arXiv subjects

Jules P. Halpern

Publications and source records attributed to Jules P. Halpern.

At least 37 records · Page 2Linked to original sources

arXiv2014

Optical Counterparts of Two Fermi Millisecond Pulsars: PSR J1301+0833 and PSR J1628-3205

Using the 1.3m and 2.4m telescopes of the MDM Observatory, we identified the close companions of two eclipsing millisecond radio pulsars discovered by the Green Bank Telescope in searches of Fermi Gamma-ray Space Telescope sources, and measured their light curves. PSR J1301+0833 is a black widow pulsar in a 6.5 hr orbit whose companion star is strongly heated on the side facing the pulsar. It varies from R = 21.8 to R > 24 around the orbit. PSR J1628-3205 is a "redback," a nearly Roche-lobe filling system in a 5.0 hr orbit whose optical modulation in the range 19.0 < R < 19.4 is dominated by strong ellipsoidal variations, indicating a large orbital inclination angle. PSR J1628-3205 also shows evidence for a long-term variation of about 0.2 mag, and an asymmetric temperature distribution possibly due to either off-center heating by the pulsar wind, or large starspots. Modelling of its light curve restricts the inclination angle to i > 55 degrees, the mass of the companion to 0.16 < M_c < 0.30 M_sun, and the effective temperature to 3560 < T_eff < 4670 K. As is the case for several redbacks, the companion of PSR J1628-3205 is less dense and hotter than a main-sequence star of the same mass.

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arXiv2014

A broadband X-ray study of the Geminga pulsar with NuSTAR and XMM-Newton

We report on the first hard X-ray detection of the Geminga pulsar above 10 keV using a 150 ks observation with the NuSTAR observatory. The double-peaked pulse profile of non-thermal emission seen in the soft X-ray band persists at higher energies. Broadband phase-integrated spectra over the 0.2-20 keV band with NuSTAR and archival XMM-Newton data do not fit to a conventional two-component model of a blackbody plus power-law, but instead exhibit spectral hardening above ~5 keV. We find two spectral models fit the data well: (1) a blackbody (kT1 ~ 42 eV) with a broken power-law (Gamma1 ~ 2.0, Gamma2 ~ 1.4 and Ebreak ~ 3.4 keV), and (2) two blackbody components (kT1 ~ 44 eV and kT2 ~ 195 eV) with a power-law component (Gamma ~ 1.7). In both cases, the extrapolation of the Rayleigh-Jeans tail of the thermal component is consistent with the UV data, while the non-thermal component overpredicts the near-infrared data, requiring a spectral flattening at E ~ 0.01 - 1 keV. While strong phase variation of the power-law index is present below ~5 keV, our phase-resolved spectroscopy with NuSTAR indicates that another hard non-thermal component with Gamma ~ 1.3 emerges above ~5 keV. The spectral hardening in non-thermal X-ray emission as well as spectral flattening between the optical and X-ray bands argue against the conjecture that a single power-law may account for multi-wavelength non-thermal spectra of middle-aged pulsars.

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arXiv2012

Variability in Quasar Broad Absorption Line Outflows III. What Happens on the Shortest Time-scales?

Broad absorption lines (BALs) in quasar spectra are prominent signatures of high-velocity outflows, which might be present in all quasars and could be a major contributor to feedback to galaxy evolution. Studying the variability in these BALs allows us to further our understanding of the structure, evolution, and basic physical properties of the outflows. This is the third paper in a series on a monitoring programme of 24 luminous BAL quasars at redshifts 1.2 < z < 2.9. We focus here on the time-scales of variability in CIV 1549A BALs in our full multi-epoch sample, which covers time-scales from 0.02-8.7 yr in the quasar rest-frame. Our sample contains up to 13 epochs of data per quasar, with an average of 7 epochs per quasar. We find that both the incidence and the amplitude of variability are greater across longer time-scales. Part of our monitoring programme specifically targeted half of these BAL quasars at rest-frame time-scales <2 months. This revealed variability down to the shortest time-scales we probe (8-10 days). Observed variations in only portions of BAL troughs or in lines that are optically thick suggest that at least some of these changes are caused by clouds (or some type of outflow substructures) moving across our lines of sight. In this crossing cloud scenario, the variability times constrain both the crossing speeds and the absorber locations. Typical variability times of order ~1 year indicate crossing speeds of a few thousand km/s and radial distances near ~1 pc from the central black hole. However, the most rapid BAL changes occurring in 8-10 days require crossing speeds of 17 000 - 84 000 km/s and radial distances of only 0.001-0.02 pc. These speeds are similar to or greater than the observed radial outflow speeds, and the inferred locations are within the nominal radius of the broad emission line region.

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arXiv2011

A Large Systematic Search for Recoiling and Close Supermassive Binary Black Holes

[ABRIDGED] We have carried out a systematic search for close supermassive black hole binaries among z < 0.7 SDSS quasars Such binaries are predicted by models of supermassive black hole and host galaxy co-evolution, therefore their census and population properties constitute an important test of these models. We used an automatic technique based on spectroscopic principal component analysis to search for broad H-beta lines that are displaced from the rest-frame of the quasar by more than 1,000 km/s This method can also yield candidates for rapidly recoiling black holes. Our search yielded 88 candidates, several of which were previously identified and discussed in the literature. The widths of the broad H-beta lines are typical among quasars but the shifts are extreme. We found a correlation between the peak offset and skewness of the broad H-beta profiles, which suggests that the profiles we have selected share a common physical explanation. The general properties of the narrow emission lines are typical of quasars. We carried out followup spectroscopic observations of 68 objects to search for changes in the peak velocities of the H-beta lines (the time interval in the observer's frame between the original and new observations is 1-10 yr). We measured significant changes in 14 objects, with resulting accelerations between -120 and +120 km/s/yr. We emphasize that interpretation of the offset broad emission lines as signatures of supermassive binaries is subject to many significant caveats. Many more followup observations over a long temporal baseline are needed to characterize the variability pattern of the broad lines and test that this pattern is indeed consistent with orbital motion. The possibility that some of the objects in this sample are rapidly recoiling black holes remains open as the available data do not provide strong constraints for this scenario.

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arXiv2008

The Rapidly Flaring Afterglow of the Very Bright and Energetic GRB 070125

We report on multi-wavelength observations, ranging from the X-ray to radio wave bands, of the IPN-localized gamma-ray burst GRB 070125. Spectroscopic observations reveal the presence of absorption lines due to O I, Si II, and C IV, implying a likely redshift of z = 1.547. The well-sampled light curves, in particular from 0.5 to 4 days after the burst, suggest a jet break at 3.7 days, corresponding to a jet opening angle of ~7.0 degrees, and implying an intrinsic GRB energy in the 1 - 10,000 keV band of around E = (6.3 - 6.9)x 10^(51) erg (based on the fluences measured by the gamma-ray detectors of the IPN network). GRB 070125 is among the brightest afterglows observed to date. The spectral energy distribution implies a host extinction of Av < 0.9 mag. Two rebrightening episodes are observed, one with excellent time coverage, showing an increase in flux of 56% in ~8000 seconds. The evolution of the afterglow light curve is achromatic at all times. Late-time observations of the afterglow do not show evidence for emission from an underlying host galaxy or supernova. Any host galaxy would be subluminous, consistent with current GRB host-galaxy samples. Evidence for strong Mg II absorption features is not found, which is perhaps surprising in view of the relatively high redshift of this burst and the high likelihood for such features along GRB-selected lines of sight.

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arXiv2007

Neutral Hydrogen Absorption Toward XTE J1810-197: the Distance to a Radio-Emitting Magnetar

We have used the Green Bank Telescope to measure HI absorption against the anomalous X-ray pulsar XTE J1810-197. Assuming a flat rotation curve, we find that XTE J1810-197 is located at a distance of 3.4(+0.5,-0.7) kpc. For a rotation curve that incorporates a model of the Galactic bar, we obtain a distance of 4.0(+0.3,-0.8) kpc. Using a rotation curve that incorporates a model of the Galactic bar and the spiral arms of the Galaxy, the distance is 3.7(+/-0.6) kpc. These values are consistent with the distance to XTE J1810-197 of about 3.3 kpc derived from its dispersion measure, and estimates of 2-5 kpc obtained from fits to its X-ray spectra. Overall, we determine that XTE J1810-197 is located at a distance of 3.5(+/-0.5) kpc, possibly not far in front of the infrared dark cloud G10.74-0.13. We also used the GBT in an attempt to measure absorption in the OH 1612, 1665, 1667, and 1720 MHz lines against XTE J1810-197. We were unsuccessful in this, mainly because of its declining radio flux density. Analysis of HI 21 cm, OH, and CO(2-1) emission toward XTE J1810-197 allows us to place a lower limit of $N_{\rm H} \ga 4.6 \times 10^{21}$ cm$^{-2}$ on the non-ionized hydrogen column density to \magnetar, consistent with estimates obtained from fits to its X-ray spectra.

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arXiv2007

The Intrinsically X-ray Weak Quasar PHL 1811. II. Optical and UV Spectra and Analysis

This is the second of two papers reporting observations and analysis of the unusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-line quasar PHL 1811. The first paper reported that PHL 1811 is intrinsically X-ray weak, and presented a spectral energy distribution (SED). Here we present HST STIS optical and UV spectra, and ground-based optical spectra. The optical and UV line emission is very unusual. There is no evidence for forbidden or semiforbidden lines. The near-UV spectrum is dominated by very strong FeII and FeIII, and unusual low-ionization lines such as NaID and CaII H&K are observed. High-ionization lines are very weak; CIV has an equivalent width of 6.6A, a factor of ~5 smaller than measured from quasar composite spectra. An unusual feature near 1200A can be deblended in terms of Lyα, NV, SiII, and CIII* using the blueshifted CIV profile as a template. Photoionization modeling shows that the unusual line emission can be explained qualitatively by the unusually soft SED. Principally, a low gas temperature results in inefficient emission of collisionally excited lines, including the semiforbidden lines generally used as density diagnostics. The emission resembles that of high-density gas; in both cases this is a consequence of inefficient cooling. PHL 1811 is very unusual, but we note that quasar surveys are generally biased against finding similar objects.

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arXiv2006

Recent Observations of EGRET Pulsar Wind Nebulae

We present recent X-ray and radio observations of pulsar wind nebulae discovered in EGRET error boxes. Two XMM-Newton observations show the X-ray extent of the rapidly moving PWN associated with the variable gamma-ray source 3EG J1809-2328, and a trail coming from the new millisecond pulsar PSR J1614-2230 at high Galactic z. We also briefly discuss three PWN that are HESS TeV sources including a new HESS source we argue is associated with the Eel nebula in 3EG J1826-1302.

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arXiv2006

The Intrinsically X-ray Weak Quasar PHL 1811. I. X-ray Observations and Spectral Energy Distribution

This is the first of two papers reporting observations and analysis of the unusually bright (m_b=14.4), luminous (M_B=-25.5), nearby (z=0.192) narrow-line quasar PHL 1811, focusing on the X-ray properties and the spectral energy distribution. Two Chandra observations reveal a weak X-ray source with a steep spectrum. Variability by a factor of 4 between the two observations separated by 12 days suggest that the X-rays are not scattered emission. The XMM-Newton spectra are modelled in the 0.3--5 keV band by a steep power law with Γ= 2.3\pm 0.1, and the upper limit on intrinsic absorption is 8.7 x 10^{20} cm^{-2}. The spectral slopes are consistent with power law indices commonly observed in NLS1s, and it appears that we observe the central engine X-rays directly. Including two recent Swift ToO snapshots, a factor of ~5 variability was observed among the five X-ray observations reported here. In contrast, the UV photometry obtained by the XMM-Newton OM and Swift UVOT, and the HST spectrum reveal no significant UV variability. The α_{ox} inferred from the Chandra and contemporaneous HST spectrum is -2.3 \pm 0.1, significantly steeper than observed from other quasars of the same optical luminosity. The steep, canonical X-ray spectra, lack of absorption, and significant X-ray variability lead us to conclude that PHL 1811 is intrinsically X-ray weak. We also discuss an accretion disk model, and the host galaxy of PHL 1811.

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arXiv2004

Long-Term Profile Variability of Double-Peaked Emmission Lines in AGNs

An increasing number of AGNs exhibit broad, double-peaked Balmer emission lines, which arise from the outer regions of the accretion disk which fuels the AGN. The line profiles vary on timescales of 5--10 years. Our group has monitored a set of 20 double-peaked emitters for the past 8 years (longer for some objects). Here we describe a project to characterize the variability patterns of the double-peaked H alpha line profiles and compare with those of two simple models: a circular disk with a spiral arm and an elliptical disk.

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arXiv2004

The Low Quiescent X-Ray Luminosity of the Neutron Star Transient XTE J2123-058

We report on the first X-ray observations of the neutron star soft X-ray transient (SXT) XTE J2123-058 in quiescence, made by Chandra and BeppoSAX, as well as contemporaneous optical observations. In 2002, the Chandra spectrum of XTE J2123-058 is consistent with a power-law model, or the combination of a blackbody plus a power-law, but it is not well-described by a pure blackbody. Using the interstellar column density, the power-law fit gives photon index of 3.1 (+0.7,-0.6) and indicates a 0.3-8 keV unabsorbed luminosity of 9(+4,-3)E31 (d/8.5 kpc)^2 ergs/s (90% confidence errors). Fits with models consisting of thermal plus power-law components indicate that the upper limit on the temperature of a 1.4 solar mass, 10 km radius neutron star with a hydrogen atmosphere is kT_eff < 66 eV, and the upper limit on the bolometric luminosity is L_infinity < 1.4E32 ergs/s, assuming d = 8.5 kpc. Of the neutron star SXTs that exhibit short (< 1 year) outbursts, including Aql X-1, 4U 1608-522, Cen X-4, and SAX J1810.8-2609, the lowest temperatures and luminosities are found for XTE J2123-058 and SAX J1810.8-2609. From the BeppoSAX observation of XTE J2123-058 in 2000, we obtained an upper limit on the 1-10 keV unabsorbed luminosity of 9E32 ergs/s. Although this upper limit allows that the X-ray luminosity may have decreased between 2000 and 2002, that possibility is not supported by our contemporaneous R-band observations, which indicate that the optical flux increased significantly. Motivated by the theory of deep crustal heating by Brown and co-workers, we characterize the outburst histories of the five SXTs. The low quiescent luminosity for XTE J2123-058 is consistent with the theory of deep crustal heating without requiring enhanced neutron star cooling if the outburst recurrence time is >~ 70 years.

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arXiv2004

PHL 1811: The Local Prototype of the Lineless High-z SDSS QSOs

Abridged) In the SDSS, several unusual QSOs have been discovered that have very blue rest-frame UV spectra but no discernible emission lines. Their UV spectra strongly resemble that of the newly discovered quasar PHL 1811 ($z=0.192$; $\rm M_{V}=-25.9$). With magnitudes of $B = 14.4$ and $R = 14.1$, PHL 1811 is the second brightest quasar known with $z>0.1$ after 3C 273. Optically it is classified as a Narrow-line Seyfert 1 galaxy (NLS1). Objects of this class are generally strong soft X-ray emitters, but a BeppoSAX observation of PHL 1811 showed that it is anomalously X-ray weak. The inferred $α_{ox}$ was 1.9--2.1, much steeper than the nominal value of 1.6 for quasars of this optical luminosity, and comparable to the X-ray weakest quasars. Follow-up {\it Chandra} observations reveal a variable, unabsorbed X-ray spectrum and confirm that it is intrinsically X-ray weak.

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arXiv2003

Long Term Profile Variability of Double-Peaked Emission Lines in AGNs

An increasing number of AGNs exhibit broad, double-peaked Balmer emission lines, which are thought to arise from the outer regions of the accretion disk which fuels the AGN. The line profiles are observed to vary on a characteristic timescales of 5-10 years. The variability is not a reverberation effect; it is a manifestation of physical changes in the disk. Our group has monitored a set of 20 double-peaked emitters for the past 8 years (longer for some objects). Here, we characterize the variability of the double-peaked H alpha line profiles in five objects from our sample. By experimenting with simple models, we find that disks with a single precessing spiral arm are able to reproduce many of the variability trends that are seen in the data.

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arXiv2003

Accurate Redshifts and Classifications for 110 Radio-Loud AGNs

We report accurate redshifts of 110 active galaxies (mostly radio-loud objects at z<0.4) observed in the course of a survey to find broad, double-peaked emission lines. These redshifts are measured from the narrow emission lines of these objects and are accurate to at least one part in 10^4 . For each object we determine a redshift from high- and low-ionization lines separately, as well as an average redshift from all the available lines. We find that in about 15% of cases, the low-ionization lines yield a slightly higher redshift than the high-ionization lines; the average redshift difference amounts to a velocity difference of approximately 80 km/s. In addition to the redshift measurements we also report revised redshifts for two objects as well as new classifications for three narrow-line objects.

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arXiv2003

Absorption-Line Systems and Galaxies in Front of the Second Brightest Quasar, PHL 1811

The extraordinarily bright quasar PHL 1811 at a redshift z = 0.192 provides an attractive opportunity to use ultraviolet absorption-line spectroscopy to study the properties of gas systems in the local universe. An R = 20,000 far-UV spectrum recorded by FUSE revealed 7 extragalactic absorption systems, one of which is a Lyman limit system at z = 0.08093 accompanied by three systems having redshifts which differ from it by less than 0.008. The abundance of O with respect to Fe in the Lyman limit system is not much different from the solar abundance ratio. Supplementary low resolution spectra recorded by STIS (on HST) at longer wavelengths helped to substantiate our identifications of systems in the FUSE spectrum and suggested the presence of an additional 4 systems that could be detected only through their Ly-alpha features. Spectroscopy at visible wavelengths of 7 galaxies within approximately 2' of PHL 1811 indicated that 2 of them are near the redshift of the quasar and 4 have redshifts within 850 km/s of the extragalactic absorption systems. The Lyman limit system is likely associated with an L* galaxy lying 23" from the sightline. Finally, in addition to prominent features at very low velocities arising from the disk of our Galaxy, the strong resonance transitions of C II and Mg II show evidence for material at v = -200 km/s; the column densities of these two species suggest that 17.7 < log N(H I) < 18.1 if the material has a solar composition.

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arXiv2002

The afterglow and complex environment of the optically dim burst GRB 980613

We report the identification of the optical afterglow of GRB 980613 in R- and I-band images obtained between 16 and 48 hours after the gamma-ray burst. Early near-infrared (NIR) H and K' observations are also reported. The afterglow was optically faint (R ~ 23) at discovery but did not exhibit an unusually rapid decay (power-law decay slope alpha < 1.8 at 2 sigma). The optical/NIR spectral index (beta_RH < 1.1) was consistent with the optical-to-X-ray spectral index (beta_RX ~ 0.6), indicating a maximal reddening of the afterglow of ~0.45 mag in R. Hence the dimness of the optical afterglow was mainly due to the fairly flat spectral shape rather than internal reddening in the host galaxy. We also present late-time HST/STIS images of the field in which GRB 980613 occurred, obtained 799 days after the burst. These images show that GRB 980613 was located close to a very compact, blue V = 26.1 object inside a complex region consisting of star-forming knots and/or interacting galaxy fragments. Therefore, GRB 980613 constitutes a strong case for the association of cosmological gamma-ray bursts with star-forming regions.

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arXiv2001

Keck Measurement of the XTE J2123-058 Radial Velocity Curve

We measured the radial velocity curve of the companion of the neutron star X-ray transient XTE J2123-058. Its semi-amplitude (K_2) of 298.5 +/- 6.9 km/s is the highest value that has been measured for any neutron star LMXB. The high value for K_2 is, in part, due to the high binary inclination of the system but may also indicate a high neutron star mass. The mass function (f_2) of 0.684 +/- 0.047 solar masses, along with our constraints on the companion's spectral type (K5V-K9V) and previous constraints on the inclination, gives a likely range of neutron star masses from 1.2 to 1.8 solar masses. We also derive a source distance of 8.5 +/- 2.5 kpc, indicating that XTE J2123-058 is unusually far, 5.0 +/- 1.5 kpc, from the Galactic plane. Our measurement of the systemic radial velocity is -94.5 +/- 5.5 km/s, which is significantly different from what would be observed if this object corotates with the disk of the Galaxy.

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arXiv2001

NGC 3065: A Certified LINER With Broad, Variable Balmer Lines

Motivated by the X-ray properties of the galaxy NGC 3065, we have obtained new optical spectra which reveal that it has a low-ionization nuclear emission-line region (LINER) as well as broad Balmer emission lines, establishing it as an active galactic nucleus. We also examined an older spectrum from the CfA Redshift Survey which, lacking broad Balmer lines, indicates that they appeared some time after 1980. Thus NGC 3065 joins the set of LINERs with broad, variable Balmer lines, which includes such well-known galaxies as NGC 1097 and M81. Inspired by the sometimes double-peaked profiles of the variable Balmer lines in other LINERs, we speculate that the broad Balmer lines of NGC 3065 also come from an accretion disk. We illustrate the plausibility of this hypothesis by fitting a disk model to the observed H$α$ profile. We also estimate the mass of the central black hole as (9+/-4) 10^7 M_sol from the properties of the host galaxy, which leads to the conclusion that the accretion rate is only 2 10^{-4} times the Eddington value, a property that appears to be common among LINERs. At such a low relative accretion rate the inner accretion disk can turn into a vertically-extended ion torus, which can illuminate the outer, thin disk and power the broad-line emission. The reason for the sudden appearance of broad Balmer lines is an open question, although we suggest two possible explanations: tidal disruption of a star or a sudden transition in the structure of the accretion disk.

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