Years Of Quiescence Research Articles

Identification of the Optical and Quiescent Counterparts to the Bright X-Ray Transient in NGC 6440

After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in 2001 August, as noted with the Rossi X-Ray Timing ExplorerAll-Sky Monitor. We carried out a short target-of-opportunity observation with the Chandra X-Ray Observatoryand are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in 2000 July with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in a globular cluster is securely identified. Since the transient is a type I X-ray burster, it is established that the compact accretor is a neutron star. Thus, this transient provides an ideal case to study the quiescent emission in the optical and X-ray of a transiently accreting neutron star while knowing the distance and reddening accurately. One model that fits the quiescent spectrum is an absorbed power-law plus neutron star hydrogen atmosphere model. We find an intrinsic neutron star radius of km and an unabsorbed bolometric luminosity for the neutron star atmosphere of 31 1712 ergs s that is consistent with predictions for a cooling neutron star.

Genetics of Childhood Disorders: XXXI. Autoimmune Disorders, Part 4: Is Sydenham Chorea an Autoimmune Disorder?

For many years Sydenham chorea (SC) has been considered the prototype of an autoimmune disorder triggered by an infectious agent. Nevertheless, despite its description more than 300 years ago by Thomas Sydenham, its defined association with rheumatic fever in 1838, and linkage to a preceding group A β-hemolytic streptococcal (GABHS) infection, there are numerous unanswered questions about the underlying pathology and pathophysiology of SC. Recently, clarification of the underlying mechanism in SC has assumed renewed importance as the disorder has been proposed as a model for a spectrum of childhood neurobehavioral disorders, including tics and obsessive-compulsive disorder, termed PANDAS (pediatric autoimmune neuropsychiatric disorders associated with streptococcal infection). The distinguishing clinical manifestation of SC is the presence of spontaneous, involuntary choreiform movements affecting the face and extremities that typically persist for several months before subsiding. Diagnosis is made strictly on clinical observation. No confirmatory laboratory test for SC is available, although an elevated titer against antistreptolysin O, anti-deoxyribonuclease B, or anti-nicotinamide adenine dinucleotidase is demonstrated in 80% of SC patients. Onset is usually between ages 5 and 15 years, and a female predominance is observed in most studies. Associated neurological symptoms may include dysarthria, hypometric saccades, hypotonia, weakness, hemiballismus, and gait disturbances that correlate with severity of chorea. Affected individuals may present with behavioral or emotional difficulties that predate the motoric abnormalities by weeks to months. For example, most patients have concomitant psychological dysfunction presenting as personality changes, emotional irritability, distractibility, and age-regressed behaviors. Studies have also shown a higher frequency of obsessive-compulsive symptoms in children with SC or nonchoreic rheumatic fever. Motor or vocal tics and oculogyric crises have also been reported in patients with SC. Despite a relatively common occurrence of EEG abnormalities, seizures are uncommon. Rheumatic valvular cardiac disease is seen in about one third of patients, although echocardiogram screenings suggest a higher prevalence. Arthritis is uncommon. The outcome in SC is quite favorable. Most cases resolve in 1 to 6 months. Some investigators have identified residual abnormal EEGs, minor lingering motor abnormalities, persistent neuropsychiatric problems, and future development of valvular heart disease. About 20% to 60% of cases have recurrent episodes of chorea, usually within 1 to 2 years after the original event. An individual may have multiple attacks with reactivation precipitated by a GABHS infection or by another environmental stimulus. Pregnancy and oral contraceptives may stimulate the reappearance of chorea after many years of quiescence. Magnetic resonance imaging (MRI) and functional imaging studies in SC have localized acute changes to the basal ganglia. In an MRI study evaluating the size of the basal ganglia in 24 patients with SC and 48 matched controls, children with SC had a 10% increase in size of the caudate and a 7% increase in size of both putamen and globus pallidus. A limited number of postmortem brain studies have demonstrated Genetics of Childhood Disorders: XXXI. Autoimmune Disorders, Part 4: Is Sydenham Chorea an Autoimmune Disorder?

Multiwavelength Observations of the Soft Gamma Repeater SGR 1900+14 during Its 2001 April Activation

The soft gamma repeater SGR 1900+14 became active on 2001 April 18 after about 2 years of quiescence; it had remained at a very low state of activity since the fall of 1998, when it exhibited extraordinary flaring. We have observed the source in the gamma-rays and X-rays with Ulysses and Chandra and in the radio with MERLIN. We report here the confirmation of a two-component X-ray spectrum (power law + blackbody), indicating emission from the neutron star surface. We have determined that there is a dust halo, due to scattering in the interstellar medium, surrounding the source that extends up to ≳100'' from the center of SGR 1900+14.

三宅島２０００年噴火に伴う火山性地震・火山性微動

The 2000 Miyakejima volcano activities started on Jun. 26 after 17 years of quiescence. The NIED Miyakejima volcano observational network, mainly equipped with seismometers and tiltmeters, successfully and continuously detected the migrations of magma. The volcanic activities consist of four stages : the dike intrusion (Stage 1 : Jun. 26-27), the subsidence of volcanic body (Stage 2 : Jun. 27-Jul. 8), tilt-steps and successive collapses of the summit crater (Stage 3 : Jul. 8-Aug. 18), and the subsidence of volcanic body and episodic eruptions (Stage 4 : Aug. 18-). This paper summarizes characteristic seismic phenomena that occurred at each stage; i.e., VT (volcanotectonic) earthquakes, LP (long-period) earthquakes, and volcanic tremor. Significant phenomena are the 50-second pulse waves associated with the tilt-steps at Stage 3. They reflect subsurface magmatic processes and their temporal and spatial variations.

Postoperative sympathetic ophthalmia.

Although uncommon, SO is a fearful postoperative complication because of its potential to blind both eyes. It can result not only from penetrating ocular surgery but also from nonpenetrating ocular procedures. Thus, it is important to consider in any patient who has undergone ocular surgery and develops bilateral uveitis, particularly because prompt, sufficient treatment is required to maximize visual outcome. It is also important to note that the disease may present with a spectrum of clinical findings, none of which is pathognomonic. Thus, suspicion is important for making the diagnosis. Treatment should address the T-cell-mediated nature of the disease. With appropriate treatment, visual acuity of no less than 20/60 is likely. However, before the start of treatment, which consists of immunosuppressants, infection must be ruled out and potential side effects of treatments must be considered. Furthermore, any patient with a history of SO needs ample immunosuppressant coverage for ocular procedures. Better understanding of the pathogenesis of the disease may lead to safer treatments that result in improved visual outcome and a cure. Meanwhile, because of its relapsing nature, SO requires continual, close surveillance, even after many years of quiescence.

Chemical (He, H2, CH4, Ne, Ar, N2) and isotopic (He, Ne, Ar, C) variations at the solfatara crater (southern italy): mixing of different sources in relation to seismic activity

Here we describe temporal variations of noble gas (He, Ne and Ar), carbon isotopes and chemical data of fluids collected at the Bocca Grande fumarole (Solfatara crater) from 1990 to 1994, before the occurrence of a seismic swarm. After more than seven years of quiescence, a sudden vertical ground deformation and a seismic swarm of 100 quakes with an Mmax = 1.5 with epicenter located beneath the Solfatara crater, was recorded from August 23 to 25 1994. Isotopic composition of He, Ne, Ar and C (from CO2) and abundances of He, Ne, Ar, H2, CH4 and N2 have been analysed from fumarolic fluids collected at the Solfatara crater. To date these isotopic ratios have not been used as precursors of earthquakes, although several cases have been reported in which the helium isotopic ratio was used to interpret volcanic unrest. All the analyzed isotopic ratios, chemical species and a calculated equilibrium temperature show a significant change a few months before the occurrence of the seismic activity and ground deformation, followed by a sudden return to values recorded prior the swarm. Migration towards the surface of deep (and hotter) gas phase related to crustal and magmatic sources with the following decrease of the surficial atmospheric component is the cause of such wide variations.

Reactivation and Precise Interplanetary Network Localization of the Soft Gamma Repeater SGR 1900+14

In 1998 May, the soft gamma repeater SGR 1900+14 emerged from several years of quiescence and emitted a series of intense bursts, one with a time history unlike any previously observed from this source. Triangulation using Ulysses, BATSE, and KONUS data gives a 1.6 arcmin2 error box near the Galactic supernova remnant G42.8+0.6. This error box contains a quiescent soft X-ray source that is probably a neutron star associated with the soft repeater.

Satellite observations of Lava Lake activity at Nyiragongo volcano, ex-Zaire, during the Rwandan refugee crisis.

In June 1994 the summit crater of Nyiragongo volcano, located in the Great Lakes region of central Africa, began to fill with new lava, ending nearly 12 years of quiescence. An earlier eruption of the volcano in 1977 had culminated in the catastrophic draining of a lava lake through fissures in the crater wall, feeding highly mobile lava flows which reached the outskirts of Goma and killed more than 70 people. By July 1994, as many as 20,000 Hutu refugees were arriving in Goma every hour, only 18 km south from the summit of Nyiragongo. The exodus brought more than one million people to the camps near the town raising fears of a repeat of the 1977 eruption. This paper examines the role that satellite remote sensing could have played in surveillance of the volcano during this time, and demonstrates the potential for monitoring this and other volcanoes in the future. Images recorded by the spaceborne Advanced Very High Resolution Radiometer (AVHRR)--freely available over the Internet--provide semi-quantitative information on the activity of the volcano. The aim of this paper is to promote the wider use of readily available technologies.

The epidemiologic trends of scabies among Israeli soldiers: a 28-year follow-up.

Scabies is not a notifiable disease in most countries. Thus, the reported rates of the disease in large populations are usually inaccurate and based on estimations. Scabies is usually reported only when large outbreaks occur. This article describes the global epidemiology of scabies in the Israel Defense Force. The data used in this study are based on the routine and mandatory reporting of every individual case of scabies to the Army Health Branch Epidemiology Department since 1968. There was a period of 13 years of quiescence from the implementation of reporting in 1968 until 1981. This period was followed by an epidemic of 15 years, peaking in 1985 and 1986, and returning to the baseline rates of the quiescent period by 1996. This prospectively observed pattern of at least 13 years of quiescence followed by a peak of 15 years is consistent with other, mostly retrospective, reports. Although the peak may have been triggered by the "Lebanon" war of 1982, it is noticeable that no peak was observed during the "Yom Kippur" war (1973). Prospective, mandatory reporting of the kind described here should help to better understand the epidemiology of scabies.

Hydrothermal fluids may offer clues about impending volcanic eruptions

On July 18, 1995, after more than 350 years of quiescence, the Soufriere Hills volcano on the West Indian island of Montserrat erupted. The eruption occurred in English's Crater in the Soufriere Hills (Figure 1). Study of the hydrothermal discharges both offshore and onshore before and during the eruption shows that the offshore hydrothermal springs experienced large compositional variations over at least a year. By contrast, no systematic compositional variations were found in onshore hydrothermal springs during the same period.

Prehistoric earthquake deformations near Masada, Dead Sea graben

Research Article| August 01, 1995 Prehistoric earthquake deformations near Masada, Dead Sea graben Shmuel Marco; Shmuel Marco 1Institute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel2Geological Survey of Israel, 30 Malkhe Israel, Jerusalem 95501, Israel Search for other works by this author on: GSW Google Scholar Amotz Agnon Amotz Agnon 1Institute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel Search for other works by this author on: GSW Google Scholar Author and Article Information Shmuel Marco 1Institute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel2Geological Survey of Israel, 30 Malkhe Israel, Jerusalem 95501, Israel Amotz Agnon 1Institute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1995) 23 (8): 695–698. https://doi.org/10.1130/0091-7613(1995)023<0695:PEDNMD>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Shmuel Marco, Amotz Agnon; Prehistoric earthquake deformations near Masada, Dead Sea graben. Geology 1995;; 23 (8): 695–698. doi: https://doi.org/10.1130/0091-7613(1995)023<0695:PEDNMD>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Earthquake-induced fluidizations and suspensions of lake sediments, associated with syndepositional faults, form a paleoseismic record in the Dead Sea graben. The association of fluidized beds with surface faulting supports the recognition of mixed layers as reliable earthquake indicators and provides a tool for the study of very long term (>70 kar) seismicity along the Dead Sea transform. The faults compose a fault zone that offsets laminated sediments of the late Pleistocene Lake Lisan. They exhibit displacements of as much as 2 m. Layers of massive mixtures of laminated fragments are interpreted as disturbed beds, each formed by an earthquake. The undisturbed laminated layers between these mixed layers represent the interseismic interval. A typical vertical slip of about 0.5 m per event is separated by several hundred years of quiescence. The fault zone lies within the Dead Sea graben, 2 km east of Masada, where archaeology and historical accounts indicate repeated strong earthquake damage. The distribution of strikes in the fault zone resembles that of the faults exposed in and around the graben, including the seismogenic ones. The excellent exposures over hundreds of metres allow an unprecedented temporal and spatial resolution of slip events on faults. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Precursory swarms of long-period events at Redoubt Volcano (1989–1990), Alaska: Their origin and use as a forecasting tool

During the eruption of Redoubt Volcano from December 1989 through April 1990, the Alaska Volcano Observatory issued advance warnings of several tephra eruptions based on changes in seismic activity related to the occurrence of precursory swarms of long-period (LP) seismic events (dominant period of about 0.5 s). The initial eruption on December 14 occurred after 23 years of quiescence and was heralded by a 23-hour swarm of LP events that ended abruptly with the eruption. After a series of vent-clearing explosions over the next few days, dome growth began on December 21. Another swarm, with LP events similar to those of the first, began on the 26th and ended in a major tephra eruption on January 2. Eruptions continued over the next two weeks and then ceased until February 15, when a large eruption initiated a long phase of repetitive dome-building and dome-destroying episodes that continued into April. Warnings were issued before the major events on December 14 and January 2, but as the eruptive sequence continued after January 2, the energy of the swarms decreased and forecasting became more difficult. A significant but less intense swarm preceded the February 15 eruption, which was not forecast. This eruption destroyed the only seismograph on the volcanic edifice and stymied forecasting until March 4, when the first of three new stations was installed within 3 km of the active vent. From March 4 to the end of the sequence on April 21, there were eight eruptions, six of which were preceded by detectable swarms of LP events. Although weak, these swarms provided the basis for warnings issued before the eruptions on March 23 and April 6. The initial swarm on December 13 had the following features: (1) short duration (23 hours); (2) a rapidly accelerating rate of seismic energy release over the first 18 hours of the swarm, followed by a decline of activity during the 5 hours preceding the eruption; (3) a magnitude range from −0.4 to 1.6; (4) nearly identical LP signatures with a dominant period near 0.5 s; (5) dilatational first motions everywhere; and (6) a stationary source location at a depth of 1.4 km beneath the crater. This occurrence of long-period events suggests a model involving the interaction of magma with groundwater in which magmatic gases, steam and water drive a fixed conduit at a stationary point throughout the swarm. The initiation of that sequence of events is analogous to the failure of a pressure-relief valve connecting a lower, supercharged magma-dominated reservoir to a shallow hydrothermal system. A three-dimensional model of a vibrating fluid-filled crack recently developed by Chouet is found to be compatible with the seismic data and yields the following parameters for the LP source: crack length, 280–380 m; crack width, 140–190 m; crack thickness, 0.05–0.20 m; crack stiffness, 100–200; sound speed of fluid, 0.8–1.3 km/s; compressional-wave speed of rock, 5.1 km/s; density ratio of fluid to rock, ≈0.4; and ratio of bulk modulus of fluid to rigidity of rock, 0.03–0.07. The fluid-filled crack is excited intermittently by an impulsive pressure drop that varies in magnitude within the range of 0.4 to 40 bar. Such disturbance appears to be consistent with a triggering mechanism associated with choked flow conditions in the crack.

Lessons from a major eruption: Mt. Pinatubo, Philippines

Small phreatic explosions on April 2, 1991, ended more than 400 years of quiescence at Mount Pinatubo. A joint Philippine‐United States team worked quickly to understand the unrest and the eruptive history of Pinatubo, and to warn those at risk. These warnings led to the evacuation of at least 58,000 people prior to the volcano's climactic eruption on June 15. Although 320 people died in that eruption, mostly due to collapse of ash‐covered roofs, the evacuations and other precautions averted a much greater loss of life and property.

The 1989”1990 eruption of Redoubt Volcano

On December 14, 1989, Redoubt Volcano, in south‐central Alaska, erupted after 23 years of quiescence. Volcanic ash and floods generated by the eruption have caused significant damage to aircraft and temporarily curtailed oil production in Cook Inlet. The long‐term effects of continued volcanic activity could be severe on the local economy. The two preliminary reports that follow, “The Eruption and Its Hazards” below and “Volcanic Ash and Aircraft” on the next page, describe the volcanic activity and some of the hazards of the first 6 weeks of the on‐going eruption.

Zonas sísmicas de Oaxaca, México: sismos máximos y tiempos de recurrencia para el periodo 1542-1988

Se revisa la literatura sobre la sismicidad (IMM≥VI en Oaxaca y/o daños en otra localidad de Oaxaca) ocurrida en el estado de Oaxaca durante el período 1542-1988. La información macrosísmica se utiliza para agrupar los temblores en familias de eventos semejantes. Basándose en el conocimiento actual sobre los procesos sismotectónicos en la región, se sugiere la definición de 8 zonas sismogenicas. En un catalogo se presentan los datos, indicando las fuentes y nuestra interpretación. Los 68 eventos mayores (mb ≥6.5, Ms≥7.0) son asignados a cada una de las zonas sísmicas; para éstas se analizan los tiempos de recurrencia y se seleccionan los sismos máximos esperados. Los resultados principales son: - En la zona de subducción pueden ocurrir terremotos de gran magnitud (Ms Ì´Í‚ 8.5), como el de marzo de 1787. - En la zona de subducción, en el periodo 1928-1988, han ocurrido once sismos (Ms ≥ 7.0), cuatro en la mitad este y siete en la oeste. En los 56 años precios a 1928 se presento un periodo de calma sísmica (Ms≥7.0); en la región de ocurrencia del gran terremoto de 1787 sólo vuelve a temblar (Ms ≥ 7.0) en 1928, después de 141 años de calma sísmica. - En 1928, en un lapso de 8 meses, ocurren cuatro eventos de subducción (Ms≥7.4); en el mismo período ocurren dos temblores intraplaca importantes (mb = 7.7) en la parte norte. En enero de 131 ocurre otro intraplaca (Ms = 8.0) en la parte sur. Este patrón de liberación de energía sísmica en un corto periodo de tiempo refuerza la afirmación de que en Oaxaca deben esperarse terremotos de gran magnitud por ruptura simultánea de varias unidades sismogénicas. - Las zonas "Huajuapan" y "Mixe" son afectadas por eventos de profundidad intermedia y falla normal (60≤H≤ 120 km, mb Ì´Í‚ 7.0-7.7) con tiempos de recurrencia bastante regulares (95 ± 11 años y 80 ± 16 años, respectivamente). - En la zona "valle" ocurren sismos someros de falla normal (25≤H≤50, Ms Ì´Í‚ 8.0) en periodos de tiempo también muy regulares (82±16 años). Posiblemente en la zona "Mixteca Media", al oeste de la anterior, también ocurren eventos de este tipo; en este caso la zona "Mixteca Media" presentaría un potencial sísmico muy alto (156 ± 22 años, ultimo temblor en 1854). - Para la zona "Istmo" se concluye que, en general, su actividad sísmica de profundidad intermedia es de magnitud moderada (mb ≤ 6.5). de especial interés es el evento de diciembre 29, 1917 (Ms = 7.1); nuestra relocalización indica que su epicentro esta situado en el Golfo de Tehuantepec, en una brecha sísmica cuyo potencial sísmico es difícil de evaluar. - El análisis de la actividad sísmica (mb ≥6.5, Ms≥7.0) ocurrida en los últimos 480 años surgiere que estudios basados en los catálogos de los últimos 150 años pueden conducir a conclusiones erróneas sobre la sismotectónica de la región, subestimar los sismos máximos y definir periodos de recurrencia con escasa información.

Space-time Clustering of Large Earthquakes: A Western U. S. Perspective with Implications for the East

Abstract Results of ongoing paleoseismicity studies along the Wasatch fault zone, Utah, and Lost River fault zone, Idaho, suggest that spatial and temporal clustering of large magnitude surface-faulting earthquakes occur on each. The timing of individual past events is based primarily on radiocarbon and thermoluminescence dating of faulted deposits and on identification of displaced volcanic ashes, all observed in trenches. Figure 1 shows the faulting history of the Wasatch for the past 6 ka. Even though there is variability, distinct patterns emerge. Five of the seven active segments ruptured between 0.3 and 1 ka; if the Salt Lake segment is included, six segments have ruptured between 0.3 and 1.5 ka. From Salt Lake south, a similar sequence of events occurred between about 4.5 and 6 ka. Figure 2 shows timing of events along the Lost River during the past 12 ka. No events occurred from at least 12 ka until middle Holocene. Between 5.5 and 8 ka the northern three segments ruptured; actual timing of events could have been considerably closer. The Thousand Springs segment produced the 1983, MS 7.3, Borah Peak earthquake. These observations suggest thousands of years of quiescence can occur between clusters of activity during which adjacent segments fail in short periods of time (hours, months, decades, a few hundred years?). Segments adjacent to historical ruptures may have a higher potential to produce the next earthquakes, especially if they have elapsed times similar to the historical rupture segment. Similar clustering with even longer periods of quiescence could characterize seismic sources in the East.

霧島火山群高千穂複合火山の噴火活動史

Takachiho composite volcano, located in south Ky ?? sh ??, Japan, is one of major members of the Kirishima volcano group. The composite volcano consists of four volcanic edifices which are partly overlapping and aligned in E-W direction. On the basis of field geology and tephrochronology, its growth-history can be divided into four stages. During the first stage, a conical stratovolcano, Futagoishi, was formed in the eastern end of the composite volcano. It is com-posed of an alternation of pyroclastic beds and lava flows. The activity of Futagoishi ended before the eruption of Ito pyroclastic flow deposit (22, 000 yBP) from the Aira caldera. The second stage activity started about 10, 000 years ago, after a repose of more than 12, 000 years. It was characterized by vulcanian eruption which continued semi-persistently through a long period of time, probably more than a thousand years. During this period a stratovolcano, Old-Takachiho, was built up on the western flank of Futagoishi, and a large amount of volcanic ash accumulated concentrically around the volcano. The third stage activity was characterized by the eruption of several thin lava flows and scoria from the vent close to Old-Takachiho Volcano. This activity resulted in formation of a small conical stratovolcano, Takachiho-no-mine, on the western slope of Old-Takachiho. At the end of this stage, enormous lava flows were successively effused from the top crater and flowed downward widely. Later lava flows, however, did not reached the foot of the slope due to decrease in effusion rate and piled up around the crater, resulting in the construction of an exogeneous lava dome on the summit. After several hundred years of quiescence, the present stage activity began about 2, 500 years ago. This activity was characterized by sub-plinian eruptions which occurred intermittently and produced thick scoria and ash beds on the eastern foot of the composite vlocano. By repeated eruptions of a great volume of scoria and lava flows, a new vlocano, Ohachi, was born on the western flank of Takachiho-no-mine volcano, and has been active since then. The growth-history of Takachiho composite volcano revealed the following two features: 1) the volcanic activity may have been controlled by the underlying E-W fracture, as suggested by the shifting of the vent position from east to west, and 2) the styles of explosive activity of the composite volcano have gradually changed with time from vulcanian eruption to sub-plinian one.

Z Andromedae: Quiescence and Activity

Z Andromedae, often considered as the prototype of symbiotic stars, experimented, after several years of quiescence, a small outburst in March-April 1984, followed by a larger one in September-October 1985. The imminence of a new activity phase was predicted by Viotti et al. (1982). Z And is, together with AG Draconis, the only symbiotic star observed both during quiescence and activity with the IUE satellite. The early photometric and spectroscopic history of Z And has been recently reviewed by Kenyon (1986).The behaviour of Z And in the ultraviolet during quiescence has been studied by Fernåndez-Castro et al. (1988), on the basis of data obtained by the International Ultraviolet Explorer from 1978 to 1982. In that period of time, the UV continuum and the emission line fluxes varied quasiperidically with a period of about two years, in phase with the Hα variability found by Altamore et al. (1979), with the UBV photometry by Belyakina (1985) and also in agreement with the ephemeris given by Kenyon and Webbink (1984). The electron density derived from Si III/C III] flux ratio, also varied in phase with the UV flux. In particular, a correlation was found between the electron density variations, and the UV continuum flux at 2900 A. This fact, together with the presence of a Balmer jump in emission, indicates that the principal contributor at those wavelengths is nebular emission, mainly free-bound transitions.

Pathophysiology and epidemiology of epidemic Kaposi's sarcoma.

The epidemic form of Kaposi's sarcoma (KS) that occurs in patients with the acquired immune deficiency syndrome (AIDS) produces lesions that, histopathologically, are indistinguishable from those of classical KS or of the endemic form of the disease seen in children and adults in certain areas of Africa. There are, however, important differences in the pathogenesis of the disease in the different groups affected by the neoplasm. Compared with classical KS in people of eastern European and Mediterranean descent, which commonly takes a protracted, indolent course, the epidemic Kaposi's sarcoma (EKS) is far more aggressive. However, the KS seen in adults in endemic areas of Africa may also become florid and rapidly progressive after years of quiescence. Some degree of immune dysfunction is thought to be a factor in all forms of KS, with immune depression being the hallmark of EKS and the setting in which it occurs. Cytomegalovirus (CMV) is thought to be at least a cofactor in the disease, but it has also been suggested that the etiologic agent of AIDS, human immunodeficiency virus (HIV), may also play a role in EKS.

Rifting of the plate boundary in north Iceland 1975–1978

A rifting episode started in 1975 on the accreting plate boundary in North Iceland after 100 years of quiescence. Horizontal extension of some 3 m has been observed in the Krafla caldera and the associated 80 km long fissure swarm. The rifting occurs periodically in short active pulses at a few months intervals. Between these active pulses, continuous inflation of 7–10 mm/day of the caldera is caused by 5 m3/s inflow of magma into a magma chamber at 3 km depth. The active pulses are caused by a sudden east‐west expansion of the fissure swarm and a contraction of zones outside the fissure swarm. Rapid flow of magma out of the magma chamber and into the fissures toward north or south is indicated. These pulses are accompanied by earthquake swarms and vertical ground movements of up to 2 m and sometimes also volcanic eruptions and formation of new fumaroles. The magma chamber below the Krafla caldera thus acts as a trigger for the plate movement along the constructive plate boundary in North Iceland.