Continuum time series of a solar active region, a plage, and a coronal hole. (Courtesy, S. White.)
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NAIC/AO Newsletter No. 19
White (Maryland), Zirker (NSO/Sac. Peak), & Kundu (Maryland) have tested a model (due to Parker) for heating the solar corona. In this, many small reconnection events take place wherever there are magnetic fields in the corona, each releasing a small amount of energy ("nanoflares"). The scale lengths for these events are subarcsec; detection of individual events is hampered by their smallness and by the likelihood that in any field of view, at any time, many such energy releases are taking place and would have to be distinguished from one another. However, they could be detectable through their influence on the characteristics of time series. Hudson (U. Hawaii) has shown that nanoflares should produce time series with a "shot noise" power spectrum, i.e., flat out to some characteristic frequency, and then falling at higher frequencies. The Arecibo observations aimed to measure power spectra from regions of the Sun's surface using as small a field of view as possible, and to detect the influence of coronal heating by comparing the power spectra of solar active regions, where heating ought to be strong, with power spectra of coronal holes, where heating is much weaker and may be fundamentally different in character. The observations were carried out at 2.4 and 4.8 GHz over a number of days. Usually, any given field of view on the Sun has strong brightness gradients at its edge, and time series are particularly sensitive to pointing variations. However, these mostly show up at frequencies above 0.3 Hz, whereas the region of the power spectrum where nanoflares should show up is at lower frequencies. As the figure below shows, the raw time series do show striking differences in character between active regions and coronal holes, as expected. However, the corresponding power spectra do not show the "shot noise" form predicted for nanoflares: instead of being flat at low frequencies, the spectrum continues to rise. The implications of this result are being investigated.
Through confirmation runs at Jodrell Bank and Green Bank, Camilo (Jodrell Bank), Nice (NRAO), Shrauner, & Taylor (Princeton) have increased the number of pulsars discovered in their 430-MHz upgrade pulsar search to 21, raising the present haul of the upgrade pulsar searches to 57. They have observed 1428 deg^2, or 53% of the area allocated to them. At high Galactic latitudes, the limiting sensitivity is » 0.4-1.1 mJy for P > 60 ms. Using Arecibo, the NRAO 140-foot telescope, and the Jodrell-Bank Lovell telescope they have timed the two binary pulsars discovered in their search, PSRs J0621+1002 (P = 28.9 ms), and J1022+1001 (P = 16.5 ms). The orbital parameters derived imply companion masses of > 0.45 and 0.73 Mo respectively. These masses are greater than those for most white-dwarf companions of millisecond pulsars, suggesting that these systems form part of a newly recognized class of intermediate-mass pulsar binaries characterized by spin periods > 10 ms and somewhat higher eccentricities than their low-mass counterparts. PSR J1022+1001 lies in the ecliptic (b = -0°.06) making it a useful probe of the solar corona. This object also shows unusual pulse-shape variations on the time scale of minutes, the cause of which is presently unclear.
Lundgren, Foster (NRL), & Camilo (Jodrell Bank) have made optical observations of 6 millisecond-pulsar companions with the Hubble Space Telescope (HST) to explore the evolution of these systems. Three of the pulsars involved were discovered in recent Arecibo searches. All companions were detected with the HST, and combining information derived from pulsar timing with optical studies of the companions, they place good constraints on the companion masses and the system ages. Masses can be determined from the observed colors, luminosities, and distances from the pulsar DMs. Using white-dwarf cooling models, ages are inferred from the temperatures and masses of the white dwarfs. Two of the systems, PSR J1640+22 and J1713+07, both discovered at Arecibo, are so old (6-9 x 10^9yr) that they may provide strong lower limits to the age of the Galaxy itself, once more accurate distances are measured from parallax. The timing precision with the upgraded Arecibo telescope is needed to measure the tiny perturbations due to parallax for these sources. Most of the systems have cooling ages significantly less than the pulsar spin-down times, implying that the pulsars were born with periods close to their current values. Hence, many millisecond pulsars were apparently not spun-up to the spin-up line, defined by the equilibrium period for accretion at the Eddington rate.
Anderson (Caltech), Wolszczan (Penn State), Kulkarni, & Prince (Caltech) have been timing the 7.95-msec binary pulsar, PSR1516+02B, offset from the center of globular cluster, M5, by 1.5 core radii. They find the orbit to be precessing. If due to general relativity, the total mass of this binary lies between 0.45 and 5.5 Mo. This limit leaves open the possibility (however slender) of a few solar-mass black-hole companion. Continued observations over the next decade will decide the issue between a low-mass binary pulsar (LMBP) and a high-mass binary pulsar (HMBP). The large number of recently discovered globular-cluster pulsars, and the even larger population inferred by these authors, indicates that globular clusters once contained a substantial population of massive stars. If so, it follows that there should be a number of black holes, the stellar remnants of even more massive stars. Thus, the above expectation is not without basis. However, dynamical interaction preferentially ejects most of these black holes from the cores over the cluster lifetime.
Rankin (Vermont) & Rathnasree (Raman Inst.) have explored the emission geometry and polarization behavior of pulsars PSRB1929+10 and B0823+26. For both, investigation of the detailed polarization behavior has employed the technique of constructing partial "mode-separated" profiles corresponding to the primary and secondary polarization modes. PSRB1929+10 emits at virtually all pulse longitudes, although least-squares fits to the polarization-angle traverse fit poorly near the main- and inter-pulse and have an inflection point far from the main-pulse center. This suggests that the position-angle traverse is an unreliable indicator of geometry in this pulsar. However, taking the inter-pulse and main-pulse comp. II widths as indicators of the magnetic latitude, it appears that PSRB1929+10 has near orthogonal spin and magnetic axes, implying that it is a two-pole interpulsar. For PSRB0823+26, the polarization modes are anything but orthogonal, with the secondary mode exhibiting structure seen neither in the primary mode nor the total profile. The distribution of power between the primary and secondary modes is similar at both 430 and 1400 MHz. Of special interest is a phase offset between the primary and secondary modes, the secondary-mode "main pulse" arriving » 1°.5 ± 0°.1 before that of the primary-mode at 430 MHz and » 1°.3 ± 0°.1 at 1400 MHz.
An anticorrelation between the degree of linear polarization dL and the total intensity I has been previously reported for some pulsars in samples of a few thousand pulses. Rathnasree & Rankin show systematic effects to be the cause of such an anticorrelation, which is also detected in the sample that they have studied. A detailed study of the variations in the total polarization of single pulses with pulse intensity has been done for a number of pulsars, with proper statistical correction applied to the linearly polarized power. This reveals very different behavior, as well as some remarkable correlations in many pulsars, which reflect directly on the nature of pulsar radio emission. All pulsars studied show the presence of "orthogonal" polarization modes. Most interestingly, the behavior of dL with I is markedly different when the data points are separated into the two constituent polarization modes. This could have implications for the factors responsible for the occurrence of orthogonal polarization modes.
Over 100 pulsars were observed at 1418 MHz by Weisberg (Carleton) and collaborators in 15 sessions between 1989 and 1993. They determined polarization parameters on all pulsars of sufficient strength, resulting in full-Stokes pulse profiles for 95 objects. The Rankin (1983) system was used to classify their sample morphologically using both their own data and published polarimetry. The spin axis of a pulsar will precess with time if the body is distributed asymmetrically with respect to this axis, a phenomenon called free precession. The resulting orientation changes would lead to secular changes of the polarization position angle (PPA). The cause of such precession could be an irregularity in the shape of the neutron star due to its cataclysmic birth. The PPAs of several strong pulsars were examined. Two, PSR B1929+10 and PSR B0540+23, were found to have steady PPAs on all relevant time scales, and were used as PPA references for the complete sample. The team searched for PPA variations with time over all the sessions. Had changes been seen, these could have been explained either by precession, or by variations in the magneto-ionic interstellar medium between the pulsar and the Earth. In practice, no significant time variability of PPA was found in the » 80 pulsars on which adequate data existed for two or more sessions. This result places upper limits on the amplitude of precession and on changes in the magneto-ionic interstellar medium.
The early hydrogen search by Avruch, Burke (MIT), Davis (NAIC), and Horowitz, Leigh, and Weintroub (Harvard) aims at detecting l21-cm HI emission from massive protocluster clouds at redshift z ~ 5, the era of galaxy formation. The search system has been gathering data since January 1995, interrupted only by the very active hurricane season of that year. Although the receiver and spectrometer are designed to monitor two feeds suspended from the telescope catwalk to use the characteristic "east-then-west" signature to distinguish celestial sources from radio-frequency interference (RFI), the system has been running mostly in single-beam mode because of feed-feed interactions. Also, the data is corrupted by RFI, which has deteriorated since the system was turned on. However, continuum sources are seen in the cleaned total power data. Observation and analysis continue in an effort to make a protocluster detection or to see what else may serendipitously show up in this unexplored search band. One useful result has been to characterize the RFI in this band. At peak RFI times, 75% of the observation band is mildly affected by interference, and 40% is strongly affected. Late at night, these figures fall to 45% and 20%, respectively. The worst RFI occurs in the sub-bands 243.5-244.5 MHz and 246-250 MHz.
Shortly before the upgrade commenced, Dell' Antonio (Harvard), Bothun (Oregon), & Geller (CfA) completed the first half of a line-width survey of all spiral galaxies located in the Great Wall (GW), the largest coherent feature yet discovered in the galaxy distribution. High-quality velocity widths were measured for some 170 spirals. In combination with I-band magnitudes, the galaxies form a Tully-Fisher relation with a scatter of 0.3-0.32 m. A rigorous investigation of the role of extinction and the amount of scatter in the TF relation strongly argues for moderate levels of internal extinction, countering the recent trend to attribute the observed scatter to significantly higher levels of extinction. Application of this high extinction to the data often results in TF relations with quite flat slopes. The GW data recovers a slope close to that predicted by application of the Virial Theorem. Since relative distances can now be measured to an accuracy of 15%, the data lend themselves to studying three main issues: 1) the amount of infall onto the GW structure, 2) the amount of large-scale flows within the GW or of the GW structure itself, and 3) the amount of shear within the GW. A robust Monte-Carlo method is used to remove any selection biases from the sample. The bias-corrected velocities strongly constrain the infall towards the GW and provide the first limits on its true spatial thickness, being best fit by an infall velocity < 150 km/s with a 90% upper limit on the mean infall velocity of 500 km/s. This translates into an upper limit on the real-space full width of the GW of 11.2 /h Mpc. These data strongly confirm that the GW is a thin, 2-D structure in both real space and redshift space. The best-fit estimate of the motion of the GW with respect to the Local Group is 725 ± 400 km/s towards the vector a = 11.7 ± 1.5 hr and dec = 37° ± 55°. This flow is consistent with the CMB dipole but slightly inconsistent with the direction of the large-scale flow reported by Lauer & Postman. Completion of the survey will substantially reduce current errors. Lastly, the detected shear across the angular extent of the GW is completely consistent with zero. The GW region is quiet and the absence of any large shear constrains the amplitude of the large-scale density fluctuations which can produce these structures.
Hoffman (Lafayette), Salpeter (Cornell), Farhat (MIT), Roos (Cornell), Williams (Delaware), & Helou (IPAC) have completed work on the mapping of HI in 70 dwarf irregular (Sm, Im, and BCD) galaxies. These were selected from, (i) the Virgo Cluster area, (ii) Field dwarfs, and (iii) a sample of nearby objects containing all such galaxies out to 6 Mpc. Of these, 20 non-interacting galaxies were sufficiently resolved to deduce rotation curve information, in most cases extending these curves well beyond the last measured point in available synthesis-array maps. Generally, they find that dwarf rotation curves rise in more-or-less solid-body fashion to the edge of the optical disk, and then remain flat as far as HI can be traced, with the outer edges in most cases being corrupted by non-circular motions (warps or turbulence). They also find that HI disks exceeding the Holmberg diameter by a factor of > 6 are rather rare; just one of the dwarfs observed, DDO154, has an HI disk > 10 times as extended as the stellar component. Salpeter & Hoffman have combined these measurements with those available for all HI-mapped dwarfs and spirals within the same spatial volume to investigate variations in Tully-Fisher relations and surface densities as functions of galaxy size and luminosity or mass. They find that the blue luminosity l µ r^2.68 µ v^3.73 µ m_H^1.35 µ m_dyn^1.16, where r is the geometric mean of the radius of the outermost detectable HI and the optical radius, v is the velocity profile half-width, m_H is the HI mass, and m_dyn = v^2 r/G. The HI surface density SH is almost constant along the sequence of size/mass/luminosity, while the blue-luminosity surface density SL increases with galaxy size. For quantities not involving HI, there is no evidence for a "break" between dwarfs and spirals, though they do find some curvature in log v vs. log r, and in the Tully-Fisher relation, log v vs. log l.
Two consequences are: (i) l ~ v^3.7 is more appropriate for the whole sequence than the l ~ v^2.5, as found for large spirals alone, and (ii) the surface density of total mass Sdyn is almost constant for lower luminosities but increases appreciably with l for higher luminosities. There is an indication that the correlations involving HI mass or radius differ between dwarfs and spirals in that irregulars have somewhat more HI mass, or slightly larger HI sizes, than spirals at a given blue luminosity, optical radius, or velocity profile width. It is unclear if this is a true morphological effect or just due to mH varying less strongly than l.
In 1994, Lauer & Postman (LP) measured the dipole of the distribution of the brightest elliptical galaxies in 119 clusters distributed over a volume of 15,000 km/s radius and with an effective depth of » 10,000 km/s. They found that the reference frame defined by the group of clusters is in motion with respect to the CMB at a velocity of 689 ± 178 km/s towards the direction of galactic coordinates (343°, +52°) (± 23°). Not only is this motion roughly orthogonal to the CMB apex and in large disagreement with that reported by other surveys, it also poses an embarrassment to many models that attempt to reproduce the characteristics of large-scale structure, as they encounter difficulties in accommodating bulk flows of large amplitude and scale. Giovanelli, Haynes (Cornell), Wegner (Dartmouth), da Costa, Freudling (ESO), & Salzer (Wesleyan) have used Tully-Fisher distances for a sample of field late spiral galaxies to test the LP result. A total of 432 galaxies were used, the HI data for many of which were measured at Arecibo. The galaxies are subdivided between two cones, of 30° semiaperture each and pointed respectively toward the apex and antapex of the LP motion. The peculiar velocities in the two data sets are inconsistent with a bulk flow of the amplitude claimed by LP. When combined in opposition, the peculiar velocity medians in shells of constant redshift width are never larger than half the amplitude of the LP bulk flow. Out to 5000 km/s the median bulk velocity in the LP apex-antapex cones is about 200 km/s or less, dropping to a value indistinguishable from zero beyond that distance. It can be excluded that field spiral galaxies within 8000 km/s partake of a bulk flow of the amplitude and direction reported by LP.
DuPrie & Schneider (UMass) report that when early-type galaxies have HI associated with them, it is often located over a very extended region. This suggests several possible origins for the gas, either intrinsic to the galaxy or from an external source. They present l21-cm HI maps around early-type galaxies to study the distribution of gas in the galaxies' vicinities and address these possibilities. They examine two sets of objects: (1) early-type galaxies with conflicting HI measurements that might indicate unusual gas distributions, and (2) galaxy groups that are dominated by early-type galaxies. They have identified several galaxies with extended HI disks, and a number of HI companions that may be interacting with the early-type galaxies, and that may be confusing HI observations. Many of the companions can be optically associated with uncataloged dwarf galaxies, but a few are quite peculiar or have no clear optical association. They discuss the implications of these results for the origin of HI in early-type galaxies.
The existence of thick, O-rich, circumstellar dust shells without detected masers continues to pose a puzzle. These color "mimics" of OH/IR stars possess the same color distribution in IR color-color plots as OH/IR stars, with mimics found at most colors at which OH/IR stars are found. However, plots of all investigated objects show that the relative number of detections decreases markedly away from the OH/IR star color-locus, while the color range of IR-bright OH/IR stars is far more circumscribed than that of the total distribution. This mainly reflects the increase in the errors of IR colors as weaker sources are consulted. Lewis (NAIC) has therefore checked whether the color distribution of the brightest mimics differs significantly from that of equally bright OH/IR stars. The sample used is the set of objects with l25-mm flux densities > 100 Jy that have been searched for 1612-MHz emission. A simple comparison of those with and without 1612-MHz masers shows a marked difference in their respective color distributions. However, when the subset of accepted carbon stars is removed, together with a widely distributed subset of emission-line objects containing a few planetary nebulae, a Wolf-Rayet star, and several supergiants with circumstellar dust, the color range of the residual sample is very much that of the OH/IR stars alone. The set (A) of 1612-MHz detections is augmented by objects with other masers (SiO, H2O, and mainline OH), most of which have the blue IR colors which characterize mira variables. This set is compared with the set (B) of clearly O-rich mimics without detected masers, which for procedural clarity are distinguished from a small set of objects that have been suggested as being potential red carbon stars. Mimics constitute 16% of the (A+B) shells at all shell colors. Over the color range of thick shells, mimics tend to exhibit differentially larger l60-mm flux densities. However, even larger differences in the mean residuals from a color relation fitted to normal shells occur in 9% of random drawings from the observed set of thick, (A+B), shells. The color range of bright mimics is statistically the same as for normal shells.
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