Research Page

Since January 1998, I have been engaged in astronomical research at a variety of locations. At first, I worked in radio astronomy. Once I moved to the University of Virginia, I began concentrating on X-ray astronomy. I am also involved in side projects regarding the large scale distribution of galaxies in the 2MASS (2 Micron All-Sky Survey) infrared catalog. In the summer of 2000, I took some time off to intern for the State of Delaware's Office for Emergency Medical Services; my work there included research in emergency medicine.

Ongoing Projects (All are Astronomy)

Low-Mass X-ray Binaries in Early-type Galaxies

The exceptional imaging quality of the Chandra X-ray Observatory has helped reveal that the X-ray emission in early-type (elliptical and lenticular galaxies) comes from both diffue interstellar gas at millions of degrees and point sources. These point sources are dominated by compact objects (neutron stars and black holes) accreting material from a low-mass binary companion star. Recent Chandra observations of these low-mass X-ray binaries were the first to image large populations of neutron stars and stellar-mass black holes in a normal elliptical galaxy. Combining X-ray and optical data, we have found that low-mass x-ray binaries are often associated with globular clusters, important tracers of galaxy evolution. Thus, low-mass x-ray binaries may help trace the evolution history of entire galaxies, as well as the population of stars that eventually form low-mass x-ray binaries. For my thesis, I will use Chandra observations of ~30 early-type galaxies to systematically characterize 1000's of low-mass X-ray binaries. With this catalog, I hope to tackle the origin of the low-mass X-ray binaries, the emission properties of low-mass X-ray binaries, and the evolution history of entire galaxies.

VLA/MERLIN Observations of 0800+608

I was a Fulbright Scholar at Jodrell Bank Observatory, University of Manchester and was enrolled in the M.Sc. program. For my thesis, 8.4 GHz VLA and 1.5 GHz MERLIN data-sets were reduced, imaged, and analyzed to interpret the peculiar structures of this quasar. Hydrodynamic simulations were explored. I am still involved in studying this atypical object.

Understanding Measurements of Brightness Temperatures in Parsec-scale AGN Jets

As a member of the Brandeis Radio Astronomy Group, I participated in a monitoring program of blazars by analyzing the results from 15 GHz and 22 GHz VLBI images.  Afterwards, I wrote a simulation of parsec-scale radio jets under the Blandford-Koenigl model and began exploring various results from the simulations. Although I already graduated from Brandeis, I am still involved in this research.

X-ray Astronomy

Low Mass X-ray Binaries Versus Globular Cluster Age in the Elliptical Galaxy NGC 4365
Sarazin, C. L., Angelini, L., & Sivakoff, G. R. 2003, Astrophysical Journal, submitted

Chandra Observations of Low Mass X-ray Binaries and Diffuse Gas in the Early-Type Galaxies NGC 4365 and NGC 4382 (M85)
Sivakoff, G. R., Sarazin, C. L., & Irwin, J. A. 2003, Astrophysical Journal, accepted

Abstract: We used the Chandra X-ray Observatory ACIS S3 to image the X-ray faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The observations resolve much of the X-ray emission into 99 and 58 sources, respectively, most of which are low-mass X-ray binaries (LMXBs) associated with each of the galaxies. Within one effective radius of NGC~4365, about 45% of the counts are resolved into sources, 30% are attributed to unresolved LMXBs, and 25% are attributed to diffuse gas. Within two effective radii of NGC 4382, about 22% of the counts are resolved into sources, 33% are attributed to unresolved LMXBs, and 45% are attributed to diffuse gas. We identify 18 out of the 37 X-ray sources in a central field in NGC 4365 with globular clusters. The luminosity functions of the resolved sources for both galaxies are best fit with cutoff power-laws whose cutoff luminosity is ≈ 0.9 - 3.1 × 10³⁹ ergs s^-1. These luminosities are much larger than those previously measured for similar galaxies; we do not find evidence for a break in the luminosity function at the Eddington luminosity of a 1.4 M_O· neutron star. The spatial distributions of the resolved sources for both galaxies are broader than the distribution of optical stars. In both galaxies, a hard power-law model fits the summed spectrum of all of the sources. The unresolved emission is best fit by the sum of a soft mekal model representing emission from diffuse gas, and a hard power-law, presumed to be from unresolved LMXBs. There is some evidence that the temperature of the diffuse gas increases with increasing radius. A standard beta model fits the radial distribution of the diffuse gas in both galaxies. In the elliptical NGC 4365, the best-fit core radius is very small, while the S0 galaxy NGC 4382 has a larger core radius. This may indicate that the gas in NGC 4382 is rotating significantly.

Galaxy Clusters & Large Scale Structure
Reiprich, T. H., Sivakoff, G. R., Chatzikos, M., Sarazin, C. L., Skrutskie, M. F., , Boehringer, H., & Retzlaff, J. 2003, at the International Astronomical Union 25th General Assembly S216

Abstract: Galaxies and galaxy clusters have been very important tools for the study of large scale structure. Since large statistical samples are becoming available we need to improve our understanding of the systematics involved. A project is under way to study systematic selection effects, and relations between cluster richness, X-ray luminosity, and mass. Here we report first results of a correlation between 10^6 infrared selected galaxies (2MASS) and 10^3 X-ray selected galaxy clusters (NORAS II + REFLEX).

Low Mass X-ray Binaries and Globular Clusters in Early-Type Galaxies
Sarazin, C. L., Kundu, A., Irwin, J. A., Sivakoff, G. R., Blanton, E. L., Randall, S. W. 2003, Astrophysical Journal, 595, in press

Abstract: A high fraction of the Low Mass X-ray Binaries (LMXBs) in early-type galaxies are associated with globular clusters (GCs). Here, we discuss the correlations between LMXBs and GCs in a sample of four early-type galaxies with X-ray source lists determined from Chandra observations. There is some evidence that the fraction of LMXBs associated with globular clusters (f_X-GC) increases along the Hubble sequence from spiral bulges (or spheroids) to S0s to Es to cDs. On the other hand, the fraction of globular clusters which contain X-ray sources appears to be roughly constant at f_GC-X ~4%. There is a strong tendency for the X-ray sources to be associated with the optically more luminous GCs. However, this correlation is consistent with a constant probability of finding a LMXB per unit optical luminosity; that is, it seems to result primarily from the larger number of stars in optically luminous GCs. The probability of finding a bright LMXB per unit optical luminosity in the GCs is about 1.5 × 10^-7 LMXBs per L_O·,I for LX >~ 1 × 10³⁸ erg s^-1 (0.3-10 keV) and rises to about 2.0 × 10^-7 LMXBs per L_O·,I at lower X-ray luminosities, LX >~ 3 × 10³⁷ erg s^-1. This frequency appears to be roughly constant for different galaxies, including the bulges of the Milky Way and M31. There is a tendency for the X-ray sources to be found preferentially in redder GCs, which is independent of optical luminosity correlation. This seems to indicate that the evolution of X-ray binaries in a GC is affected either by the metallicity or age of the GC, with younger and/or more metal rich GCs having more LMXBs. There is no strong difference in the X-ray luminosities of GC and non-GC LMXBs. There is a weak tendency for the brightest LMXBs, whose luminosities exceed the Eddington luminosity for a 1.4 M_O· neutron star, to avoid GCs. That may indicate that black hole X-ray binaries are somewhat less likely to be found in GCs, as seems to be true in our Galaxy. On the other hand, there are some luminous LMXBs associated with GCs. There is no clear evidence that the X-ray spectra or variability of GC and non-GC X-ray sources differ. We also find no evidence for a difference in the spatial distribution of GC and non-GC LMXBs. Many of these results are similar to those found in NGC 1399 and NGC 4472 by Angelini et al. and Kundu et al., respectively.

Cosmic Structure Traced by Precision Measurements of the X-ray Brightest Galaxy Clusters in the Sky
Reiprich, T. H., Sarazin, C. L., Kempner, J. C., Skrutskie, M. F., Sivakoff, G. R., Boehringer, H., & Retzlaff, J. 2003, in The Emergence of Cosmic Structure, ed. Holt, S. S. & Reynolds, C. (New York: AIP Conf. Proc.), in press

Abstract: The current status of our efforts to trace cosmic structure with 10^6 galaxies (2MASS), 10^3 galaxy clusters (NORAS II cluster survey), and precision measurements for 10^2 galaxy clusters (HIFLUGCS) is given. The latter is illustrated in more detail with results on the gas temperature and metal abundance structure for 10^0 cluster (A1644) obtained with XMM-Newton.

Radio Astronomy

Coordinated Millimeter VLBI Array Observations of R Cassiopeiae: 86 GHz SiO Masers and Envelope Dynamics
Phillips, R. B., Sivakoff, G. R., Lonsdale, C. J., & Doeleman, S. S. 2001, Astronomical Journal, 122, 2679

Abstract: Coordinated Millimeter VLBI Array measurements at 86 GHz have imaged the silicon monoxide masers of the nearby red giant R Cassiopeiae at two epochs separated by approximately 1/12 the stellar pulsation period. The v = 1, J = 2→1 transition comes from a ring at 1.5 to 2 stellar radii. Doppler velocity patterns are ordered over scales up to ~1 AU but are not consistent with simple models such as global rotation. One major masing region disappeared in the interval between the two epochs.

MERLIN & VLA Observations of QSO 0800+608
Sivakoff. G. R. M.Sc. Thesis, The University of Manchester, 2001

Abstract: MERLIN & VLA observations of the QSO 0800+608 were reduced, imaged and analysed. After a brief introduction to the characteristics of AGN, the unusual behaviour of 0800+608 is discussed. The data reduction of 21-cm (1.4-GHz) MERLIN, 18-cm (1.6- GHz) MERLIN and 3.6-cm (8.4-GHz) VLA observations is discussed and images from these data-sets and an additional VLA data-set at 6.1-cm (4.8-GHz) are presented. The unpolarized morphology, spectral behaviour and polarization of 0800+608 were analysed. A low Faraday rotation measure indicates that 0800+608 is unlikely to be a member of a dense cluster. The jet width implies that radio emission near the first three knots is dominated by the discrete knots; whereas, the radio emission after the first three knots is the sum of emission from the knots, a continuous jet and, in some regions, an extended backflow. The spectral index measurements display multiple spectral index gradients that depend on the down jet distance and the wavelength over which the spectral index is measured. No simple model of electron energy evolution, particle reacceleration or variable intrinsic spectral index can account for all of the spectral index gradients. The surface mode of a helical Kelvin-Helmholtz instability is a likely origin for the periodic transverse oscillation, which was detected after the first three knots. Force-free jet models and MHD simulations inadequately explain the oscillating polarization detected between knots 4 and 8; however, compressions across the jet may explain the polarization behaviour detected after knot 8. A periodic ejection model seems unlikely to explain the periodic knot structure of the entire jet. Steady-state fluid dynamics or perturbed fluid dynamics explain the structure better, but no current model is adequate. Computer simulations, which were performed for this thesis, of a jet-cloud collision can neither confirm nor contradict whether a jet-cloud collision could explain the counter-lobe's morphology. Future multi-frequency (radio, optical / infrared and x-ray) observations and computer simulations are suggested as follow up to the discoveries made and questions raised in this thesis.

VLA + MERLIN Imaging of 0800+608
Sivakoff, G. R. at CERES Workshop, Crete 2001.

Abstract: 0800+608 exhibits atypical structure. Initial results of VLA + MERLIN imaging are presented.

R Cassiopeaia: AGB Star Envelope Dynamics Charted by 86 Ghz SiO Masers
Phillips. R. B., Doeleman S. S. Lonsdale C. J., & Sivakoff G. R. at the 197th American Astronomical Society Meeting

Abstract: Coordinated Millimeter VLBI Array (CMVA) investigations at 86 GHz have mapped the SiO masers of the nearby red giant R Cassiopeiae at two closely-spaced epochs. Images taken one month apart (0.1 stellar period) show major changes in some maser clusters, although some features remain recognizable. The J=2 → 1 transition comes from a thin, moderately turbulent shell at 1.5 to 2 photospheric radii that shows marginal evidence for expansion. These maser clusters show variations in line-of-sight velocity comparable to shell expansion rate, indicating the stellar envelope has not yet formed a predominantly radial flow at the location of the masers. Velocity gradients are evident that evoke a hybrid of large scale photospheric convective motions with an outflowing envelope. SiO maser emission from R Cas shows considerable structure even over baselines less than 50 Mλ . Spacings interior to the shortest currently available for 86 GHz VLBI/VLBA would be helpful to describe SiO emission our study does not properly account for. Operations of the CMVA and its partner telescopes are supported by the National Science Foundation.

How Do You Measure Jet Parameters?
Sivakoff G. R., Wardle J. F. C., Homan D.C., & Ohja R. at the 2000 New England Regional Quasar and AGN Meeting

Abstract: BKJET, a computer simulator of Blandford-Koenigl jets, is used to produce ideal theoretical jets. These jets are sampled in the uv plane using real VLBI uv tracks, imaged, and then modeled in the uv plane using DIFMAP. The modeled data are compared to the theoretical jets in an attempt to understand how best to measure jet parameters. Preliminary results indicate better fits are made with multiple Gaussian components; however, using the single component fits, the ratio of major axis and positions between frequencies can help fix some jet parameters. Also, preliminary results indicate that resolution and model fitting techniques play pivotal roles in determining the brightness temperature.

Computer Simulations of Relativistic Jets
Sivakoff, G. R. Stephen Berko Memorial Prize Presentation at the 2000 Stephen Berko Memorial Symposium.

Abstract:\ A portion of the author's Senior Honors Thesis is presented. The basics of BKJET, a computer simulator of Blandford-Koenigl jets, is explained. Its initial results confirm that sub-milliarcsecond frequency dependent core shifts can be observed and measured in simulated jets with reasonable parameters.

Computer Simulations of Blandford-Koenigl Parsec-Scale Jets
Sivakoff, G. R. Senior Honors Thesis, Brandeis University, 2000

Abstract: The Blandford-Koenigl synchrotron model for jets is explored. Some more general formula are  and a new approximate formula for the optical depth of a jet's core are derived. BKJET, a computer simulator of Blandford-Koenigl jets, was created to remove the effects of some of the approximations Blandford and Koenigl used. After the program methodology and verification  is discussed, the model fits of two simulated canonical jets as viewed by a synthesis array are presented. Results seem to suggest that single component fits are more physical, but that double components can easily be fit to Blandford-Koenigl jets.

VLBI Imaging of the 3mm SiO Masers Around R Cassiopeia
Sivakoff G. R., Phillips R. B., Lonsdale, C. J., & Doeleman, S. S. at the 195th American Astronomical Society Meeting.

Abstract : In Asymptotic Giant Branch and Mira-like stars, 3mm SiO masers allow observers to probe the inner region of the circumstellar envelope close to the photosphere. In December 1998, the v=1, J=2 to 1 SiO transition of R Cas was observed at 86.2 GHz using the joint resources of the CMVA and VLBA. The images presented here are the first multi-baseline VLBI images of SiO masers in R Cas, and reveal a loosely organized ring similar to those seen around the small number of SiO stars imaged at 7mm.
We discuss the mapping techniques used and analysis of the resulting high fidelity maps which reveal approximately 57 "families" of maser features, each with well defined spatial and velocity extent. Quantifiable characteristics of these components indicate complex stellar dynamics. At a velocity resolution of 0.1 km/s, several families show velocity coherence of up to 2 km/s. These identification algorithms are being applied to test the lifetimes of families across multiple observing epochs, and our goal is to make more rigorous probes into the physical conditions within stellar mass loss envelopes.
This research was carried out under the Research Experiences for Undergraduates (REU) Program at\ Haystack. The REU and CMVA programs at Haystack are sponsored by the National Science Foundation.

3mm VLBI Observations of SiO Masers Around Evolved Stars
Lonsdale C. J., Doeleman S. S., Boboltz D. A., & Sivakoff G. R. at the XXVIth International Union Of Radio Science General Assembly.

Abstract:

Emergency Medicine

Comparing Intravenous and Intramuscular Administration of Droperidol in the Agitated Patient
C. T. Lauder, DO, M. Schnyder, NREMT-P, G. R. Sivakoff, BSc, R. E. Megargel, DO, E. D. Bailey, MD, & R. O'Connor, MP, MPH. 2001, Prehospital Emergency Care, 5, 117

Comparing Intravenous and Intramuscular Administration of Droperidol in the Agitated Patient
C. T. Lauder, DO, M. Schnyder, NREMT-P, G. R. Sivakoff, BSc, R. E. Megargel, DO, E. D. Bailey, MD, & R. O'Connor, MP, MPH at the 2001 National Association of Emergency Medical Services Physicians

Introduction: Droperidol is used more frequently in emergency medical services (EMS) as a chemical restraint for patients presenting with acute agitation or combative behavior. Intravenous (IV) access is not always feasible in agitated patients.
Objective: To examine the relative efficacy of intramuscular (IM) and IV droperidol in the prehospital setting.
Methods: This retrospective study was conducted through the state office of EMS. Standing orders for droperidol administration by advanced life support (ALS) paramedics were instituted in January 2000, permitting 2.5 to 5.0 mg, either IV or IM administration. Patients given prehospital droperidol were enrolled in the study. Paramedic reports were reviewed looking at indication Glascow comas score (GCS), scene time, route of administration, dosage, need for repeat dosing, need for adjunctive medications, and level of sedation. Sedation was determined by subjective paramedic assessment. Statistical assessment was performed using chi-square.
Results: A total of 46 patients were studies, 24 received IM dosing, and 22 received it IV. Indications for droperidol included need for control of agitation in 39 (85%), anti-emetic effect in four (9%), and to facilitate endotracheal intubations in three (6%). An initial 5-mg dose was given to 18 (75%) of the IM group, five (27%) required additional sedation. An initial 5-mg dose was given to nine (45%) of the IM group, seven (77%) required additional sedation (p < 0.05). Of the six patients given 2,5 mg IM, four (67%) required additional sedation. Of the 13 patients given 2.5 mg IV, nine (70%) required additional sedation (p = NS). Adequate sedation was achieved in 20 (83.3%) in the IM group, and 15 (68%) in the IV roup (p = NS).
Conclusions: Intramuscular administration of droperidol represents a reasonable alternative to the IV route, and achieves adequate sedation in a similar proportion of patients during the prehospital phase of care. Inability to obtain intravenous access should not preclude the use of droperidol for control of agitation.

Prehospital Intravenous Access Failure in Critically Ill or Injured Patients: Effect of Illness Severity and Impact on Field Time
E. D. Bailey, MD, C. T. Lauder, DO, R. E. Megargel, DO, M. Schnyder, NREMT-P, S. M. Vickers, RN, BSN, & G. R. Sivakoff, BSc, Prehospital Emergency Care, 5, 120-121

Prehospital Intravenous Access Failure in Critically Ill or Injured Patients: Effect of Illness Severity and Impact on Field Time
E. D. Bailey, MD, C. T. Lauder, DO, R. E. Megargel, DO, M. Schnyder, NREMT-P, S. M. Vickers, RN, BSN, & G. R. Sivakoff, BSc at the 2001 National Association of Emergency Medical Services Physicians

Objective: Critically ill and injured patients require rapid intravenous (IV) access for fluid administration and pharmacotherapy. The purpose of this retrospective observational study was to identify the incidence of failed intravenous access in critical patients in the prehospital setting. Without IV access, many potentially lifesaving interventions are not possible.
Methods: This study was a retrospective observational chart review, comparing the IV access rates for priority 1 and priority 2 patients. All patients who were categorized as priority 1 or 2 for calendar year 1999 were included. Patients not transported to the hospital and patients < 18 years old where excluded. Patients were divided into medical or trauma categories. The data collected included: chief complaint, mental status, priority, IV access success, site and catheter size, on-scene duration, and transport initiation.
Results: A total of 6,286 patients were enrolled, with 2,089 priority 1 patients (1,581 medical; 508 trauma) and 4,207 priority 2 patients (3,679 medical; 518 trauma).. Twenty-five patients were excluded as nontransports. Priority 1 medical failures = 320/1,581 (20.2%) vs priority 2 medical failures = 524/3,689 (14.2%), p<0.0001. Priority 1 trauma failures = 135/508 (26.5%) vs priority 2 trauma failures = 66/518 (12.7%), p<0.0001. Medical priority 1 successes had an on-scene tine of 14.9 minutes vs failures 12.8 minutes (95% CI 2.0-3.0), and the priority 1 trauma successes on-scene time was 11.8 minutes vs failures 10.01 minutes (CI = 0.0-2.0). Ninety-eight of 320 (30%) priority 2 medical failures and 19/135 (14%) priority 1 trauma failures had a Glasgow coma score (GCS) < 9.
Conclusions: Intravenous access on priority 1 patients can often be difficult or unachievable. These patients often require lifesaving pharmacotherapy for airway management and resuscitation as well as fluid resuscitation. Alternate methods of access need to be developed for critically ill or injured adults to ensure timely resuscitation.