arXiv subjects

Noam Soker

Publications and source records attributed to Noam Soker.

At least 343 records · Page 19Linked to original sources

arXiv2009

Correlation of Black Hole-Bulge Masses by AGN Jets

I propose a feedback model to explain the correlation between the supermassive black hole (SMBH) mass and the host galaxy bulge mass. The feedback is based on narrow jets that are launched by the central SMBH, and expel large amounts of mass to large distances. The condition is that the jets do not penetrate through the inflowing gas, such that they can deposit their energy in the inner region where the bulge is formed. For that to occur, the SMBH must move relative to the inflowing gas, such that the jets continuously encounter fresh gas. Taking into account the relative motion of the SMBH and the inflowing gas I derive a relation between the mass accreted by the SMBH and the mass that is not expelled, and is assumed to form the bulge. This relation is not linear, but rather the SMBH to bulge mass ratio increases slowly with mass. The same mechanism was applied to suppress star formation in cooling flow clusters, making a tighter connection between the feedback in galaxy formation and cooling flows.

astro-ph.CO↗

arXiv2009

Explaining the Energetic AGN Outburst of MS0735+7421 with Massive Slow Jets

By conducting axisymmetrical hydrodynamical numerical simulations (2.5 dimensional code) we show that slow, massive, wide jets can reproduce the morphology of the huge X-ray deficient bubble pair in the cluster of galaxies MS0735+7421. The total energy of the jets, composed of the energy in the bubble pair and in the shock wave, is constraint by observations conducted by McNamara et al. (2009) to be ~10^{62}erg. We show that two opposite jets that are active for ~100Myr, each with a launching half opening angle of ~70 degrees, an initial velocity of ~0.1c, and a total mass loss rate of the two jets of ~100 Mo/year, can account for the observed morphology. Rapidly precessing narrow jets can be used instead of wide jets. In our model the cluster suffered from a cooling catastrophe ~100Myr ago. Most of the mass that cooled, ~10^{10} Mo, was expelled back to the intracluster medium (ICM) by the AGN activity and is inside the bubbles now, ~10% formed stars, and ~10% of the cold gas was accreted by the central black hole and was the source of the outburst energy. This type of activity is similar to that expected to occur in galaxy formation.

astro-ph.CO↗

arXiv2009

Interpreting a Dwarf Nova Eruption as Magnetic Flare Activity

We suggest that the radio emission from the dwarf nova SS Cyg during outburst comes from magnetic activity that formed a corona (similar to coronae found in magnetically active stars), rather than from jets. We base our claim on the recent results of Laor & Behar, who found that when the ratio between radio and X-ray flux of accretion disks in radio-quiet quasars is as in active stars, Lr/Lx=10^{-5}, then most of the radio emission comes from coronae. Using observations from the literature we find that for SS Cyg during outburst Lr/Lx<10^{-5}. This does not mean jets are not launched during outbursts. On the contrary, if the magnetic activity in erupting accreting disks is similar to that in stars, then mass ejection, e.g., as in coronal mass ejection, is expected. Hence magnetic flares similar to those in active stars might be the main mechanism for launching jets in a variety of systems, from young stellar objects to massive black holes.

astro-ph.HE↗

arXiv2009

Narrow Radiative Recombination Continua: A Signature of Ions Crossing the Contact Discontinuity of Astrophysical Shocks

X-rays from planetary nebulae (PNs) are believed to originate from a shock driven into the fast stellar wind (v ~ 1000 km/s) as it collides with an earlier circumstellar slow wind (v ~ 10 km/s). In theory, the shocked fast wind (hot bubble) and the ambient cold nebula can remain separated by magnetic fields along a surface referred to as the contact discontinuity (CD) that inhibits diffusion and heat conduction. The CD region is extremely difficult to probe directly owing to its small size and faint emission. This has largely left the study of CDs, stellar-shocks, and the associated micro-physics in the realm of theory. This paper presents spectroscopic evidence for ions from the hot bubble (kT ~ 100 eV) crossing the CD and penetrating the cold nebular gas (kT ~ 1 eV). Specifically, a narrow radiative recombination continuum (RRC) emission feature is identified in the high resolution X-ray spectrum of the PN BD+30 3639 indicating bare C VII ions are recombining with cool electrons at kT_e=1.7+-1.3 eV. An upper limit to the flux of the narrow RRC of H-like C VI is obtained as well. The RRCs are interpreted as due to C ions from the hot bubble of BD+30 3639 crossing the CD into the cold nebula, where they ultimately recombine with its cool electrons. The RRC flux ratio of C VII to C VI constrains the temperature jump across the CD to Delta kT > 80 eV, providing for the first time direct evidence for the stark temperature disparity between the two sides of an astrophysical CD, and constraining the role of magnetic fields and heat conduction accordingly. Two colliding-wind binaries are noted to have similar RRCs suggesting a temperature jump and CD crossing by ions may be a common feature of stellar wind shocks.

astro-ph.HE↗

arXiv2009

Using X-Ray Observations to Explore the Binary Interaction in Eta Carinae

We study the usage of the X-ray light curve, column density toward the hard X-ray source, and emission measure (density square times volume), of the massive binary system Eta Carinae to determine the orientation of its semi-major axis. The source of the hard X-ray emission is the shocked secondary wind. We argue that, by itself, the observed X-ray flux cannot teach us much about the orientation of the semi-major axis. Minor adjustment of some unknown parameters of the binary system allows to fit the X-ray light curve with almost any inclination angle and orientation. The column density and X-ray emission measure, on the other hand, impose strong constrains on the orientation. We improve our previous calculations and show that the column density is more compatible with an orientation where for most of the time the secondary - the hotter, less massive star - is behind the primary star. The secondary comes closer to the observer only for a short time near periastron passage. The ten-week X-ray deep minimum, which results from a large decrease in the emission measure, implies that the regular secondary wind is substantially suppressed during that period. This suppression is most likely resulted by accretion of mass from the dense wind of the primary luminous blue variable (LBV) star. The accretion from the equatorial plane might lead to the formation of a polar outflow. We suggest that the polar outflow contributes to the soft X-ray emission during the X-ray minimum; the other source is the shocked secondary wind in the tail. The conclusion that accretion occurs at each periastron passage, every five and a half years, implies that accretion had occurred at a much higher rate during the 20 Great Eruption of Eta Carinae in the 19th century.

astro-ph.SR↗

arXiv2009

Very Late Thermal Pulses Influenced by Accretion in Planetary Nebulae

We consider the possibility that a mass of ~10^{-5}-10^{-3} Msun flows back from the dense shell of planetary nebulae and is accreted by the central star during the planetary nebula phase. This backflowing mass is expected to have a significant specific angular momentum even in (rare) spherical planetary nebulae, such that a transient accretion disk might be formed. This mass might influence the occurrence and properties of a very late thermal pulse (VLTP), and might even trigger it. For example, the rapidly rotating outer layer, and the disk if still exist, might lead to axisymmetrical mass ejection by the VLTP. Unstable burning of accreted hydrogen might result in a mild flash of the hydrogen shell, also accompanied by axisymmetrical ejection.

astro-ph.SR↗

arXiv2009

Triggering Eruptive Mass Ejection in Luminous Blue Variables

We study the runaway mass loss process of major eruptions of luminous blue variables (LBVs) stars, such as the 1837-1856 Great Eruption of Eta Carinae. We follow the evolution of a massive star with a spherical stellar evolution numerical code. After the star exhausted most of the hydrogen in the core and had developed a large envelope, we remove mass at a rate of 1 Mo/year from the outer envelope for 20 years. We find that after removing a small amount of mass at a high rate, the star contracts and releases a huge amount of gravitational energy. We suggest that this energy can sustain the high mass loss rate. The triggering of this runaway mass loss process might be a close stellar companion or internal structural changes. We show that a strong magnetic field region can be built in the radiative zone above the convective core of the evolved massive star. When this magnetic energy is released it might trigger a fast removal of mass, and by that trigger an eruption. Namely, LBV major eruptions might be triggered by magnetic activity cycles. The prediction is that LBV stars that experience major eruptions should be found to have a close companion and/or have signatures of strong magnetic activity during or after the eruption.

astro-ph.SR↗

arXiv2008

Prediction for the He I 10830A Absorption Wing in the Coming Event of Eta Carinae

We propose an explanation to the puzzling appearance of a wide blue absorption wing in the He I 10830A P-Cygni profile of the massive binary star Eta Carinae several months before periastron passage. Our basic assumption is that the colliding winds region is responsible for the blue wing absorption. By fitting observations, we find that the maximum outflow velocity of this absorbing material is ~2300 km/s. We also assume that the secondary star is toward the observer at periastron passage. With a toy-model we achieve two significant results. (1) We show that the semimajor axis orientation we use can account for the appearance and evolution of the wide blue wing under our basic assumption. (2) We predict that the Doppler shift (the edge of the absorption profile) will reach a maximum 0-3 weeks before periastron passage, and not necessarily exactly at periastron passage or after periastron passage.

astro-ph↗

arXiv2008

The Orientation of the Eta Carinae Binary System

We examine a variety of observations that shed light on the orientation of the semi-major axis of the Eta Carinae massive binary system. Under several assumptions we study the following observations: The Doppler shifts of some He I P-Cygni lines that is attributed to the secondary's wind, of one Fe II line that is attributed to the primary's wind, and of the Paschen emission lines which are attributed to the shocked primary's wind, are computed in our model and compared with observations. We compute the hydrogen column density toward the binary system in our model, and find a good agreement with that deduced from X-ray observations. We calculate the ionization of surrounding gas blobs by the radiation of the hotter secondary star, and compare with observations of a highly excited [Ar III] narrow line. We find that all of these support an orientation where for most of the time the secondary - the hotter less massive star - is behind the primary star. The secondary comes closer to the observer only for a short time near periastron passage, in its highly eccentric (e~0.9) orbit. Further supporting arguments are also listed, followed by discussion of some open and complicated issues.

astro-ph↗

arXiv2008

The X-Ray Spectrum of a Planetary Nebula at High Resolution: Chandra Gratings Spectroscopy of BD+30 3639

We present the results of the first X-ray gratings spectroscopy observations of a planetary nebula (PN), the X-ray-bright, young BD+30 3639. We observed BD+30 3639 for a total of 300 ks with the Chandra X-ray Observatory's Low Energy Transmission Gratings in combination with its Advanced CCD Imaging Spectrometer(LETG/ACIS-S). The LETG/ACIS-S spectrum of BD+30 3639 is dominated by H-like resonance lines of O viii and C sc vi and the He-like triplet line complexes of Ne ix and O vii. Other H-like resonance lines, such as N vii, as well as lines of highly ionized Fe, are weak or absent. Continuum emission is evident over the range 6-18 A. Spectral modeling indicates the presence of a range of plasma temperatures from T~1.7x10^6 K to 2.9x10^6 K and an intervening absorbing column N_H~2.4x10^21 cm-2. The same modeling conclusively demonstrates that C and Ne are highly enhanced, with abundance ratios of C/O~15-45 and Ne/O~3.3-5.0 (90% confidence ranges, relative to the solar ratios), while N and Fe are depleted, N/O~0.0-1.0 and Fe/O~0.1-0.4. The intrinsic luminosity of the X-ray source determined from the modeling and the measured flux (F_X = 4.1x10^-13 ergs cm-2 s-1) is L_X~8.6x10^32 erg s-1(assuming D = 1.2kpc). These gratings spectroscopy results are generally consistent with earlier results obtained from X-ray CCD imaging spectroscopy of BD+30 3639, but are far more precise. The tight constraints placed on the (nonsolar) abundances directly implicate the present-day central star -- hence, ultimately, the intershell region of the progenitor asymptotic giant branch star -- as the origin of the shocked plasma now emitting in X-rays.

astro-ph↗

arXiv2008

Sound Waves Excitation by Jet-Inflated Bubbles in Clusters of Galaxies

We show that repeated sound waves in the intracluster medium (ICM) can be excited by a single inflation episode of an opposite bubble pair. To reproduce this behavior in numerical simulations the bubbles should be inflated by jets, rather than being injected artificially. The multiple sound waves are excited by the motion of the bubble-ICM boundary that is caused by vortices inside the inflated bubbles and the backflow (`cocoon') of the ICM around the bubble. These sound waves form a structure that can account for the ripples observed in the Perseus cooling flow cluster. We inflate the bubbles using slow massive jets, with either a wide opening angle or that are precessing. The jets are slow in the sense that they are highly sub-relativistic, $v_j \sim 0.01c-0.1c$, and they are massive in the sense that the pair of bubbles carry back to the ICM a large fraction of the cooling mass, i.e., $\sim 1-50 M_\odot \yr^{-1}$. We use a two-dimensional axisymmetric (referred to as 2.5D) hydrodynamical numerical code (VH-1).

astro-ph↗

arXiv2008

Entropy Limit and the Cold Feedback Mechanism in Cooling Flow Clusters

I propose an explanation to the finding that star formation and visible filaments strong in Halpha emission in cooling flow clusters occur only if the minimum specific entropy and the radiative cooling time of the intracluster medium (ICM), are below a specific threshold. The explanation is based on the cold feedback mechanism. In this mechanism the mass accreted by the central black hole originates in non-linear over-dense blobs of gas residing in an extended region of the cooling flow region. I use the criterion that the feedback cycle period must be longer than the radiative cooling time of dense blobs for large quantities of gas to cool to low temperature. The falling time of the dense blobs is parameterized by the ratio of the infall velocity to the sound speed. Another parameter is the ratio of the blobs' density to that of the surrounding ICM. By taking the values of the parameters as in previous papers on the cold feedback model, I derive an expression that gives the right value of the entropy threshold. Future studies will have to examine in more detail the role of these parameters, and to show that the observed sharp change in the behavior of clusters across the entropy, or radiative cooling time, threshold can be reproduced by the model.

astro-ph↗

arXiv2008

Rising Jet-Inflated Bubbles in Clusters of Galaxies

We conduct two-dimensional axisymmetric (referred to as 2.5D) hydrodynamical numerical simulations of bubble evolution in clusters of galaxies. We inflate bubbles using slow, massive jets with a wide opening angle, and follow their evolution as they rise through the intra-cluster medium (ICM). We find that these jet-inflated bubbles are quite stable, and can reach large distances in the cluster while still maintaining their basic structure. The stability of the jet-inflated bubble comes mainly from the dense shell that forms around it during it's inflation stage, and from the outward momentum of the bubble and the shell. On the contrary, bubbles that are inserted by hand onto the grid and not inflated by a jet, i.e., an artificial bubble, lack these stabilizing factors, therefore, they are rapidly destroyed. The stability of the jet-inflated bubble removes the demand for stabilizing magnetic fields in the bubble.

astro-ph↗

arXiv2008

Shaping Planetary Nebulae by Light Jets

We conduct numerical simulations of axisymmetrical jets expanding into a spherical AGB slow wind. The three-dimensional flow is simulated with an axially symmetric numerical code. We concentrate on jets that are active for a relatively short time. Our results strengthen other studies that show that jets can account for many morphological features observed in planetary nebulae (PNs). Our main results are as follows. (1) With a single jet's launching episode we can reproduce a lobe structure having a `front-lobe', i.e., a small bulge on the front of the main lobe, such as that in the PN Mz~3. (2) In some runs dense clumps are formed along the symmetry axis, such as those observed in the pre-PN M1-92. (3) The mass loss history of the slow wind has a profound influence on the PN structure. (4) A dense expanding torus (ring; disk) is formed in most of our runs. The torus is formed from the inflated lobes, and not from a separate equatorial mass loss episode. (5) The torus and lobes are formed at the same time and from the same mass loss rate episode. However, when the slow wind density is steep enough, the ratio of the distance divided by the radial velocity is larger for regions closer to the equatorial plane than for regions closer to the symmetry axis. (6) With the short jet-active phase a linear relation between distance and expansion velocity is obtained in many cases. (7) Regions at the front of the lobe are moving sufficiently fast to excite some visible emission lines.

astro-ph↗

arXiv2008

Possible Implications of Mass Accretion in Eta Carinae

We apply the previously suggested accretion model for the behavior of the super-massive binary system Eta Carinae close to periastron passages. In that model it is assumed that for ~10 weeks near periastron passages one star is accreting mass from the slow dense wind blown by the other star. We find that the secondary, the less massive star, accretes ~2x10^{-6}Mo. This mass possesses enough angular momentum to form a disk, or a belt, around the secondary. The viscous time is too long for the establishment of equilibrium, and the belt must be dissipated as its mass is being blown in the reestablished secondary wind. This processes requires about half a year, which we identify with the recovery phase of Eta Carinae. We show that radiation pressure, termed radiative braking, cannot prevent accretion. In addition to using the commonly assumed binary model for Eta Carinae, we also examine alternative models where the stellar masses are larger, and/or the less massive secondary blows the slow dense wind, while the primary blows the tenuous fast wind and accretes mass for ~10 weeks near periastron passages. We end by some predictions for the next event (January-March 2009).

astro-ph↗

arXiv2008

Accretion onto the Companion of Eta Carinae During the Spectroscopic Event. V. the Infrared Decline

We propose that the decline in the near-IR flux from the massive binary system Eta Carinae during the spectroscopic event might be explained by accreted mass that absorbs the radiation from the secondary star, and by that reduces the heating of the dust that is responsible for the near-IR emission. This binary system has an orbital period of 2024 days and eccentricity of e~0.9. The emission in several bands declines for several weeks near every periastron passages, in what is termed the spectroscopic event. In the accretion model for the spectroscopic event the secondary star accretes mass from the primary's wind for ~10 weeks near every periastron passage. The mass is accreted mainly in the equatorial plane. The disk and its wind block the secondary's radiation from heating dust that does not reside within narrow cones along the symmetry axis. This, we propose, might explain the decline in the near-IR flux occurring at the beginning of each spectroscopic event. We also argue that the increase in the near-IR prior to the event might be accounted for by enhanced hot (T~1700 K) dust formation in the collision region of the winds from the two stars. This dust resides within ~60 degrees from the equatorial plane, and most of it cannot be heated by the secondary during the accretion phase.

astro-ph↗

arXiv2008

A Phenomenological Model for the Extended Zone Above AGB Stars

I suggest the existence of an extended zone above the surface of asymptotic giant branch (AGB), as well as similar stars experiencing high mass loss rates. In addition to the escaping wind, in this zone there are parcels of gas that do not reach the escape velocity. These parcels of dense gas rise slowly and then fall back. The wind and bound gas exist simultaneously to distances of ~100AU. I term this region the effervescent zone. In this phenomenological study I find that the density of the bound material in the effervescent zone falls as ~r^{-5/2}, not much faster than the wind density. The main motivation to propose the effervescent model is to allow wide binary companions to influence the morphology of the descendant planetary nebulae (PN) by accreting mass from the effervescent zone. Accretion from the effervescent zone is more efficient than accretion from the wind in forming an accretion disk around the companion. The companion might then blow two jets that will shape the descendant PN.

astro-ph↗

arXiv2008

Inflating Fat Bubbles in Clusters of Galaxies by Precessing Massive Slow Jets

We conduct hydrodynamical numerical simulations and find that precessing massive slow jets can inflate fat bubbles, i.e., more or less spherical bubbles, that are attached to the center of clusters of galaxies. To inflate a fat bubble the jet should precess fast. The precessing angle $θ$ should be large, or change over a large range $ 0 \le θ\le θ_{\max} \sim 30-70 ^\circ$ (depending also on other parameters), where $θ=0$ is the symmetry axis. The constraints on the velocity and mass outflow rate are similar to those on wide jets to inflate fat bubbles. The velocity should be $v_j \sim 10^4 \kms$, and the mass loss rate of the two jets should be $ 2 \dot M_j \simeq 1-50 \dot M_\odot \yr^{-1} $. These results, and our results from a previous paper dealing with slow wide jets, support the claim that a large fraction of the feedback heating in cooling flow clusters and in the processes of galaxy formation is done by slow massive jets.

astro-ph↗