Daryl Haggard

Amherst College, Amherst Center, Massachusetts, United States

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Publications (82)256.57 Total impact

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    ABSTRACT: Quasars that allow the study of intergalactic medium (IGM) He ii are very rare, since they must be at high redshift along sightlines free of substantial hydrogen absorption, but recent work has dramatically expanded the number of such quasars known. We analyze two dozen higher-redshift (z = 3.1–3.9) low-resolution He ii quasar spectra from the Hubble Space Telescope and find that their He ii Gunn–Peterson troughs suggest exclusion of very early and very late reionization models, favoring a reionization redshift of z ∼ 3. Although the data quality is not sufficient to reveal details such as the expected redshift evolution of helium opacity, we obtain the first ensemble measure of helium opacity at high redshift averaged over many sightlines: τ = 4.90 at z ∼ 3.3. We also find that it would be very difficult to observe the IGM red wing of absorption from the beginning of He ii reionization, but depending on the redshift of reionization and the size of ionization zones, it might be possible to do so in some objects with the current generation of UV spectrographs.
    Preview · Article · Aug 2015 · The Astrophysical Journal
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    ABSTRACT: We present a study of the X-ray flaring activity of Sgr A⋆ during all the 150 XMM–Newton and Chandra observations pointed at the Milky Way centre over the last 15 years. This includes the latest XMM–Newton and Chandra campaigns devoted to monitoring the closest approach of the very red Brγ emitting object called G2. The entire data set analysed extends from 1999 September through 2014 November. We employed a Bayesian block analysis to investigate any possible variations in the characteristics (frequency, energetics, peak intensity, duration) of the flaring events that Sgr A⋆ has exhibited since their discovery in 2001. We observe that the total bright or very bright flare luminosity of Sgr A⋆ increased between 2013 and 2014 by a factor of 2–3 (∼3.5σ significance). We also observe an increase (∼99.9 per cent significance) from 0.27 ± 0.04 to 2.5 ± 1.0 d−1 of the bright or very bright flaring rate of Sgr A⋆, starting in late summer 2014, which happens to be about six months after G2's pericentre passage. This might indicate that clustering is a general property of bright flares and that it is associated with a stationary noise process producing flares not uniformly distributed in time (similar to what is observed in other quiescent black holes). If so, the variation in flaring properties would be revealed only now because of the increased monitoring frequency. Alternatively, this may be the first sign of an excess accretion activity induced by the close passage of G2. More observations are necessary to distinguish between these two hypotheses.
    Full-text · Article · Jul 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We report on multi-frequency, wideband radio observations of the Galactic Center magnetar (SGR 1745$-$2900) with the Green Bank Telescope for $\sim$100 days immediately following its initial X-ray outburst in April 2013. We made multiple simultaneous observations at 1.5, 2.0, and 8.9 GHz, allowing us to examine the magnetar's flux evolution, radio spectrum, and interstellar medium parameters (such as the dispersion measure (DM), the scattering timescale and its index). During two epochs, we have simultaneous observations from the Chandra X-ray Observatory, which permitted the absolute alignment of the radio and X-ray profiles. As with the two other radio magnetars with published alignments, the radio profile lies within the broad peak of the X-ray profile, preceding the X-ray profile maximum by $\sim$0.2 rotations. We also find that the radio spectral index $\gamma$ is significantly negative between $\sim$2 and 9 GHz; during the final $\sim$30 days of our observations $\gamma \sim -1.4$, which is typical of canonical pulsars. The increasing radio flux has not turned over during this outburst, whereas the long-term trends in the other radio magnetars show concomitant fading of the radio and X-ray fluxes. Finally, our wideband measurements of the DMs taken in adjacent frequency bands in tandem are stochastically inconsistent with one another. Based on recent theoretical predictions, we consider the possibility that the dispersion measure is frequency-dependent. Despite having several properties in common with the other radio magnetars, such as $L_{\textrm{X,qui}}/L_{\textrm{rot}} \lesssim 1$, an increase in the radio flux during the X-ray flux decay has not been observed thus far in other systems.
    Full-text · Article · May 2015 · The Astrophysical Journal
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    ABSTRACT: In 2013 April a new magnetar, SGR 1745−2900, was discovered as it entered an outburst, at only 2.4 arcsec angular distance from the supermassive black hole at the centre of the Milky Way, Sagittarius A*. SGR 1745−2900 has a surface dipolar magnetic field of ∼2 × 1014 G, and it is the neutron star closest to a black hole ever observed. The new source was detected both in the radio and X-ray bands, with a peak X-ray luminosity LX ∼ 5 × 1035 erg s−1. Here we report on the long-term Chandra (25 observations) and XMM–Newton (eight observations) X-ray monitoring campaign of SGR 1745−2900 from the onset of the outburst in 2013 April until 2014 September. This unprecedented data set allows us to refine the timing properties of the source, as well as to study the outburst spectral evolution as a function of time and rotational phase. Our timing analysis confirms the increase in the spin period derivative by a factor of ∼2 around 2013 June, and reveals that a further increase occurred between 2013 October 30 and 2014 February 21. We find that the period derivative changed from 6.6 × 10−12 to 3.3 × 10−11 s s−1 in 1.5 yr. On the other hand, this magnetar shows a slow flux decay compared to other magnetars and a rather inefficient surface cooling. In particular, starquake-induced crustal cooling models alone have difficulty in explaining the high luminosity of the source for the first ∼200 d of its outburst, and additional heating of the star surface from currents flowing in a twisted magnetic bundle is probably playing an important role in the outburst evolution.
    Full-text · Article · Mar 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present a statistical analysis of the X-ray flux distribution of Sgr A* from the Chandra X-ray Observatory's 3 Ms Sgr A* X-ray Visionary Project (XVP) in 2012. Our analysis indicates that the observed X-ray flux distribution can be decomposed into a steady quiescent component, represented by a Poisson process with rate $Q=(5.24\pm0.08)\times10^{-3}$ cts s$^{-1},$ and a variable component, represented by a power law process ($dN/dF\propto F^{-\xi},$ $\xi=1.92_{-0.02}^{+0.03}$). This slope matches our recently-reported distribution of flare luminosities. The variability may also be described by a log-normal process with a median unabsorbed 2-8 keV flux of $1.8^{+0.9}_{-0.6}\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$ and a shape parameter $\sigma=2.4\pm0.2,$ but the power law provides a superior description of the data. In this decomposition of the flux distribution, all of the intrinsic X-ray variability of Sgr A* (spanning at least three orders of magnitude in flux) can be attributed to flaring activity, likely in the inner accretion flow. We confirm that at the faint end, the variable component contributes ~10% of the apparent quiescent flux, as previously indicated by our statistical analysis of X-ray flares in these Chandra observations. Our flux distribution provides a new and important observational constraint on theoretical models of Sgr A*, and we use simple radiation models to explore the extent to which a statistical comparison of the X-ray and infrared can provide insights into the physics of the X-ray emission mechanism.
    Full-text · Article · Dec 2014 · The Astrophysical Journal
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    ABSTRACT: AX J1745.6-2901 is a high-inclination (eclipsing) neutron star low-mass X-ray binary (LMXB) located less than ∼1.5 arcmin from Sgr A⋆. Ongoing monitoring campaigns have targeted Sgr A⋆ frequently and these observations also cover AX J1745.6-2901. We present here an X-ray analysis of AX J1745.6-2901 using a large data set of 38 XMM–Newton observations, including 11 which caught AX J1745.6-2901 in outburst. Fe K absorption is clearly seen when AX J1745.6-2901 is in the soft state, but disappears during the hard state. The variability of these absorption features does not appear to be due to changes in the ionizing continuum. The small Kα/Kβ ratio of the equivalent widths of the Fe xxv and Fe xxvi lines suggests that the column densities and turbulent velocities of the absorbing ionized plasma are in excess of NH ≃ 1023 cm−2 and vturb ≳ 500 km s−1. These findings strongly support a connection between the wind (Fe K absorber) and the accretion state of the binary. These results reveal strong similarities between AX J1745.6-2901 and the eclipsing neutron star LMXB, EXO 0748-676, as well as with high-inclination black hole binaries, where winds (traced by the same Fe K absorption features) are observed only during the accretion-disc-dominated soft states, and disappear during the hard states characterized by jet emission.
    Full-text · Article · Sep 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: The recent discovery of a milli-second radio pulsar experiencing an accretion outburst similar to those seen in low mass X-ray binaries, has opened up a new opportunity to investigate the evolutionary link between these two different neutron star manifestations. The remarkable X-ray variability and hard X-ray spectrum of this object can potentially serve as a template to search for other X-ray binary / radio pulsar transitional objects. Here we demonstrate that the transient X-ray source XMM J174457-2850.3 near the Galactic center displays similar X-ray properties. We report on the detection of an energetic thermonuclear burst with an estimated duration of ~2 hr and a radiated energy output of ~5E40 erg, which unambiguously demonstrates that the source harbors an accreting neutron star. It has a quiescent X-ray luminosity of Lx~5E32 erg/s and exhibits occasional accretion outbursts during which it brightens to Lx~1E35-1E36 erg/s for a few weeks (2-10 keV). However, the source often lingers in between outburst and quiescence at Lx~1E33-1E34 erg/s. This unusual X-ray flux behavior and its relatively hard X-ray spectrum (a power law with an index of ~1.4) could possibly be explained in terms of the interaction between the accretion flow and the magnetic field of the neutron star.
    Preview · Article · Jun 2014 · The Astrophysical Journal
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    ABSTRACT: We use Chandra and XMM–Newton observations of the globular clusters ω Cen and NGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and thus to constrain the allowed ranges of mass and radius for each. We also use Hubble Space Telescope photometry of NGC 6397 to identify a potential optical companion to the quiescent NS, and find evidence that the companion lacks hydrogen. We carefully consider a number of systematic problems, and show that the choices of atmospheric composition, interstellar medium abundances, and cluster distances can have important effects on the inferred NS mass and radius. We find that for typical NS masses, the radii of both NSs are consistent with the 10-13 km range favoured by recent nuclear physics experiments. This removes the evidence suggested by Guillot and collaborators for an unusually small NS radius, which relied upon the small inferred radius of the NGC 6397 NS.
    Full-text · Article · Jun 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We report observations using the Swift/XRT, NuSTAR, and Chandra X-ray telescopes of the transient X-ray source CXOGC J174540.0-290005, during its 2013 outburst. Due to its location in the field of multiple observing campaigns targeting Sgr A*, this is one of the best-studied outbursts of a very faint X-ray binary (VFXB; peak LX < 1036 erg/s) yet recorded, with detections in 173 ks of X-ray observations over 50 days. VFXBs are of particular interest, due to their unusually low outburst luminosities and time-averaged mass transfer rates, which are hard to explain within standard accretion physics and binary evolution. The 2013 outburst of CXOGC J174540.0-290005 peaked at Lx (2-10 keV)=5.0 × 1035 erg/s, and all data above 1034 ergs/s were well-fit by an absorbed power-law of photon index ∼ 1.7, extending from 2 keV out to ~70 keV. We discuss the implications of these observations for the accretion state of CXOGC J174540.0-290005.
    Full-text · Article · May 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We report observations using the Swift/XRT, NuSTAR, and Chandra X-ray telescopes of the transient X-ray source CXOGC J174540.0-290005 during its 2013 outburst. Due to its location in the field of multiple observing campaigns targeting Sgr A*, this is one of the best-studied outbursts of a very faint X-ray binary (VFXB; peak LX < 10^(36) erg s^(−1)) yet recorded, with detections in 173 ks of X-ray observations over 50 d. VFXBs are of particular interest, due to their unusually low outburst luminosities and time-averaged mass transfer rates, which are hard to explain within standard accretion physics and binary evolution. The 2013 outburst of CXOGC J174540.0-290005 peaked at L_X(2–10 keV) = 5.0 × 10^(35) erg s^(−1), and all data above 10^(34) erg s^(−1) were well fitted by an absorbed power law of photon index ∼1.7, extending from 2 keV out to ≳ 70 keV. We discuss the implications of these observations for the accretion state of CXOGC J174540.0-290005.
    No preview · Article · Apr 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Swift has resumed its daily X-ray monitoring campaign of the Galactic center (Atel #5006; see link below). The first XRT observation was performed on 2014 February 2 and had a duration of 1.1 ks.
    No preview · Article · Jan 2014
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    ABSTRACT: We report on continued X-ray monitoring observations of the Galactic center with the Swift/XRT (Atel #5847). Between 2014 February 2 and 6 the XRT count rate at the position of Sgr A* and the nearby transient magnetar SGR J1745-29 varied between ~1E-2 and 3E-2 counts s-1.
    No preview · Article · Jan 2014
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    ABSTRACT: We have observed SGR J1745-29 (Kennea et al. 2013, ApJ 770, L24; Mori et al. 2013, ApJ 770, L23; Rea et al. 2013, ApJ 775, L34) with the ACIS-S camera onboard Chandra for about 47ks starting on 2014-02-21 (ObsID 16508), as part of the on-going Chandra X-ray monitoring of the Galactic Center region (PIs: Haggard, Baganoff, Rea).
    No preview · Article · Jan 2014
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    ABSTRACT: The recent discovery of a dense, cold cloud (dubbed "G2") approaching Sgr A* offers an opportunity to test models of black hole accretion and its associated feedback. G2's orbit is eccentric and the cloud shows signs of tidal disruption by the black hole. High-energy emission from the Sgr A*/G2 encounter may rise toward pericenter (mid-to-late 2013, or early 2014) and continue over the next several years as the material circularizes. This encounter is also likely to enhance Sgr A*'s flare activity across the electromagnetic spectrum. We present preliminary results from our 2013 joint Chandra/XMM/VLA monitoring campaigns. Our programs aim to study the radiation properties of Sgr A* as G2 breaks up and feeds the accretion flow, to constrain the rates and emission mechanisms of faint X-ray flares, and to detect G2 itself as it is shocked and heated. We discuss the constraints these data place on theoretical models for the Sgr A*/G2 encounter and outline plans for continued monitoring with Chandra, XMM, HST, and VLA in 2014.
    No preview · Article · Jan 2014
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    ABSTRACT: Starting in 2006, the Galactic center has been monitored on a nearly daily basis with the X-ray telescope on-board the Swift satellite. The short pointed observations have offered a unique view of the long-term X-ray behavior of Sgr A*, in particular of its X-ray flaring properties. The Swift campaign also provides an excellent setup to closely monitor the interaction of the supermassive black hole with the gaseous object G2. Because of the unique daily sampling, the Swift program may prove to serve as an important trigger for other observatories at different wavelengths. I will report on the latest results of the Swift X-ray monitoring campaign of the Galactic center.
    No preview · Article · Jan 2014
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    ABSTRACT: Over the last decade, X-ray observations of Sgr A* have revealed a black hole in a deep sleep, punctuated roughly once per day by brief flares. The extreme X-ray faintness of this supermassive black hole has been a long-standing puzzle in black hole accretion. To study the accretion processes in the Galactic Center, Chandra (in concert with numerous ground- and space-based observatories) undertook a 3 Ms campaign on Sgr A* in 2012. With its excellent observing cadence, sensitivity, and spectral resolution, this Chandra X-ray Visionary Project (XVP) provides an unprecedented opportunity to study the behavior of the closest supermassive black hole. We present a progress report from our ongoing study of X-ray flares, including the brightest flare ever seen from Sgr A*. Focusing on the statistics of the flares and the quiescent emission, we discuss the physical implications of X-ray variability in the Galactic Center.
    Full-text · Article · Nov 2013 · Proceedings of the International Astronomical Union

  • No preview · Article · Nov 2013 · The Astrophysical Journal Letters
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    ABSTRACT: Most supermassive black holes (SMBHs) are accreting at very low levels and are difficult to distinguish from the galaxy centers where they reside. Our own Galaxy's SMBH provides a uniquely instructive exception, and we present a close-up view of its quiescent X-ray emission based on 3 mega-second of Chandra observations. Although the X-ray emission is elongated and aligns well with a surrounding disk of massive stars, we can rule out a concentration of low-mass coronally active stars as the origin of the emission based on the lack of predicted Fe Kalpha emission. The extremely weak H-like Fe Kalpha line further suggests the presence of an outflow from the accretion flow onto the SMBH. These results provide important constraints for models of the prevalent radiatively inefficient accretion state.
    Full-text · Article · Aug 2013 · Science
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    ABSTRACT: The center of our Galaxy hosts a supermassive black hole, Sagittarius (Sgr) A*. Young, massive stars within 0.5 pc of SgrA* are evidence of an episode of intense star formation near the black hole a few Myr ago, which might have left behind a young neutron star traveling deep into SgrA*'s gravitational potential. On 2013 April 25, a short X-ray burst was observed from the direction of the Galactic center. Thanks to a series of observations with the Chandra and the Swift satellites, we pinpoint the associated magnetar at an angular distance of 2.4+/-0.3 arcsec from SgrA*, and refine the source spin period and its derivative (P=3.7635537(2) s and \dot{P} = 6.61(4)x10^{-12} s/s), confirmed by quasi simultaneous radio observations performed with the Green Bank (GBT) and Parkes antennas, which also constrain a Dispersion Measure of DM=1750+/-50 pc cm^{-3}, the highest ever observed for a radio pulsar. We have found that this X-ray source is a young magnetar at ~0.07-2 pc from SgrA*. Simulations of its possible motion around SgrA* show that it is likely (~90% probability) in a bound orbit around the black hole. The radiation front produced by the past activity from the magnetar passing through the molecular clouds surrounding the Galactic center region, might be responsible for a large fraction of the light echoes observed in the Fe fluorescence features.
    Full-text · Article · Jul 2013 · The Astrophysical Journal Letters
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    ABSTRACT: We report a sensitive X-ray search for the proposed intermediate-mass black hole (IMBH) in the massive Galactic cluster, ω Centauri (NGC 5139). Combining Chandra X-ray Observatory data from Cycles 1 and 13, we obtain a deep (~291 ks) exposure of the central regions of the cluster. We find no evidence for an X-ray point source near any of the cluster's proposed dynamical centers, and place an upper limit on the X-ray flux from a central source of f X(0.5-7.0 keV) ≤5.0 × 10–16 erg cm–2 s–1, after correcting for absorption. This corresponds to an unabsorbed X-ray luminosity of L X(0.5-7.0 keV) ≤1.6 × 1030 erg s–1, for a cluster distance of 5.2 kpc, Galactic column density N H = 1.2 × 1021 cm–2, and power-law spectrum with Γ = 2.3. If a ~104 IMBH resides in the cluster's core, as suggested by some stellar dynamical studies, its Eddington luminosity would be L Edd ~1042 erg s–1. The new X-ray limit would then establish an Eddington ratio of L X/L Edd 10–12, a factor of ~10 lower than even the quiescent state of our Galaxy's notoriously inefficient supermassive black hole Sgr A*, and imply accretion efficiencies as low as η 10–6-10–8. This study leaves open three possibilities: either ω Cen does not harbor an IMBH or, if an IMBH does exist, it must experience very little or very inefficient accretion.
    Full-text · Article · Jul 2013 · The Astrophysical Journal Letters

Publication Stats

2k Citations
256.57 Total Impact Points

Institutions

  • 2015
    • Amherst College
      Amherst Center, Massachusetts, United States
  • 2005-2015
    • University of Washington Seattle
      • Department of Astronomy
      Seattle, WA, United States
  • 2010-2014
    • Northwestern University
      • • Department of Physics and Astronomy
      • • Center for Interdisciplinary Exploration and Research in Astrophysics
      Evanston, Illinois, United States
    • University of California, Santa Barbara
      • Kavli Institute for Theoretical Physics
      Santa Barbara, California, United States
  • 2008
    • San Francisco State University
      • Department of Physics and Astronomy
      San Francisco, CA, United States
  • 2002
    • University of Leicester
      Leiscester, England, United Kingdom