Ingrid Stairs

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Mass and geometric measurements of binary radio pulsars (2017)

One of the primary, long-term goals in high-energy astrophysics is the measurement of macroscopic parameters that constrain the equation of state for compact stellar objects. For neutron stars, known to be composed of the densest matter in the Universe, measurements of their masses and sizes are of considerable importance due to the poorly understood processes that govern their interiors. Measurements of relativistic “post-Keplerian” effects in binary systems can be used to significantly constrain viable equations of state, test modern theories of gravitation, verify binary-evolution models that predict correlations between certain binary parameters, and determine the Galactic neutron-star mass distribution that is expected to reflect different supernovae mechanisms and evolutionary paths. In this thesis, we use established pulsar-timing techniques to analyze signals from radio pulsars in 25 binary systems, as well as from one pulsar in a hierarchical triple system, in order to detect perturbations from Keplerian motion of the bodies. We characterize observed relativistic Shapiro timing delays to derive estimates of the component masses and inclination angles in 14 pulsar-binary systems, and measure a large number of secular variations due to kinematic, relativistic and/or third-body effects in the majority of binary systems studied here. We find a wide range of statistically-significant pulsar masses and make new detections of the relativistic Shapiro-delay signal in four binary systems for the first time. In the relativistic PSR B1534+12 binary system, we derive an accurate and precise rate of geodetic precession of the pulsar-spin axis -- due to secular variations of electromagnetic pulse structure -- that is consistent with the prediction from general relativity. In the PSR B1620-26 triple system, we discuss ongoing efforts to simultaneously model both “inner” and “outer” orbits and tentatively measure secular variations of all “inner-orbital” elements that we show are likely due to third-body interactions between the smaller orbit and outer companion, which can eventually be used to constrain orientation angles and possibly the pulsar mass in the near future.

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Radio observations of two binary pulsars (2012)

The study of pulsars in binary systems produces a wide variety of scientific results, including unique tests of general relativity and constraints on the equation of state of matter at extremely high densities. Through pulsar timing (which utilizes the fact that pulsars are precise clocks), it is possible to precisely measure the orbital parameters and masses of some binary pulsars, which can place constraints on their mass transfer histories. We present timing results for two binary pulsars.The intermediate-mass binary pulsar (IMBP) PSR J0621+1002 is a mildly recycled pulsar in an 8.3-day orbit around a massive white dwarf (WD) companion. It is one of only two known IMBPs with a precise mass measurement. We collected 9 days' worth of data using the 305-metre Arecibo telescope (allowing for full orbital coverage), which we used to improve constraints on the advance of periastron, and in turn, the pulsar and companion masses (to 1.53^{+0.10}_{-0.20}M☉ and 0.76^{+0.28}_{-0.07} solar masses, respectively) and inclination angle of the system. These results, combined with the relatively long orbital period, suggest a disk accretion evolutionary scenario involving short-lived periods of hyper-accretion, in which a moderate amount of mass has been transferred to the neutron star (NS).PSR J1906+0746 is a young pulsar in a compact ~4-hour orbit around a companion that was discovered in the early stages of the PALFA survey using the 305-metre Arecibo telescope. We present the timing results using data collected between 2005 to 2009 using the Green Bank, Arecibo, and Nançay telescopes. We have measured the advance of periastron, the time dilation and gravitational redshift parameter, and the orbital decay, and we find the pulsar and companion masses to be 1.323^{+0.011}_{-0.011}M☉ and 1.290^{+0.011}_{-0.011} solar masses, respectively. Although the companion may be a second NS, it is more likely to be a massive WD. The system's evolution probably involved a substantial transfer of mass from the WD progenitor onto the NS progenitor through Roche-lobe overflow accretion, followed by the formation of the WD, and a short common envelope phase, and finally the ejection of the envelope and the pulsar-forming supernova.

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Master's Student Supervision (2010 - 2018)
Timing and analysis of eclipsing black widow pulsar PSR J2256-1024 (2018)

Pulsars are rapidly spinning neutron stars emitting radiation about their magnetic field axes. Misalignment of the spin and magnetic axes causes a “lighthouse” effect where we observe radiation pulses, in time with the pulsar’s rotation. Millisecond pulsars are those which have accreted material from a companion star, spinning themselves up to rotate faster. Some millisecond pulsars occur in tight orbits with low mass companions; this combination can lead to the companion losing material, due to bombardment by energetic particles from the pulsar, presumably eventually destroying the companion. These pulsars are known as Black Widows (BWs). This thesis is an analysis of radio observations of PSR J2256-1024, a BW pulsar with a spin period of 2.294531816964939(10)ms.Observing pulsars, we can calculate the arrival times of individual pulses and compare these with those predicted from various models to find the best-fitting one. This process is known as pulsar timing. We present the timing solution for PSR J2256-1024. We find it has a 5.1091831284(9)hr binary orbit with a semimajor axis of 4.1(3)ltsec and a 0.0312(9)M⊙ companion. PSR J2256-1024 shows a radio eclipse over 7.8% of its orbit - approximately twice the size of the Roche lobe calculated for the companion. This confirms the picture of a Black Widow pulsar with material being stripped from the companion and forming a trailing cloud which blocks the pulsar signal. We also find evidence for variable clumps of material in the system.We present polarization profiles and mean flux densities at 350 MHz, 820MHz and 1500 MHz. We discuss polarization changes in the post-eclipse region, where the pulsar signal is transmitted through eclipsing material in the system, and find evidence of Faraday rotation. At one epoch, synchronous measurements of excess dispersion and rotation measure lead to a detection of a 3.9(0.6)mG line-of-sight magnetic field. This field occurs an estimated minimum 3.3(0.3) companion-Roche-lobe-radii from the companion. We believe this is the first successful detection of a magnetic field component in eclipsing material within a Black Widow system.

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Wideband timing of the double pulsar (PSR J0737-3039A) (2017)

Pulsars are neutron stars (NS) that produce beamed radio-frequency emission. Due to their rapid, steady rotation rate, this signal is detected as a series of pulses whose integrated profile is unusually stable over time. Pulsars in double neutron star (DNS) binary systems are a rare, but extremely useful, astronomical tool and have been used in tests of gravity theories in the strong-gravitational field limit. Rarer still are DNS systems in which both objects have been detected as pulsars; only one such system has been found thus far -- PSR J0737-3039A/B. Discovered over a decade ago, this system consists of one recycled pulsar, PSR J0737-3039A, and its companion, PSR J0737-3039B, which has since become undetectable.In any pulsar-related research, precise timing is necessary to produce meaningful results. The pulse time of arrivals (TOAs) are greatly affected by the medium through which the electromagnetic (EM) signal travels in both frequency-dependent and -independent ways. Even after accounting for such effects, many pulse profiles still exhibit frequency-dependent shape changes, which can greatly affect the precision of the timing results. Traditionally, corrections are applied to the TOAs after calculation in an ad hoc manner. In contrast to this, we explored the wideband timing algorithm developed by Timothy T. Pennucci and collaborators which accounts for frequency-dependent profile changes through a two-dimensional Gaussian pulse portrait model implemented in the TOA calculations. It was found that the portrait model is well-representative of the pulse profile shape over a wide frequency range. This method is also able to produce a robust set of wideband TOAs. The subsequent timing model, determined with TEMPO timing software, was found to be comparable to those produced from subbanded TOAs derived though more traditional methods. Some inconsistencies between the timing model astrometric and spin parameters of the wideband and subbanded data of this well-studied pulsar imply potential difficulties in achieving precise timing results not only for this pulsar, but for others, such as those used in pulsar timing arrays aiming to detect gravitational waves.

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The detectability of millisecond pulsars in eccentric binary systems (2013)

With new, highly sensitive telescopes, increased computational power, and improved search algorithms, the present century has seen a great increase in the discovery of pulsars in globular clusters. These are typically fast-spinning 'millisecond' pulsars, and more often than not they are members of binary systems. Unlike in the Galactic field, millisecond pulsars in globular clusters are often found in eccentric systems because of disruptions and exchanges due to the high stellar density of the cluster environment.A long-standing problem is that of characterizing our sensitivity to pulsars in binary systems, particularly those with non-zero eccentricity. A pulsar's orbital motion modulates its observed pulse period, making its detection through standard Fourier analysis difficult or impossible. A common technique to mitigate this problem is the 'acceleration search', which corrects for uniform line-of-sight acceleration, but not higher-order variations. This is often a valid approximation, and many pulsars have been found this way. However, it is not clear where such a search breaks down.This is a problem with a many-dimensional phase space that includes all of the binary parameters, the pulsar parameters, and the various search inputs. Past studies have approached the problem analytically, and have made valuable insights; however, until recently they have been restricted to circular orbits, and have not accounted for pulsar brightness or signal digitization.Here I approach the problem empirically. I simulate 1.8 million pulsars in a variety of orbital configurations and explore the frequency of pulsar recovery across various dimensions of the phase space. I find in particular that, at very short orbital periods, high eccentricities make binary systems easier to detect.

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Experimental gravity with PSR B1534+12 (2012)

We present an updated analysis of pulse profiles and their arrival-times from PSR B1534+12, a 37.9-ms pulsar that is orbiting a neutron star. Such “double-neutron-star” systems are expected to undergo various relativistic effects, such as orbital decay and precession, due to the strong-field nature of the local gravitational field (Damour & Taylor, 1992). A high-precision timing model is derived that accounts for all astrophysical processes that systematically affect pulse arrival-times. In the process of generating this model, we constrain parameters that characterize the interstellar environment, relative motion of the pulsar, its spin properties, and binary parameters. We measure five “post-Keplerian” parameters that represent relativistic corrections to the standard Keplerian quantities that describe a binary orbit. These relativistic parameters are then used to test general relativity by comparing the measured values with those predicted by Einstein’s grav- itational theory. We conclude that general relativity is confirmed to within ∼ 0.35% of its predictions. The measurement of orbital decay contains a bias due to relative acceleration in the Galactic potential, and cannot be corrected for at this time due to an unreliable measure of distance; however, we can use this bias as a means to constrain the distance the pulsar should be from Earth in order for general relativity to be the correct theory of gravity. We find this distance to be d_GR = 1.037 ± 0.012 kpc. We also present evidence for pulse “jitter” in PSR B1534+12, which indicates short-term magnetospheric activity and has significant implications for the long-term improvement of timing precision. In a separate study, we present an analysis on pulse-profile evolution that has been previously linked to relativistic spin precession (Stairs et al., 2004). The current results of our precession analysis cannot confirm general relativity using this relativistic effect, but future studies and observations are needed to constrain the precession rate of PSR B1534+12.

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