Flaring mm-wavelength object in Orion Cluster

21 Jan 2003 - text submitted to IAU Circular 8055:

     G. C. Bower, R. Plambeck, and A. Bolatto, University of
California at Berkeley, report the discovery on Jan. 20 at 86 GHz
of a flaring object in the Orion cluster, detected with the
Berkeley-Illinois-Maryland Association millimeter-wavelength array
at an average flux density of 104 mJy (with evidence that it
increased from 40 mJy at Jan. 20.188 UT to 143 mJy at Jan. 20.333).
The source was not detected in observations made on 2002 Dec. 23,
2003 Jan. 11, and 14, with 3-sigma upper limits of 6, 12, and 15
mJy, respectively.  Subarcsecond-resolution data yield a position
of R.A. = 5h35m11s.80, Decl. = -5 21'49".2 (equinox 2000.0; error <
0".1).  The source is positionally coincident with radio source A
of Garay et al. (1987, Ap.J. 314, 535) and with infrared source 573
of Hillenbrand and Carpenter (2000, Ap.J. 540, 236).  The object
may be a deeply embedded massive protostar.

21 Jan 2003 - more details of BIMA observations:

Observations were made in the BIMA A-array, providing an 0.9'' x 0.5'' synthesized
beam.  The strong v=1 J=2-1 SiO maser associated with Orion-IRc2 was used as
a phase reference.  The observations are part of a long-term project to track the
proper motion of the Becklin-Neugebauer Object relative to IRc2 - see Plambeck
et al. (1995).

Two high-quality data sets were obtained in this year's A-array, on 23 Dec 2002
and on 20 Jan 2003.  The flaring object is conspicuous in the upper right quadrant
of the 20 Jan 2003 continuum map, but is absent in the  23 Dec 2002 map.   These
maps cover a 2.5' x 2.5' region and have not been corrected for attenuation by
the 2.2' FWHM primary beam.  Note that emission from spectral lines is almost
completely resolved out in these images.  Continuum emission from 'source I',
the radio source associated with IRc2, is at the center of both maps.  The faint
arc just below source I is dust emission from the 'hot core' (mostly resolved out).
BN is the source ~ 10'' NW of source I.

The flare appeared to brighten by a factor of at least 3 during the night of 20 Jan.
However, if the flare is highly polarized this could be due to the changing
parallactic angle of the source, which varied from -38 to +45 degrees over the
course of the observations.  The BIMA receivers sensitive to linear polarization
in the vertical direction.

Maps from  11 Jan 2003  and 14 Jan 2003  were generated from poor quality
data taken in the rain.  They show no sign of the flare source.

IR data for source 573 from Hillenbrand & Carpenter (2000):

RA  05  35  11.81,  Dec -05  21  49.3  (J2000)
K magnitude 9.633,  H magnitude 11.917 (Keck/NIRC)

X-ray properties:

Coincident with source 297 in the Chandra X-ray catalog of the Orion
cluster  (Feigelson et al. 2002, ApJ, 574, 258).  It ranks among the top
40 in x-ray counts of the 1075 sources listed.  It is cataloged as a
'flare' source, meaning that there was a highly significant variation in
its x-ray flux on time scales of hours.

22 Jan 2003 - VLA observations:

Flare source detected at 22 and 43 GHz with flux densities of 87 and 80 mJy,
respectively.   No sign of linear or circular polarization to a level of a few percent.
Upper limit at 8.4 GHz  is 36 mJy.

22 Jan 2003 - Keck infrared observations:

Imke dePater observed in K, H, J bands.
K magnitude 9.61,  H magnitude 11.98,  J magnitude 16.0

23 Jan 2003 - BIMA followup observations:

Unfortunately BIMA was reconfigured from the A-array to the B-array on 21 Jan,
so followup observations will have to be done with a 5.5'' x 2.5'' synthesized beam.
Fortunately, the flare source is far enough from the Orion-KL cluster that
confusion from dust or spectral line emission should be minimal.  Data were
taken in very poor weather (rain; rmspath 500-800 microns) at about 6 UT on
23 Jan.  The resulting 86 GHz continuum map  (23 Jan 2003 )  shows no sign of
the flare source; the 3 sigma upper limit is approximately 25 mJy.

The source structure at the center of the B-array map appears very different than
on the A-array maps (overlay) because  'extended' (on scales of arcsec) dust
and spectral line emission from the hot core overwhelm the free-free radiation
from source I and BN in the B-array's larger synthesized beam.

More careful data reduction on 24 Jan, using 20 kilolambda inner cutoff,
gives rms noise of ~4 mJy in upper right quadrant of map.  Decorrelation
due to poor weather judged from amplitude of 0530+135 (short observations
interleaved with Orion): it is 1.71 mJy in the 23 Jan data, 2.15 mJy in the
higher quality 24 Jan data.  So, this gives upper limit to 86 GHz flux of
3 x 4 mJy  x 1.25 for decorrelation x 1.67 for primary beam atten = 25 mJy.

24 Jan 2003 - VLA observations:

Flux is 18.3 mJy at 15 GHz,  24.7 mJy at 22 GHz,  18.1 mJy at 43 GHz.
Clearly the flare is fading rapidly.

24 Jan 2003 - BIMA followup observations:

Observations were made in reasonably good weather from 3-8 UT on 24 Jan.
Only visibilities with u,v radius > 20 kilolambda were used to make the
continuum map ( 24 Jan 2003 ), reducing the confusion from poorly sampled
large scale structures.  The measured rms noise in the upper right quadrant
of the map is 2.6 mJy/beam.  A conservative upper limit on the flare source flux
density is 13 mJy (3 sigma, x 1.67 to correct for primary beam attenuation).

25 Jan 2003 - Nobeyama observations, from IAU Circular 8060

     K. Nakanishi and M. Saito, Nobeyama Radio Observatory,
National Astronomical Observatory of Japan; R. S. Furuya, Istituto
Nazionale di Astrofisica, Osservatorio Astrofisico di Arcetri; H.
Shinnaga, Harvard-Smithsonian Center for Astrophysics; and M.
Momose, Ibaraki University, report on follow-up observations of the
radio flaring object in Orion cluster, using the Nobeyama
Millimeter Array (NMA) at Nobeyama Radio Observatory:  The object
is located at R.A. = 5h35m11s.79, Decl. = -5 21'49".2 (equinox
2000.0; estimated uncertainty 0".2), which agrees well with that
reported on IAUC 8055.  The NMA data give continuum flux densities
of 39.1 +/- 5.1 mJy at 146.969 GHz on Jan. 25.55 UT (resolution
1".3 x 0".9) and 22.4 +/- 5.4 mJy at 97.782 GHz on Jan. 25.62
(resolution 2".5 x 1".2).  We suggest that the object is/are (a)
massive (proto)star(s) that might be involved in mass-ejection
event(s). Further interferometric observations at millimeter
wavelengths are planned, and observations at other wavelengths are
urgently requested."

27 Jan 2003 - VLA followup

Fluxes from Sunday night's VLA observations are

15 GHz:  45 mJy
22 GHz:  47 mJy
43 GHz:  39 mJy

The flare has been reenergized!

29 Jan 2003 - more VLA and VLBA time approved.  Here is the
VLA schedule:

night     prog       lst
Jan 29:  AB1080  0630-0730
Jan 30:  AB1080  0630-0730
Jan 31:  AB1080  0500-0600
Feb 1 :  AB1080  0600-0700
Feb 2 :  AB1080  0600-0800
Feb 4 :  AB1080  0700-0800
1 Feb 2003 - we learn that Chandra was observing Orion when the
flare occurred!

In response to a request by Geoff Bower and Fred Baganoff for a
target of opportunity observation by Chandra, we learn that Chandra
was observing Orion when the flare occurred on 20 Jan!

4 Feb 2003 - BIMA upper limit is ~ 6 mJy

We obtained about 3 hours of high quality BIMA data from 4-7 UT on 4 Feb.
Omitting all data < 20 kilolambda from the origin, to reduce confusion from
extended molecular line and dust emission, we produced a map with noise
level 3 mJy/beam (measured in a 1 arcmin square box centered on the flare
source).    There is no evidence of 86 GHz continuum emission at the flare
position - the measured flux in a 4 x 4'' box centered on this position is 2 mJy.
The synthesized beam is 4.6 x 1.7''.    2 sigma upper limit is ~ 6 mJy.

4 Feb 2003 - Chandra results submitted to IAU Circular, from Konstantin Getman:

On behalf of a large collaborative team, K. V. Getman, E. D.
Feigelson, G. Garmire (Penn State University), and S. S. Murray
and F. R. Harnden, Jr. (SAO) report the detection of an X-ray
flare from the radio flaring object (IAUC 8055 and 8060) in the
Orion cluster, seen with the ACIS-I detector on board the Chandra
X-ray Observatory during an ultradeep observation of the Orion
nebular covering the interval January 8-22. The X-ray variability
has a complex morphology: it is approximately constant from Jan
8.9 to Jan 16.3 around 0.04 cnts/s, rapidly rises to 0.19 cnts/s
starting Jan 16.3, falls to 0.11 cnts/s by Jan 16.7 where it
remains constant for a day, rises over several hours starting Jan
17.7 to 0.35 cnts/s and higher, falls to 0.2 cnts/s over a day,
remains at that level for a half of the day, rises again to 0.35 cnts/s
and higher starting Jan 19.7, and falls over a day to 0.2 cnts/s by the
end of the observation. Preliminary spectral analysis confirms the
previous Chandra findings (Feigelson et al. 2002, Ap.J. 574, 258)
that the source is deeply embedded in a molecular cloud with a
column density of $\log N_H \sim 22.6$ cm$^{-2}$ and with
absorption-corrected luminosities of $\log L_x \sim 31.3$ and
$\sim 32.0$ erg/s, for the quiescent and flaring phases,
respectively. As this source does not have a near-IR excess
(Muench et al. 2002, Ap.J. 573, 366), it may be an extreme example
of a magnetically active weak-line T Tauri star (such as V773 Tau,
or DoAr 21) rather than a rare radio flaring protostar (e.g., like

Here is the Chandra  Xray light curve provided by Konstantin Getman.
The solid line shows the lightcurve in the full (0.5 - 8 keV) energy band,
the dotted line - the lightcurve in the hard (2 - 8 keV) band, and the
dashed line - the lightcurve in the soft (0.5 - 2 keV) band.

5 Feb 2003 - updated radio light curve

The updated  radio light curve suggests that the flare is fading.

7 Feb 2003 - another 86 GHz detection

The flare source is detected at about the 4 sigma level in an
86 GHz map from  7 Feb  (02:25 UT to 05:15 UT), derived from
3 hours of high quality BIMA data.  The measured rms noise
level is 2.5 mJy/beam in a 60'' x 60'' box centered on the flare;
the measured flare source flux density is 11 mJy.  This is the only
86 GHz detection of the flare source we've made, apart from the
initial detection on 20 Jan!