The Galaxy Viewed at Very Short Time-Scales

with the Berkeley Visible Image Tube (BVIT)

Barry Y. Welsh, O.H.W. Siegmund, J. McPhate, D. Rogers & J.V. Vallerga

Space Sciences LaboratoryUniversity of California, Berkeley, CA

510-642-0305 (bwelsh@ssl.berkeley.edu)

What is BVIT?• The Berkeley Visible Imaging Tube (BVIT) is a

visible micro-channel plate photon counting detector designed to provide observers with very high time resolution imaging photometry.

– Enables a new time domain for astronomical observations with full imaging capability

• Time resolution (for each detected photon) ~ 1picosec

• BVIT is a simple instrument with minimal observational setup requirements and a high degree of post acquisition data flexibility.

• Designed as a user facility instrument only available at the 10m South African Large Telescope (SALT)

Microchannel Plate Detectors

Charge distribution on stripsCharge CloudMCP stackTube Window withphotocathodeγ

Photocathode converts photon to electron

MCP(s) amplify electron by 104 to 108

Rear field accelerates electrons to anode

Patterned anode measures charge centroid (X,Y) and time of photon arrival

Very similar detectors are currently in-orbit on NASA’s GALEX UV astronomy mission and COS on Hubble

Cross Delay-Line Anode Image Tube

SuperGenII

GaAs

Test mask 10µm pin-holes on 500µm centers.30µm resolution.

Photocathode Q.E.

Cross-Delay Line Anode

BVIT Demo System on SALT (Jan BVIT Demo System on SALT (Jan 2009)2009)

The Berkeley-SAAO team carried outa commissioning/ engineering test-run with the BVIT installed on SALT for 10 nights in January 2009. A further 7 nights observations were performed by SAAO staff in March 2009.

The BVIT on SALT at prime focusThe BVIT detector

THE BVIT Instrument Package on SALT

B &V + ND Filter Wheels + Shutter & BVIT

PC

HVPS

TDC electronics

LVPS

Filter wheel controllers

BVIT F.O.V = 1.9 arc min (enables source & comparison star observations)

The BVIT Demo Detector System

Resolution

BVIT 25mm tube with ampand (in rack) TDC and HVPS

Command, Control & real timeDisplay Software for BVIT

BVIT Observations on SALTBVIT Observations on SALT• Observations were carried out for a wide variety of

astronomical objects in order to investigate which types were best suited for future in-depth study

• Objects included QSO’s, AGN, Spiral galaxies, Globular Clusters, Flare Stars, CV systems, Low-Mass X-ray Binaries, Pulsars, asteroid transits

• Observational & instrumental constraints restricted observations to targets with 12.5 < Vmag < 22.0

• Some objects (CV systems) were simultaneously observed with the SAAO 74 inch telescope + hi-speed photometer by Dr. Steve Potter

• During the March run, the black hole candidate GX 339-4 (Vmag =15.5) was also observed with SALTICAM in order to compare data sets

• The BVIT+SALT observing efficiency was 58%, which is the highest value yet achieved with the SALT 10m telescope!

The SALT facilityThe SALT facility• Both the Hobby-Eberly and SALT telescopes have had more

than their fair share of engineering problems.

• SALT is presently closed for 3 months to re-engineer its tracking system

• Our data was taken when the tracking system had “glitches” and the multi-segment mirror focus system had problems

• The effect of these facility problems resulted in stellar images suddenly going in and out of focus, and the stellar image sometimes moving several arc seconds during an exposure.

• The photon counting capability of BVIT, in which every photon is assigned an X,Y and t, meant that neither of these instrumental anomalies was a problem for data reduction.

• Note that SALT cannot produce accurate photometric fluxes

Source, Background and Comparison Star Plots (time versus counts)

Samples of BVIT Reduced Data: X-ray Transients

• Low-mass X-ray binaries are systems that include a low-mass companion that transfers material onto a neutron star or black hole.

• Most LMXRB systems have orbital periods of a few hours to days and XRT’s undergo significant X-ray, optical and radio outbursts separated by long periods of quiescence

• Little information is currently known about short-time scale non-orbital emission variations in the optical. Although optical bursts generally lag behind those observed in X-rays (by re-processing), in several cases the optical burst emission LEADS the X-ray emission (perhaps due to synchrotron emission from a jet). (see Gandhi et al 2008)

• We observed the X-ray nova GX 339-4 in the B-band with BVIT on SALT for ~ 500sec with the data shown here in 0.1 sec bins

X-ray Transients with BVIT• GX 339-4: Black Hole candidate data binned at 0.1 sec

Short period flare event on the BH accretion disk

Power spectrum

Samples of BVIT Reduced Data: M-star flares– CN Leo: dMe Flare Star:

emission structure observed < 0.1 sec

Samples of BVIT Reduced Data: CV Systems

Magnetic cataclysmic variables (CVs) contain a white dwarf that is accreting material from a secondary (red dwarf) star down the field lines. In these “polar” systems the strong magnetic field of the white dwarf causes it to rotate synchronously with the orbital motion.

We observed the UZ For system in the B-band with BVIT for ~ 2000sec and captured an entire an eclipse event.

We show the BVIT data in time bins of 0.5s and compare it with similar (white light) data recorded with an STJ device by Perryman et al (2001)

Samples of BVIT Reduced Data: CV Systems

Optical pulsars and isolated neutron StarsOnly 3 optical pulsars are sufficiently bright enough to be observed with BVIT: the Crab, PSR B0540-69 and Vela.

Light curve of the Crab pulsar recorded with BVIT on the Lick 1m Nickel Telescope in 2007

We have searched the ~ 2 hrs of pulsar data for the presence of “giant” optical pulses with no success.

Optical pulsars and isolated Neutron Stars• In future observing runs we shall point the center

of the BVIT f.o.v. at the radio position of several Rotating Radio Transients (RRATs)

• To date, ~ 20 RRATs have been detected and are characterized by short bursts of 2 to 30ms duration (100mJ to 10Jy) with time intervals between recurrent bursts ranging from 3 min to 3 hours.

• Obtaining a ratio of the optical-to-radio flux can place limits on the spectral slope of any proposed emission mechanism from a plausible magnetar.

• We shall also perform limited optical follow-up observations of gamma-ray bursts to search for very fast time-structure in their optical light curve emission

New BVIT tube for SALTNew detectors for BVIT are in production, the detection efficiency is considerably better than our first detector, particularly in the red. We expect to upgrade the BVIT detector in mid-2010.

Comparison of detector photocathode efficiencyNew BVIT detector

Photon counting counts EVERYTHING!!

Meteor Detection

1.2413 sec events with rise times of only 2 milli-sec ??

Peak* Performance of the BVIT on SALTDetector FOV = 25mm circular, 1.9 arc min diamSpatial Resolution = 30 microns (0.14”)Photocathode = S20 (replace with Supergen2)Timing Resolution = Time-stamped to 25 ns

Filter selection = U, B or V-bandNeutral Density Filter = ND0 to ND4Max BVIT count rate = 2 MHz (whole detector)Max local count rate = 20 kHz per arc sec2

Stellar magnitude count rates U = 17.5 mag (1500 cts/sec)

U = 22.0 mag (25 cts/sec) B = 17.0 mag (11000 cts/sec)

B = 21.6 mag (170 cts/sec)

Sky background count rates (Dark) = 25 cts/sec (U-band)

(in 1 arc sec2) (Grey) = 170 cts/sec (B-band)

* Actual performance numbers depend on observing conditions etc etc