formula: see text], which suggests that the circumburst environment of the GRB 161017A would be an intermediate regime that are between the homogeneous interstellar medium (ISM) and wind-type medium. In addition, the optical light curve of the GRB 161017A exhibits a plateau feature and rebrightening after the onset bump, which may be related to the long-acting central engine of GRBs.

The momentum budget of clustered supernova feedback in a 3D, magnetized medium

While the evolution of superbubbles driven by clustered supernovae has been studied by numerous authors, the resulting radial momentum yield is uncertain by as much as an order of magnitude depending on the computational methods and assumed properties of the surrounding interstellar medium (ISM). In this work, we study the origin of these discrepancies, and seek to determine the correct momentum budget for a homogeneous ISM. We carry out 3D hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations of clustered supernova explosions, using a Lagrangian method and checking for convergence with respect to resolution. We find that the terminal momentum of a shell driven by clustered supernovae is dictated primarily by the mixing rate across the contact discontinuity between the hot and cold phases, and that this energy mixing rate is dominated by numerical diffusion even at the highest resolution we can complete, 0.03 $M_\odot$. Magnetic fields also reduce the mixing rate, so that MHD simulations produce higher momentum yields than HD ones at equal resolution. As a result, we obtain only a lower limit on the momentum yield from clustered supernovae. Combining this with our previous 1D results, which provide an upper limit because they allow almost no mixing across the contact discontinuity, we conclude that the momentum yield per supernova from clustered supernovae in a homogeneous ISM is bounded between $2\times 10^5$ and $3\times 10^6$ $M_\odot$ km s$^{-1}$. A converged value for the simple homogeneous ISM remains elusive.

The Redshift Dependence of the Radio Flux of Gamma-Ray Bursts and Their Host Galaxies

Abstract Using multiwavelength observations of radio afterglows, we confirm the hypothesis that the flux density of gamma-ray bursts (GRBs) at a fixed observing frequency is invariable when the distance of the GRBs increases, which means the detection rate will be approximately independent of redshift. We study this behavior theoretically and find that it can be well explained by the standard forward shock model involving a thin shell expanding in either a homogeneous interstellar medium (ISM) or a wind environment. We also found that short GRBs and supernova-associated GRBs, which are at relatively smaller distances, marginally match the flux–redshift relationship and they could be outliers. We rule out the assumption that the medium density evolves with redshift as n ∝ (1 + z)4 from the current measurements of n and z for short and long GRBs. In addition, the possible dependence of host flux on the redshift is also investigated. We find that a similar redshift independence of the flux exists for host galaxies as well, which implies that the detection rate of radio hosts might also be independent of the redshift. It is also hinted that most radio hosts have spectral indices ranging from β h ≃ −1 to 2.5 in statistics. Finally, we predict the detection rates of radio afterglows by next-generation radio telescopes such as the Five-hundred meter Aperture Spherical Telescope (FAST) and the Square Kilometer Array (SKA).

Modeling the Multiband Afterglows of GRB 060614 and GRB 060908: Further Evidence for a Double Power-law Hard Electron Energy Spectrum

Abstract Electrons accelerated in relativistic collisionless shocks are usually assumed to follow a power-law energy distribution with an index of p. Observationally, although most gamma-ray bursts (GRBs) have afterglows that are consistent with p > 2, there are still a few GRBs suggestive of a hard (p < 2) electron energy spectrum. Our previous work showed that GRB 091127 gave strong evidence for a double power-law hard electron energy (DPLH) spectrum with 1 < p 1 < 2, p 2 > 2 and an “injection break” assumed as γ b ∝ γ q in the highly relativistic regime, where γ is the bulk Lorentz factor of the jet. In this paper, we show that GRB 060614 and GRB 060908 provide further evidence for such a DPLH spectrum. We interpret the multiband afterglow of GRB 060614 with the DPLH model in a homogeneous interstellar medium by taking into account a continuous energy injection process, while, for GRB 060908, a wind-like circumburst density profile is used. The two bursts, along with GRB 091127, suggest a similar behavior in the evolution of the injection break, with q ∼ 0.5. Whether this represents a universal law of the injection break remains uncertain and more afterglow observations such as these are needed to test this conjecture.

Constraining external reverse shock physics of gamma-ray bursts from ROTSE-III limits

Assuming that the early optical emission is dominated by the external reverse shock (RS) in the standard model of gamma-ray bursts (GRBs), we intend to constrain RS models with the initial Lorentz factor $\Gamma_0$ of the outflows based on the ROTSE-III observations. We consider two cases of the RS behavior: the relativistic shock and the non-relativistic shock. For homogeneous interstellar medium (ISM) and wind circum-burst environment, the constraints can be achieved by the fact that the peak flux $F_{\rm \nu}$ at the RS crossing time should be lower than the observed upper limit $F_{\rm \nu,limit}$. We consider the different spectral regimes that the observed optical frequency $\nu_{\rm opt}$ may locate in, which are divided by the orders for the minimum synchrotron frequency $\nu_{\rm m}$ and the cooling frequency $\nu_{\rm c}$. {\bf Considering the homogeneous and wind environment around GRBs, we find that the relativistic RS case can be constrained by the (upper and lower) limits of $\Gamma_0$ in a large range from about hundreds to thousands for 36 GRBs reported by ROTSE-III. The constraints on the non-relativistic RS case are achieved with limits of $\Gamma_0$ for 26 bursts ranging from $\sim 30$ to $\sim 350$.} The lower limits of $\Gamma_0$ achieved for the relativistic RS model is disfavored based on the previously discovered correlation between the initial Lorentz factor $\Gamma_0$ and the isotropic gamma-ray energy $E_{\rm \gamma, iso}$ released in prompt phase.

THE NATURE OF GAMMA-RAY EMISSION OF TYCHO'S SUPERNOVA REMNANT

The nature of the recently detected HE and VHE gamma-ray emission of Tycho's supernova remnant (SNR) is studied. A nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) is employed to investigate the properties of Tycho's SNR and their correspondence to the existing experimental data, taking into account that the ambient interstellar medium (ISM) is expected to be clumpy. It is demonstrated that the overall steep gamma-ray spectrum observed can be interpreted as the superposition of two spectra produced by the CR proton component in two different ISM phases: The first gamma-ray component, extending up to about $10^{14}$ eV, originates in the diluted warm ISM, whereas the second component, extending up to 100 GeV, comes from numerous dense, small-scale clouds embedded in this warm ISM. Given the consistency between acceleration theory and the observed properties of the nonthermal emission of Tycho's SNR, a very efficient production of nuclear CRs in Tycho's SNR is established. The excess of the GeV gamma-ray emission due to the clouds' contribution above the level expected in the case of a purely homogeneous ISM, is inevitably expected in the case of type Ia SNe.

GRB 051028: an intrinsically faint gamma-ray burst at high redshift?

We present multiwavelength observations of the gamma-ray burst GRB 051028 detected by HETE-2 in order to derive its afterglow emission parameters and to determine the reason for its optical faintness when compared to other events. Observations were taken in the optical (2.0m Himalayan Chandra Telescope, 1.34m Tautenburg, 4.2m William Herschel Telescope) and in X-rays (Swift/XRT) between 2.7 hours and 10 days after the onset of the event. The data can be interpreted by collimated emission in a jet with a typical value of $p$ = 2.4 which is moving in an homogeneous interstellar medium and with a cooling frequency nu_{c} still above the X-rays at 0.5 days after the burst onset. GRB 051028 can be classified as a ``gray'' or ``potentially dark'' GRB. On the basis of the combined optical and Swift/XRT data, we conclude that the reason for the optical dimness is not extra absorption in the host galaxy, but rather the GRB taking place at high-redshift.We also notice the very striking similarity with the optical lightcurve of GRB 050730, a burst with a spectroscopic redshift of 3.967, although GRB 051028 is about 3 mag fainter. We suggest that the bump could be explained by multiple energy injection episodes and that the burst is intrinsically faint when compared to the average afterglows detected since 1997. The non-detection of the host galaxy down to R = 25.1 is also consistent with the burst arising at high redshift, compatible with the published pseudo-z of 3.7 +/- 1.8.

Constraints on Heterogeneous Galactic Chemical Evolution from Meteoritic Stardust

Meteorites contain presolar grains that formed in stars prior to solar system formation and preserve a record of stellar and galactic evolutionary processes. We consider a stochastic model of heterogeneous mixing of supernova ejecta into a homogeneous interstellar medium (ISM), first presented by Lugaro et al. The predictions of the model for Si, Ti, and O isotopic ratios are compared with the compositions of presolar SiC, Al2O3, and MgAl2O4 grains. Model parameters that reproduce the range of Si isotopic ratios in SiC grains fail to reproduce the strong observed correlation between SiC Si and Ti isotopic ratios and the range of 18O/16O ratios in the oxide phases. The Si-Ti correlation in SiC implies that heterogeneous mixing of SNe ejecta can account for at most 40% of the range of grain isotopic compositions. This result further implies that the ejecta from diverse SNe are well mixed to the percent level in the ISM, consistent with recent results based on stellar and interstellar abundance measurements. However, the presolar grain data provide much more stringent limits on the range of relative element ratios (e.g., Mg/Fe) than can be derived from astronomical observations. The large scatter in metallicity observed for disk stars of a given age cannot be explained by heterogeneous mixing of stellar ejecta. The range of 18O/16O ratios measured in presolar oxide grains implies either that they formed in stars with a wider range of metallicities than the SiC progenitor stars or that their parent stars experienced moderate amounts of cool bottom processing.

Analytical Light Curves in the Realistic Model for Gamma‐Ray Burst Afterglows

Afterglow light curves are constructed analytically for realistic gamma-ray burst remnants decelerating in either a homogeneous interstellar medium or a stellar wind environment, taking into account the radiative loss of the blast wave, which affects the temporal behaviors significantly. Inverse Compton scattering, which plays an important role when the energy equipartition factor epsilon(e) of electrons is much larger than that of the magnetic field (epsilon(B)), is considered. The inverse Compton effect prolongs the fast cooling phase markedly, during which the relativistic shock is semiradiative and the radiation efficiency is approximately constant, epsilon = epsilon(e). It is further shown that the shock is still semiradiative for quite a long time after it transits into the slow cooling phase because of a slow decreasing rate of the radiation efficiency of electrons. The temporal decaying index of the X-ray afterglow light curve in this semiradiative phase is (3p + 2 + 2epsilon)/(4 - epsilon) in the interstellar medium case and [3p - 2 - (p - 2)epsilon]/2(2 - epsilon) in the stellar wind case, where p is the distribution index of the shock-accelerated electrons. Taking p = 2.2-2.3 as implied from the common shock acceleration mechanism and assuming epsilon(e) similar to 1/3, the temporal index is more consistent with the observed (alpha(X)) similar to 1.3 than the commonly used adiabatic one. The observability of the inverse Compton component in soft X-ray afterglows is also investigated. To manifest as a bump or even dominant in the X-ray afterglows during the relativistic stage, the density must be larger than similar to1-10 cm(-3) in the interstellar medium case, or the wind parameter A(*) must be larger than similar to1 in the stellar wind case.

Cylindrical Jet – Wind Interaction Model of Gamma-Ray Burst Afterglows

Observations on relativistic jets in radio galaxies, active galactic nuclei, and ``microquasars'' revealed that many of these outflows are cylindrical, not conical. So it is worthwhile to investigate the evolution of cylindrical jets in gamma-ray bursts. We discuss afterglows from cylindrical jets in a wind environment. Numerical results as well as analytic solutions in some special cases are presented. Our light curves are steeper compared to those in the homogeneous interstellar medium case, carefully considered by Cheng, Huang & Lu. We conclude that some afterglows, used to be interpreted as isotropic fireballs in a wind environment, can be fitted as well by cylindrical jets interacting with a wind.

Reflection‐shocked Gas in the Cygnus Loop Supernova Remnant

We performed spectroscopic X-ray observations of the eastern and northern regions of the Cygnus Loop with the ASCA observatory. The X-ray surface brightness of these regions shows a complex structure in the ROSAT all-sky survey image. We carried out a spatially resolved analysis for both regions and found that kTe did not increase toward the center region, but showed inhomogeneous structures. Such variation cannot be explained by a blast-wave model propagating into a homogeneous interstellar medium. We thus investigated the interaction between a blast wave and an interstellar cloud. Two major emission mechanisms are plausible: a cloud evaporation model and a reflection-shock model. In both regions, only a reflection-shock model qualitatively explains our results. Our results suggest the existence of a large-scale interstellar cloud. We suppose that such a large-scale structure would be produced by a precursor.

Neutrino Afterglows and Progenitors of Gamma‐Ray Bursts

Currently popular models for progenitors of gamma-ray bursts (GRBs) are the mergers of compact objects and the explosions of massive stars. These two cases have distinctive environments for GRBs: compact object mergers occur in the interstellar medium (ISM) and the explosions of massive stars occur in the preburst stellar wind. We here discuss neutrino afterglows from reverse shocks as a result of the interaction of relativistic fireballs with their surrounding wind matter. After comparing with the analytical result of Waxman & Bahcall for the homogeneous ISM case, we find that the differential spectrum of neutrinos with energy from similar to3 x 10(15) to similar to3 x 10(17) eV in the wind case is softer by 1 power of the energy than in the ISM case. Furthermore, the expected flux of upward moving muons produced by neutrino interactions below a detector on the surface of the Earth in the wind case is similar to5 events yr(-1) km(-2), which is about 1 order of magnitude larger than in the ISM case. In addition, these properties are independent of whether the fireballs are isotropic or beamed. Therefore, neutrino afterglows, if detected, may provide a way of distinguishing between GRB progenitor models based on the differential spectra of neutrinos and their event rates in a detector.

The abundance of radioactive nuclei under different physical environments around the solar system

Abstract Differential equations describing the propagation of cosmic-ray nuclei from 7Be to 56Fe in a homogeneous interstellar medium (ISM) were solved simultaneously in the time domain, so as to determine the escape path length using the observed B/C ratio as a constraint. The spectra of nuclei thus obtained were then used to obtain the 10Be/9Be ratio as a function of energy. From a comparison with the data, we determined the mean density of nH in the confinement volume of cosmic rays to be 0.2 atom/cc and the corresponding escape life time of 30 My. The effect of the solar system being in a “local bubble” of low density was examined. It was found that if cosmic-ray nuclei have spent considerable amount of time in the local bubble, the mean value of nH in the Galaxy needs to be raised to account for the observation. The ratio of 26Al/27Al and 36Cl/Cl are more sensitive to the propagation in the local hole and our study showed that in order that the data are consistent with each other, the diffusion co-efficient in the local bubble should be > 1029cm2/s. This clearly demonstrates the need to include in the diffusion models the variation of diffusion co-efficient with gas density. We then examined the effect of the local hole on the abundance ratio of 14C/13C and bring out the need to measure 14C to further constrain the diffusion co-efficient in the local hole.