Engineering topics

Halpern, Jules P.

Publications and source records attributed to Halpern, Jules P..

At least 19 records

NASA NTRS2002

Testing for X-ray Periodicities in Seyfert Galaxies

The Deep Survey instrument on the Extreme Ultraviolet Explorer obtained long, continuous light-curves of 10 Seyfert galaxies with durations of 5-33 days each. We present a uniform reduction of these data, which account for a total of 209 days of observation. Several of the light curves are uniquely suited to a search for periodicity or QPOs in the range of hours to days that might be expected from dynamical effects in the inner accretion disks around approximately 10(exp 8) solar mass black holes. Power spectra show features in three of the longest observations that could be transient periods: 0.9 days in RX J0437.4-4711, 2.1 days in Ton S180, and 5.8 days in 1H 0419-577. These period values seem to be unrelated to the length of the observations, which are similar in the three cases, but they do roughly scale as the luminosity of the objects, which would be expected in a dynamical scenario if the black hole masses also scale with luminosity. The significance of these periods will be evaluated in a future publication by using the method of Timmer & Konig (1995), which properly takes into account the red-noise properties of AGN light curves.

Halpern, Jules P.↗

NASA NTRS2001

Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High-Energy Gamma-Ray Source?

Most of the EGRET high-energy gamma-ray sources remain unidentified. It is highly likely that many of these are fainter blazars or pulsars, but there may also be new types of sources to be discovered. We have focussed our search for novel gamma-ray sources on 3EG 1835+5918, which is the brightest and most accurately positioned of the unidentified EGRET sources at high Galactic latitude (l, b = 89 deg, 25 deg). In this talk, we will summarize the results of X-ray, radio, and optical surveys of this location. In particular, we have made complete optical identifications of all of the ROSAT and ASCA sources in this region to a flux limit of approximately 1 x 10(exp -13) ergs/sq cm s. All of the X-ray sources within the EGRET error circle are radio-quiet quasars or coronally emitting stars. Previous radio pulsar searches have been unsuccessful. We set an upper limit of 3.8 mJy (at 1.4 GHz) on any possible radio counterpart to 3EG 1835+5918. We also find several quasars and white dwarfs using optical color selection, and we have monitored the entire field for variable optical objects on short and long time scales. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, we assert that 3EG 1835+5918 must be lacking in one or more of the physically essential attributes of those classes of gamma-ray emitters. In particular, its radio flux is at least two orders of magnitude fainter than any of the securely identified EGRET blazars, and its soft X-ray flux is at least 30 times fainter than that of Geminga and other EGRET pulsars. If it is an AGN it lacks the beamed radio emission of blazars. If it is an isolated neutron star, it lacks both the thermal X-rays from a cooling surface and the magnetospheric non-thermal X-ray emission that is characteristic of all EGRET pulsars. As such, it is more problematic physically than Geminga, which is an ordinary pulsar that only lacks radio emission. As a pulsar, 3EG 1835+5918 would have to be either older or more distant than Geminga, and probably an even more efficient gamma-ray engine.

Mirabal, N.↗

NASA NTRS2001

Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High-Energy Gamma-Ray Source

The EGRET source 3EG J1835+5918 is the brightest and most accurately positioned of the as-yet unidentified high-energy gamma-ray sources at high Galactic latitude (l, b = 89 deg, 25 deg). We present a multiwavelength study of the region around it, including X-ray, radio, and optical imaging surveys, as well as optical spectroscopic classification of most of the active objects in this area. Identifications are made of all but one of the ROSAT and ASCA sources in this region to a flux limit of approximately 5 x 10(exp -14) erg/sq cm s, which is 10(exp -4) of the gamma-ray flux. The identified X-ray sources in or near the EGRET error ellipse are radio-quiet QSOs, a galaxy cluster, and coronal emitting stars. We also find eight quasars using purely optical color selection, and we have monitored the entire field for variable optical objects on short and long time scales without any notable discoveries. The radio sources inside the error ellipse are all fainter than 4 mJy at 1.4 GHz. There are no flat-spectrum radio sources in the vicinity; the brightest neighboring radio sources are steep-spectrum radio galaxies or quasars. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, 3EG J1835+5918 must be lacking one or more of the physically essential attributes of these known classes of gamma-ray emitters. If it is an AGN it lacks the beamed emission radio of blazars by at least a factor of 100 relative to identified EGRET blazars. If it is an isolated neutron star, it lacks the steady thermal X-rays from a cooling surface and the magnetospheric non-thermal X-ray emission that is characteristic of all EGRET pulsars. If a pulsar, 3EG J1835+5918 must be either older or more distant than Geminga, and probably an even more efficient or beamed gamma-ray engine. One intermittent ROSA T source falls on a blank optical field to a limit of B greater than 23.4, V greater than 23.3, and R greater than 22.5. In view of this conspicuous absence, RX J1836-2+5925 should be examined further as a candidate for identification with 3EG J1835+5918 and possibly the prototype of a new class of high-energy gamma-ray source.

Mirabal, N.↗

NASA NTRS2001

Jets in Gamma-Ray Bursts

In the afterglows of several gamma-ray bursts (GRBs), rapid temporal decay, which is inconsistent with spherical (isotropic) blast-wave models, is observed. In particular, GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, with t(sup -2.05 +/- 0.04) in optical as well as in X-rays. We show that such temporal decay is more consistent with the evolution of a jet after it slows down and spreads laterally, for which t(sup -P) decay is expected (where p is the index of the electron energy distribution). Such a beaming model would relax the energy requirements on some of the more extreme GRBs by a factor of several hundred. It is likely that a large fraction of the weak- (or no-) afterglow observations are also due to the common occurrence of beaming in GRBs and that their jets have already transitioned. to the spreading phase before the first afterglow observations were made. With this interpretation, a universal value of p approx. = 2.4 is consistent with all data.

Sari, R.↗

NASA NTRS2001

Supernova 1998bw In ESO 184-G82

Over the wavelength range 390-750 nm, we measure intrinsic linear polarization of 0.53 +/- 0.08 percent at position angle 49 +/- 3 deg, after correcting for Galactic interstellar polarization using the star HD 184100, which has polarization of 0.75 +/- 0.01 percent at p.a. 176.5 +/- 2.5 deg. This measured interstellar polarization is consistent with the Galactic extinction in this direction, estimated to be E(B-V) = 0.059 from IRAS maps, or E(B-V) = 0.079 from 21-cm H I. Interstellar polarization in the host galaxy ESO 184-G82 is expected to be negligible based on the relative absence of Na I D absorption at z = 0.00841 +/- 0.00005, the redshift of the environment of the supernova from narrow H II region emission lines in its spectrum. Polarization appears highest in between emission features in the total flux spectrum, which strengthens the interpretation of the polarization as intrinsic to the supernova. This modest polarization is less than that of some type-II supernovae, but greater than that of type-Ia supernovae, which are generally unpolarized. This supports the interpretation of SN 1998bw, a peculiar type-Ic supernova, as a core-collapse event in which the observed polarization is due to moderate asymmetry in either the photosphere of the ejecta or an overlying scattering envelope. However, this result does not strongly constrain arguments about whether some supernovae emit gamma-ray bursts, since such emission may come from a mildly relativistic shock associated with the radio emission and above the optical photosphere, without any requirements on beaming or orientation." A. V. Filippenko, University of California at Berkeley, comments on the total flux spectrum obtained above: "The spectrum most closely resembles those of the peculiar SN 1997ef, but perhaps evolving more slowly. It is not typical of type-Ic supernovae; indeed, the spectrum does not match any of the known spectral classes, but perhaps 'peculiar type Ic' is the best choice at this time. The object appears to be starting a transition to the nebular phase, but identifications of the broad emission features are uncertain. The feature at 650 nm is probably [Fe III], as is the one at 455 nm (perhaps with some contribution from Mg I]). That at 590 nm may be a mixture of [Co III] and Na D. The strongest feature, at 545 nm, may consist of Fe, Co, and Ti lines."

Kay, L. E.↗

NASA NTRS2001

X-Ray Emission from the Millisecond Pulsar J1012+5307

The recently discovered 5.3 ms pulsar J1012+5307 at a distance of 520 pc is in an area of the sky which is particularly deficient in absorbing gas. The column density along the line of sight is less than 7.5 x 10(exp 19)/sq cm, which facilitates soft X-ray observations. Halpern reported a possible ROSAT PSPC detection of the pulsar in a serendipitous, off-axis observation. We have now confirmed the X-ray emission of PSR J1012+5307 in a 23 ksec observation with the ROSAT HRI. A point source is detected within 3 sec. of the radio position. Its count rate of 1.6 +/- 0.3 x 10(exp -3)/s corresponds to an unabsorbed 0.1 - 2.4 keV flux of 6.4 x 10(exp -14) ergs/sq cm s, similar to that reported previously. This counts-to-flux conversion is valid for N(sub H) = 5 x 10(exp 19)/sq cm, and either a power-law spectrum of photon index 2.5 or a blackbody of kT = 0.1 keV. The implied X-ray luminosity of 2.0 x 10(exp 30) ergs/ s is 5 x 10(exp -4) of the pulsar's spin-down power E, and similar to that of the nearest millisecond pulsar J0437-4715, which is nearly a twin of J1012+5307 in P and E. We subjected the 37 photons (and 13 background counts) within the source region to a pulsar search, but no evidence for pulsation was found. The pulsar apparently emits over a large fraction of its rotation cycle, and the absence of sharp modulation can be taken as evidence for surface thermal emission, as favored for PSR J0437-4715, rather than magnetospheric X-ray emission which is apparent in the sharp pulses of the much more energetic millisecond pulsar B1821-24. A further test of of the interpretation will be made with a longer ROSAT observation, which will increase the number of photons collected by a factor of 5, and permit a more sensitive examination of the light curve for modulation due to emission from heated polar caps. If found, such modulation will be further evidence that surface reheating by the impact of particles accelerated along open field lines operates in these approx. 10(exp 9) yr old pulsars.

Halpern, Jules P.↗

NASA NTRS2001

A Broad-Band X-Ray Study of the Geminga Pulsar

We present a comprehensive study of the Geminga pulsar at energies 0.1-10 keV using data from the ASCA, ROSAT, and EUVE satellites. The bulk of the soft X-ray flux can be parameterized as a blackbody of T = (5.6 +/- 0.6) x 10(exp 5) K, occupying a fraction 0. 10 - 0.64 of the surface area of the neutron star at the parallax distance of 160 pc. The ASCA detection of Geminga resolves the nature of the harder X-ray component previously discovered by ROSAT in favor of nonthermal emission, rather than thermal emission from a heated polar cap. The hard X-ray spectrum can be fitted by a power-law of energy index 1.0 +/- 0.5. The hard X-ray light curve has a strong main peak and a weak secondary peak; its total pulsed fraction is = 55%. Three ROSAT PSPC observations show significant variability of Geminga's light curve. In particular, a peculiar energy dependence of the modulation in the soft X-ray component, dubbed the "Geminga effect" in the original PSPC data, is not present in later observations. In addition, fine structure in the soft X-ray light curve, interpreted as eclipses due to cyclotron resonance scattering by a plasma screen on the closed magnetic field lines, almost disappeared in the most recent observations. All of the variable properties of Geminga can probably be associated with the nonthermal process that supplies e(sup +, sup -) pairs to its inner magnetosphere.

Halpern, Jules P.↗

NASA NTRS2001

The Geminga Pulsar: Soft X-Ray Variability and an EUVE Observation

We observed the Geminga pulsar with the EUVE satellite, detecting pulsed emission in the Deep Survey imager. Joint spectral fits of the EUVE flux with ROSAT PSPC data are consistent with thermal plus power-law models in which the thermal component makes the dominant contribution to the soft X-ray flux seen by EUVE and ROSAT. The data are consistent with blackbody emission of T = (4 - 6) x 10(exp 5) K over most of the surface of the star at the measured parallax distance of 160 pc. Although model atmospheres are more realistic, and can fit the data with effective temperatures a factor of 2 lower, current data would not discriminate between these and blackbody models. We also find evidence for variability of Geminga's soft X-ray pulse shape. Narrow dips in the light curve that were present in 1991 had largely disappeared in 1993/1994, causing the pulsed fraction to decline from 32% to 18%. If the dips are attributed to cyclotron resonance scattering by an e1 plasma on closed magnetic field lines, then the process that resupplies that plasma must be variable.

Halpern, Jules P.↗

NASA NTRS2001

Multiwavelength Examination of the COS B Field 2CG 075+00 Yields a Blazar Identification for 3EG J2016+3657

We present a high-energy study of the intriguing COS B gamma-ray field, 2CG 075+00, in order to search for possible counterparts. New EGRET data show that the COS B emission probably corresponds to two localized gamma-ray sources, 3EG J2016+3657 and 3EG J2021+3716. Spectral fits to these EGRET sources, assuming a power-law model, yield photon indices of approx. 2 for each object. We examine archival ROSAT and ASCA X-ray data that overlap both EGRET error boxes and find several point sources in the region to a flux limit of approximately 6.5 x 10(exp -13) ergs/sq cm s. We conclude that the most probable candidate for 3EG J2016+3657 is the compact, variable, flat-spectrum radio and millimeter source B2013+370 (G74.87+1.22), which has blazar like properties. The other source, 3EG J2021+3716, remains unidentified.

Mukherjee, R.↗

NASA NTRS2001

Discovery of a Hand X-Ray Source, SAX J0635+0533, in the Error Box of the Gamma-Ray Source 2EG J0635+0521

We have discovered an X-ray source, SAX J0635+0533, with a hard spectrum within the error box of the GeV gamma-ray source in Monoceros, 2EG J0635+0521. The unabsorbed flux from the source is 1.2 x 10(exp -11) ergs /sq cm s in the 2-10 keV band. The X-ray spectrum is consistent with a simple power-law model with absorption. The photon index is 1.50 +/- 0.08, and we detect emission out to 40 keV. Optical observations identify a counterpart with a V magnitude of 12.8. The counterpart has broad emission lines and the colors of an early B-type star. If the identification of the X-ray/optical source with the gamma-ray source is correct, then the source would be a gamma-ray-emitting X-ray binary.

Kaaret, P.↗

NASA NTRS2001

The Rapidly Fading Afterglow of GRB 980519

GRB 980519 had the most rapidly fading of the well-documented GRB afterglows, consistent with t(sup -2.05 +/- 0.04) in BVRI as well as in X-rays during the two days in which observations were made. We report VRI observations from the MDM 1.3m and WIYN 3.5m telescopes, and we synthesize an optical spectrum from all of the available photometry. The optical spectrum alone is well fitted by a power law of the form nu(sup - 1.20 +/- 0.25), with some of the uncertainty due to the significant Galactic reddening in this direction. The optical and X-ray spectra together are adequately fitted by a single power law nu(sup 1.05 +/- 0.10). This combination of steep temporal decay and flat broad-band spectrum places a severe strain on the simplest afterglow models involving spherical blast waves in a homogeneous medium. Instead, the rapid observed temporal decay is more consistent with 2 models of expansion into a medium of density n(r) proportional to r(sup -2), or with predictions of the evolution of a jet after it slows down and spreads laterally. The jet model would relax the energy requirements on some of the more extreme GRBs, of which GRB 980519 is likely to be an example because of its large gamma-ray fluence and faint host galaxy.

Halpern, Jules P.↗

NASA NTRS2001

Magnetospheric Geometry in PSR B1929+10 from Radio/X-Ray Alignment

We have conducted a study of two rotation- powered pulsars that emit at both radio and x-ray wavelengths, PSR B0531+21 and PSR B1929+10. Using absolute phase information, we have phase-aligned x-ray and radio profiles from these pulsars. Observations were done using the Green Bank 140ft telescope, and ASCA. The 0531+21 X-ray profile is sharp and lines up well with the radio profile confirming that the X-ray emission from this pulsar is magnetospheric in origin. The 1929+10 profile is approximately sinusoidal with the peak of the emission arriving 67+/-23 degrees after the maximum in the radio emission. The controversy to which the PSR B1929+10 result adds fuel, is whether this "inter" -pulsar, is an "aligned" or "orthogonal" rotator -- describing the alignment of the magnetic axis to the rotation axis. Do the two peaks in the radio profile (the pulse and interpulse) come from a double crossing of a thin hollow cone nearly aligned with rotation axis, or alternatively do they come from from opposite poles of an "orthogonal" rotator where the spin axis is perpendicular to the magnetic axis? The radio to X-ray alignment we find favors the former explanation: if the X-ray hot spot is the result of return currents to the surface from the outward current that generates radio emission, then in the "double-crossing" model, the hot spot phase is expected to lie between the main pulse and interpulse as observed.

Somer, A. L.↗

NASA NTRS2001

The Optical Spectrum of the Geminga Pulsar

We obtained an optical spectrum of the isolated pulsar Geminga at the Keck Observatory. The optical object is at the limit of spectroscopic capability of any telescope, with a continuum flux that is approx. 0.5% of the dark sky on Mauna Kea. With particular attention paid to the dominant systematics of sky subtraction in our observing and analysis methods, we attained approx. 0.1% systematics in heavily binned spectra. The resulting spectrum spanning 3700 - 8000 A has a flat power-law shape f(sub nu) proportional to nu(exp -0.8) and a broad dip over 6300 - 6500 A. Thermal radiation cannot explain the optical spectrum of Geminga. The dominant component can be modeled as either electron synchrotron emission and ion (proton) cyclotron absorption, or ion cyclotron emission, the latter in a 10(exp 11) G magnetic field.

Martin, Christopher↗

NASA NTRS2001

ASCA Observations of PSR 1929+10 and PSR 0950+08

The ASCA X-ray spectra of the old pulsars PSR 1929+10 and PSR 0950+08 are adequately fitted by simple blackbodies. Between 0.5 and 5.0 keV, PSR 1929+10 is best fitted with T = (5.14 +/- 0.53) x 10(exp 6) K and L = 1.54 x 10(exp 30) ergs/s; PSR 0950+08 has T = (5-70 +/- 0.63) x 10(exp 6) K and L = 4.67 x 10(exp 29) ergs/s. The inferred areas are 2 orders of magnitude smaller than canonical polar cap sizes. There is some evidence for a dearth of photons above 5 keV, which would disfavor alternative power-law models for the spectra. Pulsed emission is detected from PSR 1929+10 with 99.8% significance and pulsed fraction 0.35 +/- 0.18. Marginal evidence of pulsed X-ray emission from PSR 0950+08 is present with 99.0% significance and pulsed fraction 0.34+/- 0.18. Relativistic ray tracing shows that if the geometry deduced from the polarization of the radio pulse is adopted for the surface thermal emission, a central dipole cannot reproduce the large pulsed modulations observed unless the radius of the neutron star is as large as 18 km. A mechanism of polar cap heating by the impact of relativistic particles is suggested that can produce small areas with T approx. 5 x 10(exp 6) K on old, spun-down pulsars with weak magnetic fields (B approx. 3 x 10(exp 11) G).

Wang, F. Y.-H.↗

NASA NTRS2001

Timing the Geminga Pulsar with EGRET Data

The pulsation of Geminga has been detected to date only at high energies (E greater than 0.1 keV). Since X-ray exposures are short and Geminga is at best only marginally detected in gamma-rays at E less than 30 MeV, the primary means of timing Geminga is with high-energy gamma-rays. The EGRET observations of Geminga now span 4 years. These data are analyzed to determine the 1995 ephemeris for Geminga which is provided here. We continue to count every revolution of Geminga during the GRO mission with a rotational phase resolution which improves with additional exposure. Proper motion is now apparent in the gamma-ray timing, consistent with the optical measurement of Bignami et al. With improved statistics, two addition peaks are tentatively detected in the "minor bridge" region. More exposure is required to confirm them. If found to be real, they are difficult to understand with polar cap models, but are expected for the outer gap model, and provide sorely needed constraints.

Mattox, J. R.↗

NASA NTRS2001

A Possible X-Ray Detection of the Binary Millisecond Pulsar J1012+5307

A possible X-ray detection of the newly discovered binary millisecond radio pulsar PSR J1012+5307 was obtained from an archival ROSAT observation. The 80 +/- 24 photons detected correspond to a 0.1 - 2.4 keV luminosity of approx. = 2.5 x 10(exp 30) erg/s at the nominal dispersion-measure distance of 520 pc. This luminosity is a factor of 2 less than that of PSR J0437-4715, a near twin of PSR J1012+5307 in its spin parameters and energetics, and the only millisecond pulsar from which pulsed X-rays have definitely been detected. PSR J1012+5307 is also within 6 deg of the "HI hole" in Ursa Major, providing a new estimate of the electron column density through this region which confirms that the ionized column density is also low. The small neutral column density to PSR J1012+5307, N(sub H) less than 7.5 x 10(exp 19)/sq cm, will facilitate future soft X-ray study, which will help to discriminate between thermal and nonthermal origins of the X-ray emission in millisecond pulsars.

Halpern, Jules P.↗

NASA NTRS2001

Timing the Geminga Pulsar with Gamma-Ray Observations

We present the COS-B/EGRET 1997 ephemeris for the rotation of the Geminga pulsar. This ephemeris is derived from high-energy gamma-ray observations that span 24 yr. The recently obtained accurate position and proper motion are assumed. A cubic ephemeris predicts the rotational phase of Geminga with errors smaller than 50 milliperiods for all existing high-energy gamma-ray observations that span a 24.2 yr timing baseline. The braking index obtained is 17 +/- 1. Further observation is required to ascertain whether this high value truly reflects the rotational energy loss mechanism, or whether it is a manifestation of timing noise. The ephemeris parameters are sufficiently constrained so that timing noise will be the limitation on forward extrapolation. If Geminga continues to rotate without a glitch, as it has for at least 23 yr, we expect this ephemeris to continue to describe the phase, with an error of less than 100 milliperiods, until 2008. Statistically significant timing residuals are detected in the EGRET data that depart from the cubic ephemeris at a level of 30 milliperiods. Although this could simply be an additional manifestation of timing noise, the EGRET timing residuals appear to have a sinusoidal modulation that is consistent with a planet of mass 1.7/sin i solar mass, orbiting Geminga at a radius of 3.3 AU.

Mattox, J. R.↗

NASA NTRS2001

A Possible X-Ray and Radio Counterpart of the High-Energy Gamma-Ray Source 3EG J2227+6122

The identity of the persistent EGRET sources in the Galactic plane is largely a mystery. For one of these, 3EG J2227+6122, our complete census of X-ray and radio sources in its error circle reveals a remarkable superposition of an incomplete radio shell with a flat radio spectrum, and a compact, power-law X-ray source with photon index Gamma = 1.5 and with no obvious optical counterpart. The radio shell is polarized at a level of approx. = 25%. The anomalous properties of the radio source prevent us from deriving a completely satisfactory theory as to its nature. Nevertheless, using data from ROSAT, ASCA, the VLA, and optical imaging and spectroscopy, we argue that the X-ray source may be a young pulsar with an associated wind-blown bubble or bow shock nebula, and an example of the class of radio-quiet pulsars which are hypothesized to comprise the majority of EGRET sources in the Galaxy. The distance to this source can be estimated from its X-ray absorption as 3 kpc. At this distance, the X-ray and gamma-ray luminosities would be approx. = 1.7 x 10(exp 33) and approx. = 3.7 x 10(exp 35) erg/s, respectively, which would require an energetic pulsar to power them. If, on the contrary, this X-ray source is not the counterpart of 3EG J2227+6122, then by process of elimination the X-ray luminosity of the latter must be less than 10(exp -4) of its gamma-ray luminosity, a condition not satisfied by any established class of gamma-ray source counterpart. This would require the existence of at least a quantitatively new type of EGRET source, as has been suggested in studies of other EGRET fields.

Halpern, Jules P.↗