arXiv subjects

J. P. Finley

Publications and source records attributed to J. P. Finley.

At least 145 records · Page 8Linked to original sources

arXiv2002

Recent Results from the VERITAS Collaboration

A decade after the discovery of TeV gamma-rays from the blazar Mrk 421 (Punch et al. 1992), the list of TeV blazars has increased to five BL Lac objects: Mrk 421 (Punch et al. 1992; Petry et al. 1996; Piron et al. 2001), Mrk 501 (Quinn et al. 1996; Aharonian et al. 1999; Djannati-Atai et al. 1999), 1ES2344+514 (Catanese et al. 1998), H1426+428 (Horan et al. 2000, 2002; Aharonian et al. 2002; Djannati-Atai et al. 2002) and 1ES1959+650 (Nishiyama et al. 1999; Konopelko et al. 2002; Holder et al. 2002). In this paper we report results from recent observations of Mrk 421, H1426+428 and 1ES1959+650 using the Whipple Observatory 10 m telescope.

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arXiv2002

The TeV spectrum of H1426+428

The BL Lac object H1426+428 was recently detected as a high energy gamma-ray source by the VERITAS collaboration (Horan et al. 2002). We have reanalyzed the 2001 portion of the data used in the detection in order to examine the spectrum of H1426+428 above 250 GeV. We find that the time-averaged spectrum agrees with a power law of the shape dF/dE = 10^(-7.31 +- 0.15(stat) +- 0.16(syst)) x E^(-3.50 +- 0.35(stat) +- 0.05(syst)) m^(-2)s^(-1)TeV^(-1) The statistical evidence from our data for emission above 2.5 TeV is 2.6 sigma. With 95% c.l., the integral flux of H1426+428 above 2.5 TeV is larger than 3% of the corresponding flux from the Crab Nebula. The spectrum is consistent with the (non-contemporaneous) measurement by Aharonian et al. (2002) both in shape and in normalization. Below 800 GeV, the data clearly favours a spectrum steeper than that of any other TeV Blazar observed so far indicating a difference in the processes involved either at the source or in the intervening space.

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arXiv2002

Discovery of Spectral Variability of Markarian 421 at TeV Energies

The detection of spectral variability of the gamma-ray blazar Mrk 421 at TeV energies is reported. Observations with the Whipple Observatory 10m gamma-ray telescope taken in 2000/2001 revealed exceptionally strong and long-lasting flaring activity. Flaring levels of 0.4 to 13 times that of the Crab Nebula flux provided sufficient statistics for a detailed study of the energy spectrum between 380 GeV and 8.2 TeV as a function of flux level. These spectra are well described by a power law with an exponential cutoff. There is no evidence for variation in the cutoff energy with flux, and all spectra are consistent with an average value for the cutoff energy of 4.3 TeV. The spectral index varies between 1.89 in a high flux state and 2.72 in a low state. The correlation between spectral index and flux is tight when averaging over the total 2000/2001 data set. Spectral measurements of Mrk~421 from previous years (1995/96 and 1999) by the Whipple collaboration are consistent with this flux-spectral index correlation, which suggest that this may be a constant or a long-term property of the source. If a similar flux-spectral index correlation were found for other gamma-ray blazars, this universal property could help disentangle the intrinsic emission mechanism from external absorption effects.

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arXiv2001

Cutoff in the TeV Energy Spectrum of Markarian 421 During Strong Flares in 2001

Exceptionally strong and long lasting flaring activity of the blazar Markarian 421 (Mrk 421) occurred between January and March 2001. Based on the excellent signal-to-noise ratio of the data we derive the energy spectrum between 260 GeV - 17 TeV with unprecedented statistical precision. The spectrum is not well described by a simple power law even with a curvature term. Instead the data can be described by a power law with exponential cutoff: $\rm {{dN}\over{dE}} \propto \rm E^{-2.14 \pm 0.03_{stat}} \times e^{-E/E_{0}} m^{-2} s^{-1} TeV^{-1}$ with $\rm E_{0} = 4.3 \pm 0.3_{stat} TeV$. Mrk 421 is the second $γ$-ray blazar that unambiguously exhibits an absorption-like feature in its spectral energy distribution at 3-6 TeV suggesting that this may be a universal phenomenon, possibly due to the extragalactic infra-red background radiation.

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arXiv2001

ASCA Detection of Pulsed X-ray Emission from PSR J0631+1036

ASCA's long look at the 288 millisecond radio pulsar, PSR J0631+1036, reveals coherent X-ray pulsation from this source for the first time. The source was first detected in the serendipitous Einstein observation and later identified as a radio pulsar. Possible pulsation in the gamma-ray band has been detected from the CGRO EGRET data (Zepka, et al. 1996). The X-ray spectrum in the ASCA band is characterized by a hard power-law type emission with a photon index of about 2.3, when fitted with a single power-law function modified with absorption. An additional blackbody component of about 0.14 keV increases the quality of the spectral fit. The observed X-ray flux is 2.1e-13 ergs/s/cm2 in the 1-10 keV band. We find that many characteristics of PSR J0631+1036 are similar to those of middle-aged gamma-ray pulsars such as PSR B1055-52, PSR B0633+17 (Geminga), and PSR B0656+14.

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arXiv2000

Gamma-ray Observations of the Galactic Plane at Energies E > 500 GeV

In 1998 and 1999 the Whipple Observatory 10 m telescope was used to search for diffuse gamma ray emission from the Galactic Plane. No signifiant evidence of emission was found. Assuming the TeV emission profile matches EGRET observations above 1 GeV with a differential spectral index of 2.4, we derive an upper limit of {$\rm {3.0\cdot10^{-8}cm^{-2}s^{-1}sr^{-1}}$} for the average diffuse emission above {$\rm500 GeV$} in the galactic latitude range from {$\rm-2^o$ to $\rm +2^o$} at galactic longitude {$\rm 40^o$}. Comparisons with EGRET observations provide a lower limit of 2.31 for the differential spectral index of the diffuse emission, assuming there is no break in the spectrum between 30 GeV and 500 GeV. This constrains models for diffuse emission with a significant inverse Compton contribution.

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arXiv1999

Search for Pulsed TeV Gamma-ray Emission from the Crab Pulsar

We present the results of a search for pulsed TeV emission from the Crab pulsar using the Whipple Observatory's 10 m gamma-ray telescope. The direction of the Crab pulsar was observed for a total of 73.4 hours between 1994 November and 1997 March. During this period the Whipple 10 m telescope was operated at its lowest energy threshold to date. Spectral analysis techniques were applied to search for the presence of a gamma-ray signal from the Crab pulsar over the energy band 250 GeV to 4 TeV. We do not see any evidence of the 33 ms pulsations present in other energy bands from the Crab pulsar. The 99.9% confidence level upper limit for pulsed emission above 250 GeV is derived to be 4.8x10^-12 cm^-2 s^-1 or <3% of the steady flux from the Crab Nebula. These results imply a sharp cut-off of the power-law spectrum seen by the EGRET instrument on the Compton Gamma-Ray Observatory. If the cut-off is exponential, it must begin at 60 GeV or lower to accommodate these upper limits.

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arXiv1999

Whipple Observations of BL Lac Objects

Only BL Lac objects have been detected as extragalactic sources of very high energy (E > 300 GeV) gamma rays. Using the Whipple Observatory Gamma-ray Telescope, we have attempted to detect more BL Lacs using three approaches. First, we have conducted surveys of nearby BL Lacs, which led to the detections of Mrk 501 and 1ES 2344+514. Second, we have observed X-ray bright BL Lacs when the RXTE All-Sky Monitor identifies high state X-ray emission in an object, in order to efficiently detect extended high emission states. Third, we have conducted rapid observations of several BL Lacs and QSOs located close together in the sky to search for very high flux, short time-scale flare states such as have been seen from Mrk 421. We will present the results of a survey using the third observational technique.

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arXiv1999

The Very Energetic Radiation Imaging Telescope Array System (VERITAS)

We give an overview of the current status and scientific goals of VERITAS, a proposed hexagonal array of seven 10 m aperture imaging Cherenkov telescopes. The selected site is Montosa Canyon (1390 m a.s.l.) at the Whipple Observatory, Arizona. Each telescope, of 12 m focal length, will initially be equipped with a 499 element photomultiplier camera covering a 3.5 degree field of view. A central station will initiate the readout of 500 MHz FADCs upon receipt of multiple telescope triggers. The minimum detectable flux sensitivity will be 0.5% of the Crab Nebula flux at 200 GeV. Detailed simulations of the array's performance are presented elsewhere at this meeting. VERITAS will operate primarily as a gamma-ray observatory in the 50 GeV to 50 TeV range for the study of active galaxies, supernova remnants, pulsars and gamma-ray bursts.

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arXiv1999

A Search for Pulsed TeV Gamma Ray Emission from the Crab Pulsar

We present the results of a search for pulsed TeV emission from the Crab pulsar using the Whipple Observatory's 10m gamma-ray telescope. The direction of the Crab pulsar was observed for a total of 73.4 hours between 1994 November and 1997 March. Spectral analysis techniques were applied to search for the presence of a gamma-ray signal from the Crab pulsar over the energy band 250GeV to 4TeV. At these energies we do not see any evidence of the 33ms pulsations present at lower energies from the Crab pulsar. The 99.9% confidence level upper limit for pulsed emission above 250GeV is derived to be 4.8 10^-12 cm^-2 s^-1 or <3% of the steady flux from the Crab Nebula. These results imply a sharp cut-off of the power-law spectrum seen by the EGRET instrument on the Compton Gamma-Ray Observatory. If the cut-off is exponential, it must begin at 60GeV or lower to accommodate these upper limits.

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arXiv1999

Hour-Scale Multiwavelength Variability in Markarian 421

Markarian 421 was observed for about four days with BeppoSAX and the Whipple Observatory gamma-ray telescope in April 1998. A pronounced, well-defined, flare with hour-scale variability was observed simultaneously in X-rays and very high energy gamma-rays. These data provide the first evidence that the X-ray and TeV intensities are well correlated on time-scales of hours. While the rise of the flare occurred on a similar time-scale in the two wavebands, the decay of the flare was much more rapid in gamma rays, providing the first clear indication that the X-ray and gamma-ray emission may not be completely correlated in Markarian 421.

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arXiv1999

VHE gamma-ray observations of Markarian 501

Markarian 501, a nearby (z=0.033) X-ray selected BL Lacertae object, is a well established source of Very High Energy (VHE, E>=300 GeV) gamma rays. Dramatic variability in its gamma-ray emission on time-scales from years to as short as two hours has been detected. Multiwavelength observations have also revealed evidence that the VHE gamma-ray and hard X-ray fluxes may be correlated. Here we present results of observations made with the Whipple Collaboration's 10 m Atmospheric Cerenkov Imaging Telescope during 1999 and discuss them in the context of observations made on Markarian 501 during the period from 1996-1998.

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arXiv1999

Search for TeV Gamma-Rays from Shell-Type Supernova Remnants

If cosmic rays with energies <100 TeV originate in the galaxy and are accelerated in shock waves in shell-type supernova remnants (SNRs), gamma-rays will be produced as the result of proton and electron interactions with the local interstellar medium, and by inverse Compton emission from electrons scattering soft photon fields. We report on observations of two supernova remnants with the Whipple Observatory's 10 m gamma-ray telescope. No significant detections have been made and upper limits on the >500 GeV flux are reported. Non-thermal X-ray emission detected from one of these remnants (Cassiopeia A) has been interpreted as synchrotron emission from electrons in the ambient magnetic fields. Gamma-ray emission detected from the Monoceros/Rosette Nebula region has been interpreted as evidence of cosmic-ray acceleration. We interpret our results in the context of these observations.

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arXiv1999

The Flux Variability of Markarian 501 in Very High Energy Gamma Rays

The BL Lacertae object Markarian 501 was identified as a source of gamma-ray emission at the Whipple Observatory in March 1995. Here we present a flux variability analysis on several times-scales of the 233 hour data set accumulated over 213 nights (from March 1995 to July 1998) with the Whipple Observatory 10 m atmospheric Cherenkov imaging telescope. In 1995, with the exception of a single night, the flux from Markarian 501 was constant on daily and monthly time-scales and had an average flux of only 10% that of the Crab Nebula, making it the weakest VHE source detected to date. In 1996, the average flux was approximately twice the 1995 flux and showed significant month-to-month variability. No significant day-scale variations were detected. The average gamma-ray flux above ~350 GeV in the 1997 observing season rose to 1.4 times that of the Crab Nebula -- 14 times the 1995 discovery level -- allowing a search for variability on time-scales shorter than one day. Significant hour-scale variability was present in the 1997 data, with the shortest, observed on MJD 50607, having a doubling time of ~2 hours. In 1998 the average emission level decreased considerably from that of 1997 (to ~20% of the Crab Nebula flux) but two significant flaring events were observed. Thus, the emission from Markarian 501 shows large amplitude and rapid flux variability at very high energies as does Markarian 421. It also shows large mean flux level variations on year-to-year time-scales, behaviour which has not been seen from Markarian 421 so far.

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arXiv1998

Tev Blazars: Status of Observations

The close relation between ground-based TeV observations and satellite borne $γ$-ray measurements has been important for the understanding of blazars. The observations which involve the TeV component in blazar studies are reviewed.

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arXiv1998

Evidence for the Galactic X-ray Bulge II

A mosaic of 5 \ros~PSPC pointed observations in the Galactic plane ($l\sim25^{\circ}$) reveals X-ray shadows in the $0.5-2.0$ keV band cast by distant molecular clouds. The observed on-cloud and off-cloud X-ray fluxes indicate that $\sim15$% and $\sim37$% of the diffuse X-ray background in this direction in the \tq~keV and 1.5 keV bands, respectively, originates behind the molecular gas which is located at $\sim$3 kpc from the Sun. The implication of the derived background X-ray flux beyond the absorbing molecular cloud is consistent with, and lends further support to recent observations of a Galactic X-ray bulge.

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arXiv1997

Discovery of >350 GeV Gamma Rays from the BL Lacertae Object 1ES 2344+514

We present the discovery of >350 GeV gamma-ray emission from the BL Lacertae object 1ES 2344+514 with the Whipple Observatory 10m gamma-ray telescope. This is the third BL Lac object detected at gamma-ray energies above 300 Gev, the other two being Markarian 421 (Mrk 421) and Mrk501. These three active galactic nuclei are all X-ray selected and have the lowest known redshifts of any BL Lac objects currently identified. The evidence for emission derives primarily from an apparent flare on December 20, 1995 when a 6 sigma excess was detected with a flux approximately 63% of the very high energy gamma-ray emission from the Crab Nebula, the standard candle for TeV gamma-ray sources. Excluding the flare, observations between October 1995 and January 1996 yield a 4 sigma detection corresponding to 11% of the VHE Crab Nebula flux. Observations spanning September 1996 to January 1997 failed to yield a significant detection of a steady flux or any flaring. For this period, the 99.9% confidence level upper limit is <8% of the Crab Nebula. The low baseline emission level and variations in nightly and yearly flux of 1ES 22344+514 are the same as the VHE emission characteristics of Mrk 421 and Mrk 501

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arXiv1997

Multiwavelength Observations of a Flare from Markarian 501

We present multiwavelength observations of the BL Lacertae object Markarian 501 (Mrk 501) in 1997 between April 8 and April 19. Evidence of correlated variability is seen in very high energy (VHE, E > 350 GeV) gamma-ray observations taken with the Whipple Observatory gamma-ray telescope, data from the Oriented Scintillation Spectrometer Experiment of the Compton Gamma-Ray Observatory, and quicklook results from the All-Sky Monitor of the Rossi X-ray Timing Explorer while the Energetic Gamma-Ray Experiment Telescope did not detect Mrk 501. Short term optical correlations are not conclusive but the U-band flux observed with the 1.2m telescope of the Whipple Observatory was 10% higher than in March. The average energy output of Mrk 501 appears to peak in the 2 keV to 100 keV range suggesting an extension of the synchrotron emission to at least 100 keV, the highest observed in a blazar and ~100 times higher than that seen in the other TeV-emitting BL Lac object, Mrk 421. The VHE gamma-ray flux observed during this period is the highest ever detected from this object. The VHE gamma-ray energy output is somewhat lower than the 2-100 keV range but the variability amplitude is larger. The correlations seen here do not require relativistic beaming of the emission unless the VHE spectrum extends to >5 TeV.

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