PhD Thesis (PDF) - Department of Astronomy - University of Virginia

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of a galaxy, the low density implies that essentially all of the binary systems are primordial systems which formed together. In the Milky Way, the ∼ 140 LMXBs found in the field (hereafter, Field-LMXBs) are most likely primordial binaries. Roughly half of the Milky Way stars are located in binaries. However, if one compares the number of stars in the Milky Way (total stellar luminosity of L∗ ∼ 10 10 L⊙) with the number of Field-LMXBs, it is clear that it must be relatively difficult to produce an active LMXB from a primordial binary. This is not terribly surprising; the binary progenitor of a primordial LMXB must initially be sufficiently separated to avoid having the stars merge, and the binary must survive the supernova event associated with the formation of the compact object. Despite the likely loss of a significant frac- tion of the mass and gravitational binding energy in the supernova, the binary must end up close enough that the binary system can evolve to the point where Roche-lobe overflow begins. On the other hand, galaxies also contain very dense, spherical star clusters containing up to millions of stars. These systems are called globular clusters (GCs). The stellar densities in GCs are much higher than in the field. Among Galactic GCs, the median central stellar density is 4 × 10 3 M⊙ pc −3 , and the densest GC has a central stellar density of nearly 10 6 M⊙ pc −3 . At such high densities, stellar interactions can occur. These stellar interactions can create (and destroy) binaries. This raises the possibility that a binary system can be created (through two-body tidal capture, or three-body or four-body exchange interactions) in GCs. This can allow a single NS or BH to acquire a donor star, or a compact object in a wide binary can move to a more compact system. For example, a typical three-body exchange interaction tends to create a tighter binary containing the more massive stars. Since most normal stars in GCs have masses of 0.8 M⊙, and NS and BHs always are more massive than 8
this, the result of dynamical interactions in GCs will tend to be the creation of tight binaries containing compact objects. In fact, in the Milky Way, LMXBs are much more likely to be found in GCs than in the field, given the number of stars in GCs; ∼ 10% of Galactic LMXBs are found in GCs, while GCs contain only ∼ 0.06% of the optical light of the Milky Way. Clark (1975) suggested that dynamical interactions in GCs could account for this association of LMXBs with GCs. At least thirteen LMXBs are associated with 12 GCs. (If the two transient LMXBs in NGC 6440 were different LMXBs, then there are 14 LMXBs in 12 GCs.) We list the LMXB names and corresponding GC names in Table 1.1. Nearly all of these sources show Type-I X-ray bursts. These bursts are thermonuclear flashes that must occur on a physical surface. This indicates that the compact objects in the bursting LMXBs are NSs. A few of these sources are transient and are not currently active. Given these relatively low numbers of LMXBs and associated GCs, it is difficult to perform detailed studies of the Galactic LMXB-GC connection; however some patterns have emerged. First, the Galactic GCs with LMXBs tend to have high central densities (Grindlay 1993). This suggests that dynamical interactions play a role in the GC- LMXB connection. This result is supported by recent work by Bregman et al. (2006), which indicated that the Galactic GCs with LMXBs tend to have a higher dynamical interaction rate than those without LMXBs. The second pattern is that the GCs with LMXBs tend to be found in the disk of our Galaxy (Grindlay 1993). This may indicate that tidal shocking and disruptive encounters with the disk play a role in the formation of LMXBs. Perhaps it is more likely that this is due to a preference of LMXBs for GCs with higher metallicities, which is a property of the disk GCs. A much larger sample is needed to probe the LMXB-GC connection. The com- bination of Chandra X-ray Observatory and Hubble Space Telescope observations of 9
Page 1 and 2: LOW-MASS X-RAY BINARIES, DIFFUSE GA
Page 3 and 4: smaller GCs are more likely to cont
Page 5 and 6: GO2-3099X, GO3-4099X, AR3-4005X, GO
Page 7 and 8: her room (my apologies to her offic
Page 9 and 10: US Court expert from India), for al
Page 11 and 12: Table of contents Abstract ii Ackno
Page 13 and 14: 5.2.1 Chandra X-ray Observatory . .
Page 15 and 16: List of Figures 1.1 Hubble Tuning F
Page 17 and 18: List of Tables 1.1 Galactic Low-Mas
Page 19 and 20: Chapter 1 General Introduction 1.1
Page 21 and 22: Fig. 1.1.— Hubble Tuning Fork, co
Page 23 and 24: 2000). In most HMXBs, the accreted
Page 25: not get a complete picture of the L
Page 29 and 30: is the Advanced CCD Imaging Spectro
Page 31 and 32: Chapter 2 Chandra Observations of L
Page 33 and 34: keV (Sarazin et al. 2000, 2001). Wi
Page 35 and 36: agreed with positions from the USNO
Page 37 and 38: 24:00 22:00 7:20:00 18:00 16:00 18:
Page 39 and 40: Table 2.1. Discrete X-ray Sources i
Page 41 and 42: Table 2.1—Continued Source d Coun
Page 43 and 44: Table 2.2. Discrete X-ray Sources i
Page 45 and 46: Table 2.2—Continued Source d Coun
Page 47 and 48: optical/IR positions of R.A. = 12 h
Page 49 and 50: log N(>L) 2.0 1.5 1.0 0.5 0.0 NGC 4
Page 51 and 52: luminosity function for luminositie
Page 53 and 54: H31 1.0 0.5 0.0 -0.5 -1.0 NGC 4365
Page 55 and 56: 2.4.5 Variability We searched for v
Page 57 and 58: Number 10 8 6 4 2 NGC 4365 Optical
Page 59 and 60: LMXBs has an effective radius of 12
Page 61 and 62: component (“MEKAL”) was used fo
Page 63 and 64: Table 2.4. X-ray Spectral Fits of N
Page 65 and 66: Fig. 2.9.— Top panels: Cumulative
Page 67 and 68: we extended the spectrum down to 0.
Page 69 and 70: We also examined whether the unreso
Page 71 and 72: esolved sources in the galaxy. This
Page 73 and 74: temperature ∼0.3 keV. Matsumoto e
Page 75 and 76: log I X (counts/sec/arcmin 2 ) -1.0
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∼45% of the counts are resolved i
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Chapter 3 Chandra Observations of D
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noninteracting members (de Vaucoule
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used for spectroscopic analysis. We
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02:00.0 03:00.0 04:00.0 05:00.0 06:
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Table 3.1. Discrete X-ray Sources i
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Table 3.1—Continued d a Count Rat
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We also attempted detections in mul
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sociated with extended X-ray or opt
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Fig. 3.3.— Histogram of the obser
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chip. The number of ULX candidates
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these luminosities among previously
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to 2-6 keV. Since the 6-10 keV rang
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absorption and varying power-law in
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counts occur within ∼ 4 ks. Sourc
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log I X (counts/sec/arcmin 2 ) 0.5
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tion), and βouter = 0.36 +0.01 −
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the east and northeast of NGC 1600.
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group, we assumed that they were at
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the group gas than we calculate fro
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04:55 -5:05:00 05 -5:05:10 15 -5:05
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for the spectrum: “Field” impli
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90% confidence level errors. Bracke
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we adopted the power-law model as o
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sources could be a non-trivial sour
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jection to include the emission fro
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Fig. 3.13.— Estimated gas and gra
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ackground source population cannot
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NGC 1600 that has a small spatial s
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(HMXBs). LMC X-4, an HMXB pulsar, h
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searched for the shortest time tn o
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with NGC 4697. Columns (1)-(4) list
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duration, ∆tflat, beginning at a
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the uncertainties quoted do not inc
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to persistent rate for short flares
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Chapter 5 Multi-Epoch Chandra X-ray
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the field, these different populati
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luminous (MB < −20) elliptical (E
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Although Chandra is known to encoun
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42:00.0 44:00.0 46:00.0 48:00.0 -5:
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Fig. 5.3.— Adaptively smoothed Ch
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1 × 10 −5 count s −1 arcsec
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Table 5.1—Continued Source R.A. D
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source list against which to regist
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ate from Observation 0784 alone (Pa
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Table 5.2. : Optical Properties of
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actually be an AGN. Sources 79, 80,
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two potential GC counterparts. We l
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Table 5.3—Continued X-ray Source
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Fig. 5.6.— Color (G475−Z850) -
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mass segregation of GCs with the sa
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Table 5.4. Fits to the Expected Num
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dependence are allowed to vary. Var
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NGC 4697 to convert the observed so
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Table 5.6—Continued Source LA LB
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Table 5.6—Continued Source LA LB
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Table 5.7. Combined Luminosities &
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Table 5.7—Continued Source Not De
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Fig. 5.8.— Cumulative luminosity
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LF, there are enough datapoints tha
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Fig. 5.10.— Cumulative luminosity
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Fig. 5.11.— Merged luminosities (
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Fig. 5.12.— X-ray color-color dia
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5.9 Spectral Analysis We performed
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some data loss, it allows for direc
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for 2 dof and the f-test indicates
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ple to LMXBs interior to that semi-
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est-fit (χ 2 = 56.4 for 65 dof) by
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5.10.1 Intraobservation Variability
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Fig. 5.14.— Binned lightcurve usi
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Table 5.10. Possible Flaring Source
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Fig. 5.16.— Cumulative lightcurve
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pairs of observations. In Figure 5.
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Table 5.11—Continued Source PL,AB
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Fig. 5.17.— Percentage of Analysi
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dofs and the number of combinations
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Table 5.12—Continued Source Obs.S
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of states as transient candidates.
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Long-Term Hardness Ratio Variabilit
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In the final observation, there wer
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limits). The 0.3-10 keV X-ray lumin
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atio is expected only from the most
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eak (Kim et al. 2006a). We find mar
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LMXBs in Galactic GCs (Heinke et al
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in the field of the Milky Way and a
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6.2 Sample 6.2.1 Galaxy Sample Tabl
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Table 6.2. Properties of Chandra Ob
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identification of point sources, AC
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the QE degradation in the ACIS dete
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Table 6.3—Continued Galaxy NX LX,
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The positions of the GCs were used
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Fig. 6.2.— Scatter plot of GC gal
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Fig. 6.3.— Scatter plot of estima
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Fig. 6.5.— Scatter plot of estima
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isophote from the galaxy optical su
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6.3.1 Luminosity and Mass Prior obs
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compare their χ 2 fits to test whi
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GCs without LMXBs; however, there a
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following dependence on GC properti
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of this probability with individual
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Fig. 6.9.— Identical to Figure 6.
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We used the indices from our best-f
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(20.3 for half-mass radius, both wi
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most are steeper than Z ∼0.4 . Ei
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Fig. 6.11.— (Left:) Two-dimension
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differently by the properties of th
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dependent IMF (Grindlay 1987), or e
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a 1.4M⊙ neutron star (NS). They s
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equivalent to a dependence on the b
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variability timescale, but it is no
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7.2.1 X-ray Properties of the GC-LM
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of the metallicity dependence. As i
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Appendix A Encounter Rate Parameter
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Fig. A.1.— Dimensionless quantity
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and four; these positions do not in
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Table B.1—Continued RA Dec. d GC
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Table B.2. Optical Properties of Gl
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investigations into improving the h
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C.4 Sivakoff Signature Soy Sauce Sk
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Blanton, E. L., Sarazin, C. L., & I
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Frogel, J. A., Persson, S. E., Matt
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Johnston, H. M. & Verbunt, F. 1996,
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Makishima, K. et al. 2000, ApJ, 535
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Porter, A. C. 1993, PASP, 105, 1250
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Verner, D. A., Ferland, G. J., Kori
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