Eric Pfahl
Massachusetts Institute of Technology
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Featured researches published by Eric Pfahl.
The Astrophysical Journal | 2002
Philipp Podsiadlowski; Saul Rappaport; Eric Pfahl
We present the results of a systematic study of the evolution of low- and intermediate-mass X-ray binaries (LMXBs and IMXBs). Using a standard Henyey-type stellar evolution code and a standard model for binary interactions, we have calculated 100 binary evolution sequences containing a neutron star and a normal-type companion star, where the initial mass of the secondary ranges from 0.6 to 7 M☉ and the initial orbital period from ~4 hr to ~100 days. This range samples the entire range of parameters one is likely to encounter for LMXBs and IMXBs. The sequences show an enormous variety of evolutionary histories and outcomes, where different mass transfer mechanisms dominate in different phases. Very few sequences resemble the classical evolution of cataclysmic variables, where the evolution is driven by magnetic braking and gravitational radiation alone. Many systems experience a phase of mass transfer on a thermal timescale and may briefly become detached immediately after this phase (for the more massive secondaries). In agreement with previous results (Tauris & Savonije 1999), we find that all sequences with (sub)giant donors up to ~2 M☉ are stable against dynamical mass transfer. Sequences where the secondary has a radiative envelope are stable against dynamical mass transfer for initial masses up to ~4 M☉. For higher initial masses, they experience a delayed dynamical instability after a stable phase of mass transfer lasting up to ~106 yr. Systems where the initial orbital period is just below the bifurcation period of ~18 hr evolve toward extremely short orbital periods (as short as ~10 minutes). For a 1 M☉ secondary, the initial period range that leads to the formation of ultracompact systems (with minimum periods less than ~40 minutes) is 13-18 hr. Since systems that start mass transfer in this period range are naturally produced as a result of tidal capture, this may explain the large fraction of ultracompact LMXBs observed in globular clusters. The implications of this study for our understanding of the population of X-ray binaries and the formation of millisecond pulsars are also discussed.
The Astrophysical Journal | 2004
Ph. Podsiadlowski; N. Langer; A.J.T. Poelarends; S.M. Rappaport; Alexander Heger; Eric Pfahl
We systematically examine how the presence in a binary affects the final core structure of a massive star and its consequences for the subsequent supernova explosion. Interactions with a companion star may change the final rate of rotation, the size of the helium core, the strength of carbon burning, and the final iron core mass. Stars with initial masses larger than � 11 Mthat experience core collapse will generally have smaller iron cores at the point of explosion if they lost their envelopes through a binary interaction during or soon after core hydrogen burning. Stars below � 11 M� , on the other hand, can end up with larger helium and metal cores if they have a close companion, since the second dredge-up phase that reduces the helium core mass dramatically in single stars does not occur once the hydrogen envelope is lost. We find that the initially more massive stars in binary systems with masses in the range 8-11 Mare likely to undergo an electron-capture supernova, while single stars in the same mass range would end as ONeMg white dwarfs. We suggest that the core collapse in an electron-capture supernova (and possibly in the case of relatively small iron cores) leads to a prompt or fast explosion rather than a very slow, delayed neutrino-driven explosion and that this naturally produces neutron stars with low-velocity kicks. This leads to a dichotomous distribution of neutron star kicks, as inferred previously, where neutron stars in relatively close binaries attain low kick velocities. We illustrate the consequences of such a dichotomous kick scenario using binary population synthesis simulations and discuss its implications. This scenario has also important consequences for the minimum initial mass of a massive star that becomes a neutron star. For single stars the critical mass may be as high as 10-12 M� , while for close binaries it may be as low as 6-8 M� .T hese critical masses depend on the treatment of convection, the amount of convective overshooting, and the metal- licity of the star, and will generally be lower for larger amounts of convective overshooting and lower metallicity. Subject heading
The Astrophysical Journal | 2002
Eric Pfahl; Saul Rappaport; Philipp Podsiadlowski; Hendrik C. Spruit
We investigate an interesting new class of high-mass X-ray binaries (HMXBs) with long orbital periods (Porb > 30 days) and low eccentricities (e 0.2). The orbital parameters suggest that the neutron stars in these systems did not receive a large impulse, or kick, at the time of formation. After considering the statistical significance of these new binaries, we develop a self-consistent phenomenological picture wherein the neutron stars born in the observed wide HMXBs receive only a small kick (50 km s-1), while neutron stars born in isolation, in the majority of low-mass X-ray binaries, and in many of the well-known HMXBs with Porb 30 days receive the conventional large kicks, with a mean speed of ~300 km s-1. Assuming that this basic scenario is correct, we discuss a physical process that lends support to our hypothesis, whereby the magnitude of the natal kick to a neutron star born in a binary system depends on the rotation rate of its immediate progenitor following mass transfer—the core of the initially more massive star in the binary. Specifically, the model predicts that rapidly rotating precollapse cores produce neutron stars (NSs) with relatively small kicks, and vice versa for slowly rotating cores. If the envelope of the NS progenitor is removed before it has become deeply convective, then the exposed core is likely to be a rapid rotator. However, if the progenitor becomes highly evolved prior to mass transfer, then a strong magnetic torque, generated by differential rotation between the core and the convective envelope, may cause the core to spin down to the very slow rotation rate of the envelope. Our model has important implications for the dynamics of stellar core collapse, the retention of neutron stars in globular clusters, and the formation of double neutron star systems in the Galaxy.
The Astrophysical Journal | 2005
Michael P. Muno; Eric Pfahl; F. K. Baganoff; W. N. Brandt; Andrea M. Ghez; Jessica R. Lu; Mark R. Morris
During 5 years of Chandra observations, we have identified seven X-ray transients located within 23 pc of Sgr A*. These sources each vary in luminosity by more than a factor of 10 and have peak X-ray luminosities greater than 5 × 1033 ergs s-1, which strongly suggests that they are accreting black holes or neutron stars. The peak luminosities of the transients are intermediate between those typically considered outburst and quiescence for X-ray binaries. Remarkably, four of these transients lie within only 1 pc of Sgr A*. This implies that, compared to the numbers of similar systems located between 1 and 23 pc, transients are overabundant by a factor of 20 per unit stellar mass within 1 pc of Sgr A*. It is likely that the excess transient X-ray sources are low-mass X-ray binaries that were produced, as in the cores of globular clusters, by three-body interactions between binary star systems and either black holes or neutron stars that have been concentrated in the central parsec through dynamical friction. Alternatively, they could be high-mass X-ray binaries that formed among the young stars that are present in the central parsec.
The Astrophysical Journal | 2003
Eric Pfahl; Saul Rappaport; Philipp Podsiadlowski
We present the first study that combines binary population synthesis in the Galactic disk and detailed evolutionary calculations of low- and intermediate-mass X-ray binaries (L/IMXBs). Our approach allows us to follow completely the formation of incipient L/IMXBs and their evolution through the mass-transfer phase to the point when they become binary millisecond pulsars (BMPs). We show that the formation probability of IMXBs with initial donor masses of 1.5-4 M☉ is typically 5 times higher than that of standard LMXBs with initial donor masses of less than 1.5 M☉. Since IMXBs evolve to resemble observed LMXBs, we suggest that the majority of the observed systems may have descended from IMXBs. Distributions at the current epoch of the orbital periods, donor masses, and mass accretion rates of L/IMXBs have been computed, as have orbital-period distributions of BMPs. This is a major step forward over previous theoretical population studies of L/IMXBs that utilized only crude representations of the binary evolution through the X-ray phase. Several significant discrepancies between the theoretical and observed distributions are discussed. We find that the total number of luminous (LX > 1036 ergs s-1) X-ray sources at the current epoch and the period distribution of BMPs are very sensitive to the parameters in the analytic formula describing the common-envelope phase that precedes the formation of the neutron star. The orbital-period distribution of observed BMPs strongly favors cases in which the common envelope is more easily ejected. However, this leads to an approximately hundred-fold overproduction of the theoretical number of luminous X-ray sources relative to the total observed number of LMXBs. As noted by several groups prior to our study, X-ray irradiation of the donor star may result in a dramatic reduction in the X-ray active lifetime of L/IMXBs, and we suggest that irradiation may resolve the overproduction problem as well as the long-standing BMP/LMXB birthrate problem.
Monthly Notices of the Royal Astronomical Society | 2005
S. Rappaport; Ph. Podsiadlowski; Eric Pfahl
Ultraluminous X-ray sources (ULXs) with Lx>10^{39} ergs/s have been discovered in great numbers in external galaxies with ROSAT, Chandra, and XMM. The central question regarding this important class of sources is whether they represent an extension in the luminosity function of binary X-ray sources containing neutron stars and stellar-mass black holes (BHs), or a new class of objects, e.g., systems containing intermediate-mass black holes (100-1000 Msun). We have carried out a theoretical study to test whether a large fraction of the ULXs, especially those in galaxies with recent star formation activity, can be explained with binary systems containing stellar-mass black holes. To this end, we have applied a unique set of binary evolution models for black-hole X-ray binaries, coupled to a binary population synthesis code, to model the ULXs observed in external galaxies. We find that for donor stars with initial masses>10 Msun the mass transfer driven by the normal nuclear evolution of the donor star is sufficient to potentially power most ULXs. This is the case during core hydrogen burning and, to an even more pronounced degree, while the donor star ascends the giant branch, though the latter phases lasts only ~5% of the main sequence phase. We show that with only a modest violation of the Eddington limit, e.g., a factor of ~10, both the numbers and properties of the majority of the ULXs can be reproduced. One of our conclusions is that if stellar-mass black-hole binaries account for a significant fraction of ULXs in star-forming galaxies, then the rate of formation of such systems is ~3 x 10^{-7} per year normalized to a core-collapse supernova rate of 0.01 per year.
The Astrophysical Journal | 2004
Eric Pfahl; Abraham Loeb
The supermassive black hole at the Galactic center harbors a bound cluster of massive stars that should leave neutron star remnants. Extrapolating from the available data, we estimate that ~1000 radio pulsars may currently orbit Sgr A* with periods of 100 yr. Optimistically, 1-10 of the most luminous of these pulsars may be detectable with current telescopes in periodicity searches at frequencies near 10 GHz, where the effects of interstellar scattering are alleviated. Long-term timing observations of such a pulsar would clearly reveal its Keplerian motion and possibly show the effects of relativistic gravity. We briefly discuss how pulsar timing can be used to study the dynamical and interstellar environment of the central black hole and speculate on the prospects for astrometric observations of an orbiting pulsar.
The Astrophysical Journal | 2000
Frederic A. Rasio; Eric Pfahl; Saul Rappaport
We present a new dynamical scenario for the formation of short-period binary millisecond pulsars in globular clusters. Our work is motivated by the recent observations of 20 radio pulsars in 47 Tuc. In a dense cluster such as 47 Tuc, most neutron stars acquire binary companions through exchange interactions with primordial binaries. The resulting systems have semimajor axes in the range approximately 0.1-1 AU and neutron star companion masses approximately 1-3 M middle dot in circle. For many of these systems, we find that when the companion evolves off the main sequence and fills its Roche lobe, the subsequent mass transfer is dynamically unstable. This leads to a common envelope phase and the formation of short-period neutron star-white dwarf binaries. For a significant fraction of these binaries, the decay of the orbit due to gravitational radiation will be followed by a period of stable mass transfer driven by a combination of gravitational radiation and tidal heating of the companion. The properties of the resulting short-period binaries match well those of observed binary pulsars in 47 Tuc.
The Astrophysical Journal | 2001
Hugo Delgado-Marti; Alan M. Levine; Eric Pfahl; Saul Rappaport
We have observed the Be/X-ray pulsar binary system X Per/4U 0352+30 on 61 occasions spanning an interval of 600 days with the PCA instrument on board the Rossi X-Ray Timing Explorer (RXTE). Pulse timing analyses of the 837 s pulsations yield strong evidence for the presence of orbital Doppler delays. We confirm the Doppler delays by using measurements made with the All Sky Monitor (ASM) on RXTE. We infer that the orbit is characterized by a period Porb = 250 days, a projected semimajor axis of the neutron star ax sin i = 454 lt-s, a mass function f(M) = 1.61 M?, and a modest eccentricity e = 0.11. The measured orbital parameters, together with the known properties of the classical Be star X Per, imply a semimajor axis a = 1.8-2.2 AU and an orbital inclination i ~ 26?-33?. We discuss the formation of the system in the context of the standard evolutionary scenario for Be/X-ray binaries. We find that the system most likely formed from a pair of massive progenitor stars and probably involved a quasi-stable and nearly conservative transfer of mass from the primary to the secondary. We find that the He star remnant of the primary most likely had a mass 6 M? after mass transfer. If the supernova explosion was completely symmetric, then the present orbital eccentricity indicates that 4 M? was ejected from the binary. If, on the other hand, the neutron star received at birth a kick of the type often inferred from the velocity distribution of isolated radio pulsars, then the resultant orbital eccentricity would likely have been substantially larger than 0.11. We have carried out a Monte Carlo study of the effects of such natal kicks and find that there is less than a 1% probability of a system like that of X Per forming with an orbital eccentricity e 0.11. We speculate that there may be a substantial population of neutron stars formed with little or no kick. Finally, we discuss the connected topics of the wide orbit and accretion by the neutron star from a stellar wind.
The Astrophysical Journal | 2002
Eric Pfahl; Saul Rappaport; Philipp Podsiadlowski
We explore the possibility that neutron stars accreting from the winds of main-sequence stellar companions account for a significant fraction of low-luminosity, hard X-ray sources (LX 1035 ergs s-1; 1-10 keV) in the Galaxy. This work was motivated by recent Chandra observations of the Galactic center by Wang, Gotthelf, & Lang. Our calculations indicate that many of the discrete X-ray sources detected in this survey may be wind-accreting neutron stars and that many more may be discovered with deeper X-ray observations. We propose that an infrared observing campaign be undertaken to search for the stellar counterparts of these X-ray sources. If future observations reveal that a large fraction of the X-ray sources are wind-accreting neutron stars, this should provide an important calibration point for massive binary population synthesis studies.