Weak Flares Research Articles

The RPS-1 X-ray CdTe spectrometer onboard the CORONAS-F satellite

Some of the results of X-ray observations of the Sun in the energy interval from 3 to 31.5 keV using a semiconductor spectrometer are reported. A brief description is given of the instrument, in which a not deeply cooled cadmium telluride detector is used for the first time for spectrometric measurements under cosmic conditions. The main parameters of the instrument and the technique employed for processing the data are described as well. The capabilities of the spectrometer made it possible to track the evolution of the X-ray spectra of weak solar flares registered under favorable background conditions. It is also shown that the intensity and hardness of the radiation of the quiet Sun increases with the level of its activity.

Analysis and modeling of high temporal resolution spectroscopic observations of flares on AD Leonis

We report the results of a high temporal resolution spectroscopic monitoring of the flare star AD Leo. During 4 nights, more than 600 spectra were taken in the optical range using the Isaac Newton Telescope (INT) and the Intermediate Dispersion Spectrograph (IDS). We have observed a large number of short and weak flares occurring very frequently (flare activity > 0.71 hours-1). This is in favour of the very important role that flares can play in stellar coronal heating. The detected flares are non white-light flares and, though most of solar flares belong to this kind, very few such events had been previously observed on stars. The behaviour of different chromospheric lines (Balmer series from H_alpha to H_11, Ca II H & K, Na I D_1 & D_2, He I 4026 AA and He I D_3) has been studied in detail for a total of 14 flares. We have also estimated the physical parameters of the flaring plasma by using a procedure which assumes a simplified slab model of flares. All the obtained physical parameters are consistent with previously derived values for stellar flares, and the areas - less than 2.3% of the stellar surface - are comparable with the size inferred for other solar and stellar flares. Finally, we have studied the relationships between the physical parameters and the area, duration, maximum flux and energy released during the detected flares.

The onset of solar energetic particle events: prompt release of deka-MeV protons and associated coronal activity

Between 1996 and 2002, the COSTEP particle instrument aboard the SoHO mission observed 18 solar energetic particle events where the release times of protons up to 54 MeV could be determined to within an uncertainty of about ten minutes, and where metre wave imaging observations with the Nancay Radioheliograph were available. In six of them the proton release starts, within the uncertainty of the method, simultaneously with the release of electron beams observed through their type III emission at decametric-to-kilometric wavelengths, during flares in Ha or EUV. These well-connected events are studied to identify the coronal processes that occur when the particle release starts at the Sun. Given that big events are not reliably measured by COSTEP due to its large geometric factor, they are rather weak with peak intensities (9 x 10 -4 - 5 x 10 -2 ) [(cm 2 s sr MeV) -1 ] in the (8-25) MeV range. All are accompanied by fast (400-1300 km s -1 ) and narrow coronal mass ejections (CMEs). While half of the events is accompanied by significant microwave bursts, the other half shows little or no evidence of electron acceleration in the low corona and rather weak soft X-ray bursts. The association with weak flares may point to different acceleration sites of the interacting and the escaping particles. From the radio observations of the type III bursts and the associated metre wave emission, the escaping electrons, and by inference also the first escaping protons, are accelerated roughly between 0.1 R ○. and 0.5 R ○. above the photosphere, a height range shown to be well behind the front of the CMEs at the time of acceleration. The data show no evidence that the shock waves, which are presumably driven by at least the fastest of these CMEs, participate in the early acceleration of the escaping protons or affect the escape of the protons accelerated at lower height.

The X-ray emission from Young Stellar Objects in the ρ Ophiuchi cloud core as seen by XMM-Newton

We observed the main core F of the rho Ophiuchi cloud, an active star-forming region located at ~140 pc, using XMM-Newton with an exposure of 33 ks. We detect 87 X-ray sources within the 30' diameter field-of-view of the it EPIC imaging detector array. We cross-correlate the positions of XMM-Newton X-ray sources with previous X-ray and infrared (IR) catalogs: 25 previously unknown X-ray sources are found from our observation; 43 X-ray sources are detected by both XMM-Newton and Chandra; 68 XMM-Newton X-ray sources have 2MASS near-IR counterparts. We show that XMM-Newton and Chandra have comparable sensitivity for point source detection when the exposure time is set to ~30 ks for both. We detect X-ray emission from 7 Class I sources, 26 Class II sources, and 17 Class III sources. The X-ray detection rate of Class I sources is very high (64 %), which is consistent with previous Chandra observations in this area. We propose that 15 X-ray sources are new class III candidates, which doubles the number of known Class III sources, and helps to complete the census of YSOs in this area. We also detect X-ray emission from two young bona fide brown dwarfs, GY310 and GY141, out of three known in the field of view. GY141 appears brighter by nearly two orders of magnitude than in the Chandra observation. We extract X-ray light curves and spectra from these YSOs, and find some of them showed weak X-ray flares. We observed an X-ray flare from the bona fide brown dwarf GY310. We find as in the previous Chandra observation of this region that Class I sources tend to have higher temperatures and heavier X-ray absorptions than Class II and III sources.

X‐Ray Flares from Markarian 501

Motivated by the recent finding of hierarchical X-ray flaring phenomenon in Mrk 421, we conducted a systematic search for X-ray flares from Mrk 501, another well-known TeV blazar, by making use of the rich {\em RXTE} archival database. We detected flares over a wide range of timescales, from months down to minutes, as in the case of Mrk 421. However, the flares do not seem to occur nearly as frequently in Mrk 501 as in Mrk 421 on any of the timescales. The flaring hierarchy also seems apparent in Mrk 501, suggesting that it might be common among TeV blazars. The results seem to imply a scale-invariant physical origin of the flares (large or small). The X-ray spectrum of the source shows a general trend of hardening toward the peak of long-duration flares, with indication of spectral hysteresis, which is often seen in TeV blazars. However, the data are not of sufficient quality to allow us to draw definitive conclusions about spectral variability associated with more rapid but weaker flares. We critically examine a reported sub-hour X-ray flare from Mrk 501, in light of intense background flaring activity at the time of the observation, and concluded that the flare is likely an artifact. On the other hand, we did identify a rapid X-ray flare that appears to be real. It lasted only for about 15 minutes, during which the flux of the source varied by about 30%. Sub-structures are apparent in its profile, implying variabilities on even shorter timescales. Such rapid variabilities of Mrk 501 place severe constraints on the physical properties of the flaring region in the jet, which have serious implications on the emission models proposed for TeV blazars.

The Magnetic Properties of an L Dwarf Derived from Simultaneous Radio, X‐Ray, and Hα Observations

We present the first simultaneous, multiwavelength observations of an L dwarf, the L3.5 candidate brown dwarf 2MASS J00361617+1821104, conducted with the Very Large Array, the Chandra X-Ray Observatory, and the Kitt Peak 4 m telescope. We detect strongly variable and periodic radio emission (P = 3 hr) with a fraction of about 60% circular polarization. No X-ray emission is detected to a limit of LX/Lbol 2 × 10-5, several hundred times below the saturation level observed in early M dwarfs. Similarly, we do not detect Hα emission to a limit of LHα/Lbol 2 × 10-7, the deepest for any L dwarf observed to date. The ratio of radio to X-ray luminosity is at least 4 orders of magnitude in excess of that observed in a wide range of active stars (including M dwarfs), providing the first direct confirmation that late-M and L dwarfs violate the radio/X-ray correlation. The radio emission is due to gyrosynchrotron radiation in a large-scale magnetic field of about 175 G, which is maintained on timescales longer than 3 yr. The detected 3 hr period may be due to (1) the orbital motion of a companion at a separation of about 5 stellar radii, similar to the configuration of RS CVn systems, (2) an equatorial rotation velocity of about 37 km s-1 and an anchored, long-lived magnetic field, or (3) periodic release of magnetic stresses in the form of weak flares. In the case of orbital motion, the magnetic activity may be induced by the companion, possibly explaining the unusual pattern of activity and the long-lived signal. We conclude that fully convective stars can maintain a large-scale and stable magnetic field, but the lack of X-ray and Hα emission indicates that the atmospheric conditions are markedly different than in early-type stars and even M dwarfs. Similar observations are therefore invaluable for probing both the internal and external structure of low-mass stars and substellar objects, and for providing constraints on dynamo models.

X-ray observation of SS 433 with RXTE

Apart from regular monitoring with the All Sky Monitor, the compact object SS 433 was observed with RXTE several times over the last 2 or 3 yr. We present the first analysis of these observations. We also include the results of the recent exciting Target of Opportunity campaign made during donor inferior (orbital phase φ = 0) and superior (φ = 0.5) conjunctions which took place on 2003 October 2 and 2004 March 13, respectively, when the jet was directly pointing towards us (i.e. precessional phase ψ ∼ 0). Generally, we found that two distinct lines fit the spectra taken on these days. We present some of the light curves and the X-ray spectra, and show that the Doppler shifts of the emitted lines roughly match those predicted by the kinematic model for the jets. We find that the line with a higher energy can be best identified with a Fe XXVI Lyman-α transition while the line with lower energy can be identified with a Fe XXV (1s2p‐1s 2 ) transition. We observe that the X-ray flux on 2004 March 13 (when the base of the jet was exposed) is more than twice that on 2003 October 2 (when the base is covered by the companion). We find that the flux continues to remain high at least until another orbital period. We believe that this is because SS 433 was undergoing a weak flaring activity during the recent observation. Ke yw ords: black hole physics ‐ methods: observational ‐ stars: individual: SS 433 ‐ X-rays: individual: SS 433.

The X-ray emission from Young Stellar Objects in theρOphiuchi cloud core as seen by XMM-Newton

We observed the main core F of the rho Ophiuchi cloud, an active star-forming region located at ~140 pc, using XMM-Newton with an exposure of 33 ks. We detect 87 X-ray sources within the 30' diameter field-of-view of the it EPIC imaging detector array. We cross-correlate the positions of XMM-Newton X-ray sources with previous X-ray and infrared (IR) catalogs: 25 previously unknown X-ray sources are found from our observation; 43 X-ray sources are detected by both XMM-Newton and Chandra; 68 XMM-Newton X-ray sources have 2MASS near-IR counterparts. We show that XMM-Newton and Chandra have comparable sensitivity for point source detection when the exposure time is set to ~30 ks for both. We detect X-ray emission from 7 Class I sources, 26 Class II sources, and 17 Class III sources. The X-ray detection rate of Class I sources is very high (64 %), which is consistent with previous Chandra observations in this area. We propose that 15 X-ray sources are new class III candidates, which doubles the number of known Class III sources, and helps to complete the census of YSOs in this area. We also detect X-ray emission from two young bona fide brown dwarfs, GY310 and GY141, out of three known in the field of view. GY141 appears brighter by nearly two orders of magnitude than in the Chandra observation. We extract X-ray light curves and spectra from these YSOs, and find some of them showed weak X-ray flares. We observed an X-ray flare from the bona fide brown dwarf GY310. We find as in the previous Chandra observation of this region that Class I sources tend to have higher temperatures and heavier X-ray absorptions than Class II and III sources.

Flare‐Associated Coronal Mass Ejections with Large Accelerations

It is well known that while flare-associated coronal mass ejections (CMEs) show higher speeds and little acceleration in the corona, filament-associated CMEs have lower speeds and large accelerations. In this paper, we examine three flare-associated CMEs with relatively large accelerations as counterexamples of the former tendency. The estimated accelerations are all larger than 45 m s-2 below 15 R☉. By analyzing SOHO EIT, SOHO LASCO, and GOES data, we attempt to find out what kind of physical characteristics control such strong accelerations. The first event is the 1999 July 9 event associated with a C1.1 flare. Considering the fact that its CME appearance, seen in the LASCO running difference imagery, is quite similar to the shape of a helmet streamer, we speculate that its eruption is related to the destabilization of a helmet streamer, which may induce the weak X-ray flare. The second event is the 1999 August 17 event associated with a C2.6 flare. The CME speed abruptly increased from 232 to 909 km s-1 for 1 hr, and the strong acceleration is coincident with the occurrence of a subsequent flare/CME. The third event is the 2000 November 24 event associated with a C4.1 flare. The CME speed first decreased and then constantly accelerated for 3 hr. The start of such an acceleration is also coincident with a subsequent CME/flare event. For the last two CME events, the Lorentz forces acting on the subsequent events may play an important role in accelerating CMEs. Our results show that large accelerations of flare-associated CMEs, as counterexamples of the two classes of CMEs, seem to be caused by other solar activities, such as helmet streamer disruptions or subsequent CMEs/flares.

Determination of the location and effective angular size of solar flares with a 210 GHz multibeam radiometer

We report on the study and successful application of an improved measurement method for solar flares at millimeter wavelengths. A 210 GHz multibeam receiver for the observation of solar bursts was installed in the KOSMA 3 m telescope on the Gornergrat. It consists of three radiometer channels, with a fourth beam synthesised from the other three. The four inter- secting beams allow measurements of source locations with arcsecond resolution and, for the first time, also the determination of the effective source size at short millimeter waves. The typical sensitivity of <1.5 sfu allows also the detection of weak flares at millisecond time resolution. In this paper we present the instrument and the numerical method for the determination of the source flux density, position and effective size, as well as simulations to asses the validity of the method. First observational results were obtained for the GOES X17.2 flare on October 28, 2003. The event reached a peak flux density of 11 000 sfu at 210 GHz and exhibited a slowly varying, time-extended emission from an extended source (effective diameter ≈ 60 arcsec), as well as a short-lived component from a compact source (<10 arcsec) originating from a different location.

Flares from small to large: X-ray spectroscopy of Proxima Centauri with XMM-Newton

We report results from a comprehensive study of the nearby M dwarf Proxima Centauri with the XMM-Newton satellite, using simultaneously its X-ray detectors and the Optical Monitor with its U band filter. We find strongly variable coronal X-ray emission, with flares ranging over a factor of 100 in peak flux. The low-level emission is found to be continuously variable on at least three time scales (a slow decay of several hours, modulation on a time scale of 1 hr, and weak flares with time scales of a few minutes). Several weak flares are characteristically preceded by an optical burst, compatible with predictions from standard solar flare models. We propose that the U band bursts are proxies for the elusive stellar non-thermal hard X-ray bursts suggested from solar observations. In the course of the observation, a very large X-ray flare started and was observed essentially in its entirety. Its peak luminosity reached erg s-1 [0.15–10 keV], and the total X-ray energy released in the same band is derived to be ergs. This flare has for the first time allowed to measure significant density variations across several phases of the flare from X-ray spectroscopy of the O vii He-like triplet; we find peak densities reaching up to cm-3 for plasma of about 1–5 MK. Abundance ratios show little variability in time, with a tendency of elements with a high first ionization potential to be overabundant relative to solar photospheric values. Using Fe xvii lines with different oscillator strengths, we do not find significant effects due to opacity during the flare, indicating that large opacity increases are not the rule even in extreme flares. We model the large flare in terms of an analytic 2-Ribbon flare model and find that the flaring loop system should have large characteristic sizes (1) within the framework of this simplistic model. These results are supported by full hydrodynamic simulations. Comparing the large flare to flares of similar size occurring much more frequently on more active stars, we propose that the X-ray properties of active stars are a consequence of superimposed flares such as the example analyzed in this paper. Since larger flares produce hotter plasma, such a model also explains why, during episodes of low-level emission, more active stars show hotter plasma than less active stars.

High Temporal Resolution Spectroscopic Observations of the Flare Star V1054 Oph

We present the results of a high temporal resolution spectroscopic monitoring of the flare (UV Cet type) star V1054 Oph (Wolf 630AB), classified as a dM3.5e visual binary system. Intermediate resolution spectra have been taken during four nights (2–5 April 2001) using the IDS spectrograph of the 2.5 m Isaac Newton Telescope (INT) (La Palma, Canary Islands, Spain). The V1054 Oph spectra show very strong emission lines even in its quiescent state. The analysis of the temporal evolution of the observed emission lines (from Hβ to H11 and the Ca II H and K lines) reveals four strong flares and several weak flares. We have studied in detail the behaviour of the chromospheric lines during the different phases (pre-flare, impulsive and gradual decay) of these flares. The observed flares last from ∼25 to 95 min. The equivalent width of the Hβ line changes by a factor up to ∼2.3. Broad wings and asymmetric (red-shifted) lines are observed as well.

Tether‐cutting Energetics of a Solar Quiet‐Region Prominence Eruption

We study the morphology and energetics of a slowly evolving quiet-region solar prominence eruption occurring on 1999 February 8-9 in the solar north polar crown region, using soft X-ray data from the soft X-ray telescope (SXT) on Yohkoh and Fexv EUV 284 Angstrom data from the EUV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO). After rising at approximately equal to l kilometer per second for about six hours, the prominence accelerates to a velocity of approximately equal to 10 kilometers per second, leaving behind EUV and soft X-ray loop arcades of a weak flare in its source region. Intensity dimmings occur in the eruption region cospatially in EUV and soft X-rays, indicating that the dimmings result from a depletion of material. Over the first two hours of the prominences rapid rise, flare-like brightenings occur beneath the rising prominence that might correspond to tether-cutting magnetic reconnection. These brightenings have heating requirements of up to approximately 10(exp 28)-10(exp 29) ergs, and this is comparable to the mechanical energy required for the rising prominence over the same time period. If the ratio of mechanical energy to heating energy remains constant through the early phase of the eruption, then we infer that coronal signatures for the tether cutting may not be apparent at or shortly after the start of the fast phase in this or similar low-energy eruptions, since the plasma-heating energy levels would not exceed that of the background corona.

Flare Heating in Stellar Coronae

An open question in the field of solar and stellar astrophysics is the source of heating that causes stellar coronae to reach temperatures of millions of degrees. One possibility is that the coronae are heated by a large number of small flares. On the Sun, flares with energies as low as those of microflares are distributed with energy as a power law of the form ∝ E-α, with α ≈ 1.8, and α appears to increase to values of 2.2-2.7 for flares of lower energy. If the slope exceeds the critical value of 2, then in principle the entire coronal energy input can be ascribed to flares that are increasingly less energetic but are more numerous. Previous analyses of flares in light curves of active stars have shown that this index generally is greater than 2, although it may be as low as 1.6 when strong flares alone are considered. Here we investigate the contribution of very weak flares, covering the milliflare energy range, to the coronal luminosity of low-mass active stars. We analyze Extreme Ultraviolet Explorer/Deep Survey events data from FK Aqr, V1054 Oph, and AD Leo and conclude that in all these cases, the coronal emission is dominated by flares to such an extent that in some cases, the entire emission can be ascribed to flare heating. We have developed a new method to directly model for the first time stochastically produced flare emission, including undetectable flares, and their effects on the observed photon arrival times. We find that αFK Aqr = 2.60 ± 0.34, αV1054 Oph = 2.74 ± 0.35, and αAD Leo = 2.03-2.32, and the flare component accounts for a large fraction (generally greater than 50%) of the total flux.

Network flares as agents of coronal heating

Network flares are small heating events occurring outside the active regions, mostly over the network junctions, where coronal bright points are most likely to form. The amount of thermal energy released into the flaring plasma is of the order of 10 26−27 ergs per event and then can be classed as micro-flares when compared to very large solar flares (10 32 ergs). In this paper we present our efforts to improve our knowledge about the network flaring ratio in X-rays, which now is estimated to be approximately 8000 events per day over the whole Sun, and gives the total rate of energy released in these events 8 × 10 25 ergs s −1. This value is about one order of magnitude less than the energy rate needed to heat up the quiescent corona. However the number distribution of events with energy shows the prevalence of weak flares strong enough to expect that the heating requirements of the quiescent corona could be achieved when extrapolating the distribution below the detectability treshold.

The flux deficits in star spots

The bolometric flux deficits of the photospheres of spotted stars are derived for the first time in the framework of zonal spottedness models for red dwarfs computed at the Crimean Astrophysical Observatory. The resulting flux deficits are compared to the estimated radiative losses from the chromospheres and coronas measured during quasi-simultaneous observations. A linear correlation is found between the logarithms of these quantities, with the Sun fitting these relations. Radiative losses from the outer stellar atmospheres in quiescence and during individual sporadic flares are significantly lower than the bolometric deficits of the spotted photospheres of active stars. This suggests that the flux deficit due to spots leads to global reconstruction of the atmospheres of red dwarfs, analogous to the local atmospheric reconstruction that occurs during solar and stellar flares. This process may be realized via the superposition of a large number of weak impulsive flares and other dynamic events, which develop on these stars and heat their coronas (i.e., in this view, microflaring is favored as the principal coronal heating mechanism for these stars). A brief analysis of the long-term variations in the chromospheric and photospheric radiation of F-K stars from the HK project and of the Sun suggests that such dynamical reconstruction of the outer atmosphere by energy associated with the flux deficit of the spotted photosphere occurs at times of increased surface activity in all F-M stars.

Flares at the Cool End of the M Dwarfs: The Case of LHS 2065

LHS 2065 is an M9 very low mass star at a distance of 8.5 pc. It was known to have Hα emission, but no other emission lines had been seen before. We observed it on 1998 December 12 and found a rich emission spectrum (He I, Na D, and Balmer series). Additional spectra were obtained after 1 hr, 2 hr, and 1 day. They showed progressively weaker emission lines. Using our data and a compilation of the literature, we find that, while strong flares in LHS 2065 are rare (their recurrence rate is ≤0.03 hr-1), weak flares may be common (about 0.5 hr-1). Our observations add to the growing evidence that many, if not all, of the coolest M dwarfs have flare activity.

A Radio Study of the Evolution of Spatial Structure of an Active Region and Flare Productivity

We present the results of a radio study of the evolution of an active region through its flare productivity. The radio study was carried out with data obtained by the Nobeyama Radio Heliograph at 17 GHz. We chose the active region AR 7515, which appeared at the east limb on 1993 May 23 and then evolved during its passage across the disk. We followed its evolution until June 2. This region produced many small flares. We consider this region to be a typical active region in the sense that it did not produce any large flares, but a large number of weak flares. We investigate the optical and magnetic development of the region and show how this affects the locations of the flaring activity. We discuss a number of events in detail in order to investigate the roles of nonthermal and thermal radio emission in the flares. The nonthermal gyrosynchrotron emission generally occurs in regions of strong magnetic fields, is generally circularly polarized, and often varies rapidly in time. On the other hand, gradual radio components tend to be thermal and only weakly polarized, if at all. An interesting aspect of evolution of the flares in this region is that many of the flares in the early phase of the evolution show strong but brief nonthermal radio emission in the impulsive phase followed by gradual thermal emission, whereas in the last 3 days more gradual events without a strong spike of radio emission in the impulsive phase tend to be seen. Correspondingly, the flare images suggest that the radio sources are more compact during the early phases and more extended in the last half of the period covered. The most dominant component of the preflare region is often not the component that undergoes immediate flaring. Sometimes a number of components in the preflare region participate in the flare process together. We speculate that these component sources are unresolved compact bipolar loops that flare in sequence. Loop-loop interactions occurring at many different sites at the same time seems to be a less plausible explanation of these events.

A Gamma‐Ray Flare of Quasar CTA 26

During the first 3 years of the Compton Gamma Ray Observatory mission, the blazar CTA 26 was observed 10 times by EGRET and not significantly detected. We report an observation in 1995 when CTA 26 flared to a peak γ-ray flux of (4.9 ± 1.5) × 10-6 cm-2 s-1 (E > 100 MeV), the third brightest of all EGRET blazars. Following the γ-ray flare, extensive VLBA and single-dish radio observations were obtained. We find two components of a milliarcsecond jet moving with apparent transverse velocities of 12 ± 1 h-1 c, and 5 ± 2 h-1 c (H0 = 100 h km s-1 Mpc-1, q0 = 0.1). The position angle of VLBI components appears to change with time. The slowest VLBI component's motion is consistent with ejection at the time of the 1995 γ-ray flare. A weak radio flare is also seen in Metsahovi millimeter radio monitoring data, peaking within weeks of the γ-ray flare.

The Acute and Sub-chronic Effects of Levocetirizine, Cetirizine, Loratadine, Promethazine and Placebo on Cognitive Function, Psychomotor Performance, and Weal and Flare

SummaryAim: To compare the central and peripheral H1 inhibitory effects of acute and sub-chronic doses of levocetirizine (l-CTZ), cetirizine (CTZ), loratadine (LOR) and promethazine (PRM) versus placebo, using a battery of psychomotor and cognitive tests together with measures of the weal and flare reaction. PRM was included in the study as a positive internal control to validate the sensitivity of the psychometric test battery to the CNS effects of the various treatments.Methods: Twenty healthy volunteers (18–50 years) received l-CTZ 5 mg, CTZ 10 mg, LOR 10 mg, PRM 30 mg and placebo once daily for four days in a five-way, double-blind, crossover study. For each treatment condition, subjects were assessed using a psychometric test system and a pinprick weal and flare response to 100mg/ml histamine solution at baseline and at 1, 2, 3,4, 6,8,10 and 12 hours post-dose on days 1 and 4. The psychometrics comprised critical flicker fusion (CFF), choice reaction time (CRT), a continuous tracking task (CTT) and subjective rating scales for sedation (LARS). On days 2 and 3, subjects took their medication at pre-designated times while out of the unit.Results: The verum (PRM) established the sensitivity of the test battery: a significant overall reduction in CFF thresholds on both days 1 and 4 (p < 0.05); an overall significant increase (impairment) in recognition, motor and total reaction times on day 1 (p < 0.05); a significant impairment of both the tracking accuracy and reaction time aspects of the CTT task on day 1 (p < 0.005) and significantly higher ratings of subjective sedation on day 1 (p < 0.05). l-CTZ, CTZ and LOR were not distinguishable from placebo in any of the objective and subjective tests at any time point on either day 1 or day 4. With regards to the peripheral inhibitory effects, l-CTZ inhibited both the weal and flare reaction, with maximum inhibition (almost 100%) occurring within two hours of drug ingestion. CTZ also showed evidence of potent peripheral inhibition of histamine, whereas PRM, and especially LOR, showed only a weak weal and flare reaction which had completely attenuated at day 4.Conclusions: In a study where the psychometric assessments were shown to be sensitive to impairment, l-CTZ 5 mg was found not only to be a potent inhibitor of the histamine-induced weal and flare reaction,