arXiv subjects

Nir J. Shaviv

Publications and source records attributed to Nir J. Shaviv.

At least 37 records · Page 2Linked to original sources

arXiv2011

A Lower Limit on the Halo Mass to form Supermassive Black Holes

We consider a scenario where supermassive black holes form through direct accumulation of gas at the centre of proto-galaxies. In the first stage, the accumulated gas forms a super-massive star whose core collapses when the nuclear fuel is exhausted, forming a black hole of $M_{\rm BH} \approx 100 M_{\sun}$. As the black hole starts accreting, it inflates the surrounding dense gas into an almost hydrostatic self-gravitating envelope, with at least 10-100 times the mass of the hole. We find that these "quasistars" suffer extremely high rates of mass loss through winds from their envelopes, in analogy to very massive stars such as eta-Carinae. Only for envelope masses greater than 2.8 \times 10^{5} (M_{\rm BH}/100 M_{\sun})^{9/11} is the envelope evaporation time-scale longer than the accretion time-scale of the black hole. This relation thus constitutes a "threshold growth line" above which quasistars can grow their internal black holes. Accretion rates can be 10 to 100 times the Eddington rate. The quasistars born in this "growth region" with 10^7-10^8 M_{\sun} can grow black holes with masses between 10^4$ to $10^5 M_{\sun}, before crossing the threshold growth line and dispersing their envelopes in less than $10^4$ yr. This scenario therefore predicts that massive black hole seeds can be found only in dark matter halos with total masses larger than about 10^9 M_{\sun}, which can provide sufficiently high accretion rates to form such massive quasistars.

astro-ph.HE↗

arXiv2010

Super Eddington Slim Accretion Disks with Winds

We construct Super-Eddington Slim Disks models around both stellar and super-massive black holes by allowing the formation of a porous layer with a reduced effective opacity. We show that at high accretion rates, the inner part of the disks become radiation pressure dominated. However, unlike the standard scenario in which the disks become thick, super-Eddington disks remain slim. In addition, they accelerate a significant wind with a "thick disk" geometry. We show that above about 1.5 times the standard critical mass accretion rate (needed to release the Eddington luminosity), the net luminosity released is above Eddington. At above about 5 times the standard critical rate, the central BH accretes more than the Eddington accretion rate. Above about 20 m-dot_crit, the disk remains slim but the wind becomes spherical, and the present model breaks down.

astro-ph.HE↗

arXiv2009

Inhomogeneity in the Supernova Remnants as a Natural Explanation of the PAMELA/ATIC Observations

Recent measurements of the positron/electron ratio in the cosmic ray (CR) flux exhibits an apparent anomaly, whereby this ratio increases between 10 and 100 GeV. In contrast, this ratio should decrease according to the standard scenario, in which CR positrons are secondaries formed by hadronic interactions between the primary CR protons and the interstellar medium (ISM). The positron excess is therefore interpreted as evidence for either an annihilation/decay of weakly interacting massive particles, or for a direct astrophysical source of pairs. The common feature of all proposed models is that they invoke new physics or new astrophysical sources. However, this line of argumentation relies implicitly on the assumption of a relatively homogeneous CR source distribution. Inhomogeneity of CR sources on a scale of order a kpc, can naturally explain this anomaly. If the nearest major CR source is about a kpc away, then low energy electrons (~GeV) can easily reach us. At higher energies (> 10 GeV), the source electrons cool via synchrotron and inverse-Compton before reaching the solar vicinity. Pairs formed in the local vicinity through the proton/ISM interactions can reach the solar system also at high energies, thus increasing the positron/electron ratio. A natural origin of source inhomogeneity is the strong concentration of supernovae to the galactic spiral arms. Assuming supernova remnants (SNRs) as the sole primary source of CRs, and taking into account their concentration near the galactic spiral arms, we consistently predict the observed positron fraction between 1 and 100 GeV, while abiding to different constraints such as the observed electron spectrum and the CRs cosmogenic age. An ATIC like spectral excess at ~600 GeV can be explained, in this picture, as the contribution of a few known nearby SNRs.

astro-ph.HE↗

arXiv2009

Inhomogeneity in the Supernova Remnant Distribution as the Origin of the PAMELA Anomaly

Recent measurements of the positron/electron ratio in the cosmic ray (CR) flux exhibits an apparent anomaly, whereby this ratio increases between 10 and 100 GeV. We show that inhomogeneity of CR sources on a scale of order a kpc, can naturally explain this anomaly. If the nearest major CR source is about a kpc away, then low energy electrons ($\sim 1$ GeV) can easily reach us. At higher energies ($\gtrsim 10$ GeV), the source electrons cool via synchrotron and inverse-Compton before reaching Earth. Pairs formed in the local vicinity through the proton/ISM interactions can reach Earth also at high energies, thus increasing the positron/electron ratio. A natural origin of source inhomogeneity is the strong concentration of supernovae in the galactic spiral arms. Assuming supernova remnants (SNRs) as the sole primary source of CRs, and taking into account their concentration near the galactic spiral arms, we consistently recover the observed positron fraction between 1 and 100 GeV. ATIC's electron excess at $\sim 600$ GeV is explained, in this picture, as the contribution of a few known nearby SNRs. The apparent coincident similarity between the cooling time of electrons at 10 GeV (where the positron/electron ratio upturn), $\sim 10$ Myr, and the CRs protons cosmogenic age at the same energy is predicted by this model.

astro-ph.HE↗

arXiv2008

On the behaviour of stellar winds that exceed the photon-tiring limit

Stars can produce steady-state winds through radiative driving as long as the mechanical luminosity of the wind does not exceed the radiative luminosity at its base. This upper bound on the mass loss rate is known as the photon-tiring limit. Once above this limit, the radiation field is unable to lift all the material out of the gravitational potential of the star, such that only part of it can escape and reach infinity. The rest stalls and falls back toward the stellar surface, making a steady-state wind impossible. Photon-tiring is not an issue for line-driven winds since they cannot achieve sufficiently high mass loss rates. It can however become important if the star exceeds the Eddington limit and continuum interaction becomes the dominant driving mechanism. This paper investigates the time-dependent behaviour of stellar winds that exceed the photon-tiring limit, using 1-D numerical simulations of a porosity moderated, continuum-driven stellar wind. We find that the regions close to the star show a hierarchical pattern of high density shells moving back and forth, unable to escape the gravitational potential of the star. At larger distances, the flow eventually becomes uniformly outward, though still quite variable. Typically, these winds have a very high density but a terminal flow speed well below the escape speed at the stellar surface. Since most of the radiative luminosity of the star is used to drive the stellar wind, such stars would appear much dimmer than expected from the super-Eddington energy generation at their core. The visible luminosity typically constitutes less then half of the total energy flow and can become as low as ten percent or less for those stars that exceed the photon-tiring limit by a large margin.

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arXiv2006

A remark on ``On the Formation and Progenitor of PSR J0737-3039: New Constraints on the Supernova Explosion Forming Pulsar B" by Willems et al., astro-ph/0602024

Shortly after the discovery of the binary system PSR J0737-3039 we predicted that it has a small proper motion and suggested that this implies that the progenitor mass of the younger pulsar (B) must be around 1.45M_sun. This is in contradiction to standard evolutionary scenarios that suggest a He-star progenitor with M >2.1-2.3M_sun and in sharp contrast to a velocity of ~100 km/s predicted by these models. It requires a new previously unseen type of gravitational collapse. Our (generally ignored) prediction of a low peculiar motion was confirmed later by pulsar timing observations that found v_\bot < 30 km/s. Recently Willems et al. put forward an elaborate simulation suggesting that in spite of these observations, the most likely natal kick is large (50-170 km/s) and the corresponding most likely progenitor mass is large as well. We refute this claim and show that Willems et al. implicitly assume the result that they obtain.

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arXiv2005

The Origin of the Binary Pulsar J0737-3039

It is generally accepted that neutron stars form in core collapse events that are accompanied by a supernovae (types II or Ib or Ic). Typical progenitors are, therefore, larger than $\sim 2.1 \Ms$. We suggest \cite{PS04,PS05} that the binary pulsar J0737-3039 provides evidence for a new formation channel: collapse of a light progenitor. This binary pulsar J0737-3039 has several remarkable features including among others: a very tight orbit with a Keplerian velocity of ~600km/sec, a low eccentricity, and a location ~50pc from the Galactic plane implying that the system has, at high likelihood, a small (compared to Keplerian) center of mass velocity. A significant mass loss during the formation of the second pulsar would have lead either to an eccentric orbit or to a large center of mass velocity or to both. Therefore, we can set a strong upper limit on the progenitor's mass. A progenitor more massive than 1.9$\Ms$ is ruled out (at 97% confidence). The kinematically favored option is of a progenitor mass around 1.45$\Ms$. Recent evidence for a rather low velocity proper motion supports this prediction and decreases the likelihood of the standard (high mass progenitor) scenario. Lack of variations in the pulses' profiles (which indicate no significant geodetic precession) provides further support for the no kick and low progenitor mass formation scenario.

astro-ph↗

arXiv2005

Open Cluster Birth Analysis and Multiple Spiral Arm Sets in the Milky Way

The Milky Way spiral arm dynamics is studied using the birth place of open clusters. We separately study the nearby spiral arms, and find evidence for multiple spiral sets. In particular, the Sagittarius-Carina arm appears to be a superposition of two sets. The first has a pattern speed of Omega_P,Carina,1=16.5^{+1.2}_{-1.4sys}+/-1.1_{stat} (km/s)/kpc, while the second has Omega_P,Carina,2=29.8^{+0.6}_{-1.3sys}+/-1.3_{stat} (km/s)/kpc. The Perseus arm located at larger galactic radii exhibits only one clear set with Omega_P,Perseus=20.0^{+1.7}_{-1.2sys}+/-1.6_{stat} (km/s)/kpc. The Perseus and "slower" Carina arms are most probably part of an m=4 set. The Orion "armlet" appears to have a pattern speed of Omega_P,Orion=28.9 ^{+1.3}_{-1.2sys}+/-0.8_{stat} (km/s)/kpc. In other words, both the "faster" Carina arm and the Orion arm are nearly corotating with the solar system.

astro-ph↗

arXiv2005

The High-Energy Polarization-Limiting Radius of Neutron Star Magnetospheres II -- Magnetized Hydrogen Atmospheres

In the presence of strong magnetic fields, the vacuum becomes a birefringent medium. We show that this QED effect couples the direction of the polarization of photons leaving the NS surface, to the direction of the magnetic field along the ray's path. We analyze the consequences that this effect has on aligning the polarization vectors to generate large net polarizations, while considering thermal radiation originating from a thermal hydrogen atmosphere. Counter to previous predictions, we show that the thermal radiation should be highly polarized even in the optical. When detected, this polarization will be the first demonstration of vacuum birefringence. It could be used as a tool to prove the high magnetic field nature of AXPs and it could also be used to constrain physical NS parameters, such as $R/M$, to which the net polarization is sensitive.

astro-ph↗

arXiv2004

The Origin of the Binary Pulsar J0737-3039B

Evolutionary scenarios suggest that the progenitor of the new binary pulsar J0737-3039B \cite{ref1,ref2} was a He-star with $M > 2.1-2.3~\Ms$ \cite{ref3,ref4}. We show that this case implies that the binary must have a large ($>120$ km/s) center of mass velocity. However, the location, $\sim 50$ pc from the Galactic plane, suggests that the system has, at high likelihood, a significantly smaller center of mass velocity and a progenitor more massive than 2.1~$\Ms$ is ruled out (at 97% c.l.). A progenitor mass around 1.45~$\Ms$, involving a new previously unseen gravitational collapse, is kinematically favored. The low mass progenitor is consistent with the recent scintillations based velocity measurement of 66$\pm 15$ km/s \cite{ref12new} (and which also rules out the high mass solution at 99% c.l.) and inconsistent with the higher earlier estimates of 141$\pm 8.5$ km/s \cite{ref11new}. Direct proper motion measurements, that should be available within a year or so, should better help to distinguish between the two scenarios.

astro-ph↗

arXiv2004

On climate response to changes in the cosmic ray flux and radiative budget

We examine the results linking cosmic ray flux (CRF) variations to global climate change. We then proceed to study various periods over which there are estimates for the radiative forcing, temperature change and CRF variations relative to today. These include the Phanerozoic as a whole, the Cretaceous, the Eocene, the Last Glacial Maximum, the 20th century, as well as the 11-yr solar cycle. This enables us to place quantitative limits on climate sensitivity to both changes in the CRF, Phi_CR, and the radiative budget, F, under equilibrium. Under the assumption that the CRF is indeed a climate driver, we find that the sensitivity to CRF variations is consistently fitted with mu := -Phi_0 (dT_global/ d Phi_CR) = 6.5 +/- 2.5 K (where Phi_0 is the CR energy flux today). Additionally, the sensitivity to radiative forcing changes is lambda := dT_global/ dF_0 = 0.35 +/- 0.09 K/(W/m^2), at the current temperature, while its temperature derivative is negligible with d lambda / dT_0 = 0.01 +/- 0.03 1/(W/m^2). If the observed CRF/climate link is ignored, the best sensitivity obtained is lambda = 0.54 +/- 0.12 K/(W/m^2) and d lambda / dT_0 = -0.02 +/- 0.05 1/(W/m^2). The CRF/climate link therefore implies that the increased solar luminosity and reduced CRF over the previous century should have contributed a warming of 0.37+/-0.13 K, while the rest should be mainly attributed to anthropogenic causes. Without any effect of cosmic rays, the increase in solar luminosity would correspond to an increased temperature of 0.16+/-0.04 K.

physics.ao-ph↗

arXiv2004

A Porosity-Length Formalism for Photon-Tiring-Limited Mass Loss from Stars Above the Eddington Limit

We examine radiatively driven mass loss from stars near and above the Eddington limit (Ledd). We begin by reviewing the instabilities that are expected to form extensive structure near Ledd. We investigate how this "porosity" can reduce the effective coupling between the matter and radiation. Introducing a new "porosity-length'' formalism, we derive a simple scaling for the reduced effective opacity, and use this to derive an associated scaling for the porosity-moderated, continuum-driven mass loss rate from stars that formally exceed Ledd. For a simple super-Eddington model with a single porosity length that is assumed to be on the order of the gravitational scale height, the overall mass loss is similar to that derived in previous porosity work. This is much higher than is typical of line-driven winds, but is still only a few percent of the photon tiring limit--for which the luminosity becomes insufficient to carry the flow out of the gravitational potential. To obtain still stronger mass loss that approaches observationally inferred values near this limit, we introduce a power-law-porosity model in which the associated structure has a broad range of scales. We show that the mass loss rate can be enhanced by a factor that increases with the Eddington parameter Gamma, such that for moderately large Gamma (> 3-4), mass loss rates could approach the photon tiring limit. Together with the ability to drive quite fast outflow speeds, the derived mass loss could explain the large inferred mass loss and flow speeds of giant outbursts in eta Carinae and other LBV stars.

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arXiv2004

On the Low Progenitor Mass of the Binary Pulsar J0737-3039B

The observed orbital parameters of the recently discovered binary pulsar J0737-3039 are used to constrain the progenitor system. In particular, the small observed eccentricity and the small inferred peculiar velocity imply that during the formation of the second NS in the system, a very small mass Delta M <~ 0.15 Msun, was ejected. A progenitor more massive than 1.4 Msun is unlikely and a progenitor more massive than 2.3 Msun is practically ruled out. We therefore argue that the companion removed most of the progenitor envelope prior to its collapse. The second collapse may have been a SN of Type Ib, or more probably, of Type Ic. Alternatively, it could have been a bare collapse, an option which is kinematically favored. Future constraints on the peculiar motion could solidify this constraint on the mass loss. A future determination of the pulsars' spin vectors and proper motion could allow us the complete determination of the conditions during the second collapse.

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arXiv2003

Towards a Solution to the Early Faint Sun Paradox: A Lower Cosmic Ray Flux from a Stronger Solar Wind

Standard solar models predict a solar luminosity that gradually increased by about 30% over the past 4.5 billion years. Under the faint sun, Earth should have been frozen solid for most of its existence. Yet, running water is observed to have been present since very early in Earth's history. This enigma is known as the faint sun paradox. We show here that it can be partially resolved once we consider the cooling effect that cosmic rays are suspected to have on the global climate and that the younger sun must have had a stronger solar wind, such that it was more effective at stopping cosmic rays from reaching Earth. The paradox can then be completely resolved with the further contribution of modest greenhouse gas warming. When we add the cosmic ray flux modulation by a variable star formation rate in the Milky Way, we recover the long term glacial activity on Earth. As to the future, we find that the average global temperature will increase by typically 10K in the coming 2 Gyr.

astro-ph↗

arXiv2003

Photo-evaporation by thermal winds in dwarf galaxies

We revisit the evaporation process of gas from dwarf galaxies after it has been photo-ionized by the UV flux from the first stars and AGNs and heated to T~10^4K or 2x10^4K respectively. Earlier estimates, based on the balance between pressure and gravity, indicated that dark haloes of virial velocity lower than Vevap ~ 11-13 km/s have lost most of their gas in a dynamical time. We follow the continuous evaporation by a thermal wind during the period when the ionizing flux was effective. We find that the critical virial velocity for significant evaporation is significantly higher. For example, if the ionization starts at z-ion=10 and is maintained until z=2, a mass loss of one e-fold occurs in haloes of Vevap ~ 25 (or 35 km/s) for T ~ 10^4K (or 2x10^4K). Haloes of Vevap ~ 21 km/s (or 29 km/s) lose one e-fold within the first Hubble time at z=10. Any dwarf galaxies with virial velocities smaller than Vevap must have formed their stars from a small fraction of their gas before z-ion, and then lost the rest of the gas by photo-evaporation. This may explain the gas-poor, low surface brightness dwarf spheroidal galaxies. By z<1, most of the IGM gas was evaporated at least once form dwarf galaxies, thus providing a lower bound to its metallicity.

astro-ph↗

arXiv2003

The High-Energy Polarization-Limiting Radius of Neutron Star Magnetospheres I -- Slowly Rotating Neutron Stars

In the presence of strong magnetic fields, the vacuum becomes a birefringent medium. We show that this QED effect decouples the polarization modes of photons leaving the NS surface. Both the total intensity and the intensity in each of the two modes is preserved along a ray's path through the neutron-star magnetosphere. We analyze the consequences that this effect has on aligning the observed polarization vectors across the image of the stellar surface to generate large net polarizations. Counter to previous predictions, we show that the thermal radiation of NSs should be highly polarized even in the optical. When detected, this polarization will be the first demonstration of vacuum birefringence. It could be used as a tool to prove the high magnetic field nature of AXPs and it could also be used to constrain physical NS parameters, such as $R/M$, to which the net polarization is sensitive.

astro-ph↗

arXiv2002

The Spiral Structure of the Milky Way, Cosmic Rays, and Ice Age Epochs on Earth

The short term variability of the Galactic cosmic ray flux (CRF) reaching Earth has been previously associated with variations in the global low altitude cloud cover. This CRF variability arises from changes in the solar wind strength. However, cosmic ray variability also arises intrinsically from variable activity of and motion through the Milky Way. Thus, if indeed the CRF climate connection is real, the increased CRF witnessed while crossing the spiral arms could be responsible for a larger global cloud cover and a reduced temperature, thereby facilitating the occurrences of ice ages. This picture has been recently shown to be supported by various data (Shaviv, 2001). In particular, the variable CRF recorded in Iron meteorites appears to vary synchronously with the appearance ice ages. Here we expand upon the original treatment with a more thorough analysis and more supporting evidence. In particular, we discuss the cosmic ray diffusion model which considers the motion of the Galactic spiral arms. We also elaborate on the structure and dynamics of the Milky Way's spiral arms. In particular, we bring forth new argumentation using HI observations which imply that the galactic spiral arm pattern speed appears to be that which fits the glaciation period and the cosmic-ray flux record extracted from Iron meteorites. In addition, we show that apparent peaks in the star formation rate history, as deduced by several authors, coincides with particularly icy epochs, while the long period of 1 to 2 Gyr before present, during which no glaciations are known to have occurred, coincides with a significant paucity in the past star formation rate.

astro-ph↗

arXiv2002

Classical Novae as Super-Eddington Objects

Several of the inconsistencies plaguing the field of novae are resolved once we consider novae to be steady state super-Eddington objects. In particular, we show that the super-Eddington shell burning state is a natural consequence of the equations of stellar structure, and that the predicted mass loss in the super-Eddington state agrees with nova observations. We also find that the transition phase of novae can be naturally explained as "stagnating" winds.

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