Gamma-ray Burst 990520 = Transient 1SAX J0835.9+5118

On May 20 at 2:02:58 UT a fast X-ray transient, which was designated 1SAX J0835.9+5118, was detected by one of the BeppoSAX WFC. Follow-up observations from the ground and an untriggered BATSE event suggest that this transient was the X-ray afterglow of a GRB.

(All information courtesy of the instrument teams.)

Previous IAU Circulars

• IAUC 7174 BeppoSAX WFC discovery

• BeppoSAX GRB alert n. 99/09 = GCN notice #333:
  Note: we have decided to distribute this info to the BeppoSAX GRB alert list due to the potential association of these events (or some of those) with GRB.
  On May 20 at 2:02:58 UT a fast X-ray transient, which we designate 1SAX J0835.9+5118, was detected by one of the BeppoSAX WFC (n.1). The event lasted about 10 s., with a peak flux of about 1 Crab in the 2-25 keV range. The position is (equinox 2000) R.A.=128.98, decl=51.31, with an error radius of 3'. No previously known X-ray source lies in the error box. At a preliminary analysis no simultaneous signal was detected by the GRBM (40-700 keV). Follow up observations are encouraged to investigate the nature of this object.
• GCN notice #334:
  D. A. Frail (NRAO), S. R. Kulkarni (Caltech) and C. H. Young (MSU, NRAO) report:
  "On 1999 May 20.69 UT we carried out VLA observations at 4.8 GHz of the fast X-ray transient detected by BeppoSAX (GCN 333). There is only one radio source detected in the 3-arcminute error circle. Its position is (equinox 2000) R.A.=8 36 0.7 (+/-0.1s), decl.=+51 17 8.4 (+/-1"). There is no source at this position in the 1.4 GHz NRAO VLA Sky Survey taken in 1993 November. Further VLA observations are underway to determine whether this source is related to the X-ray transient.
• GCN notice #335
  J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski (Caltech) and D. A. Frail (NRAO) report on behalf of the larger Caltech-NRAO-CARA GRB collaboration:
  We have examined the Digital Sky Survey at the position of the radio source reported by Frail et al. (GCN #334) found in the error circle of SAX J0835.9+5118 (GCN #333). An astrometric plate solution for the field was obtained using the USNO A2.0 Catalogue with an r.m.s. uncertainty of 0.23 arcsec (ra) and 0.26 arcsec (dec). Near the position of the radio source is a faint source, probably a galaxy with R ~ 19.5. The centroid position of this optical source is RA: 08h36m1s.1, Dec: 51o17'06".9 (J2000) which is ~1.5 arcsec to the East of the radio position. Though the radio source appears to be offset from the galaxy, given the uncertainties in the absolute position of the radio source we cannot rule out the possibility that the radio source is in the nucleus of the galaxy.
  A finding chart can obtained at: http://astro.caltech.edu/~jsb/XRB990520/.
• GCN notice #336
  A. Castro-Tirado (LAEFF-INTA Madrid and IAA-CSIC Granada), J. Greiner (AIP Potsdam), S. Phleps (MPIA Heidelberg), E. Pian (ITESRE Bologna) and E. Costa (IAS Frascati) report on behalf of a larger collaboration:
  We have obtained two 20-minute R-band exposures of the 1SAX J0835.9 +5118 error box (Gandolfi et al. IAUC 7174) starting at 19:18 UT 20 May 1999 (17.25 hours after the trigger) with the 2.2 m telescope at the German-Spanish Calar Alto Observatory (CAHA). After a visual comparison with the the Digital Sky Survey, no new sources were seen to the DSS-2 limit. At the position of the radiosource reported by Frail et al. (GCN 334) we see a point-like, faint object which is also present in the DSS-2, as pointed out by Bloom et al. (GCN 335). This object does not seem to have changed significantly in brightness. Our frames reach significantly deeper than the DSS-2 image. If the above-mentioned radiosource is not the counterpart to the SAX source, a more detailed analysis and comparison with deeper images in the next few days might yield a counterpart.
• GCN notice #337
  D. A. Frail (NRAO), S. R. Kulkarni (Caltech), C. H. Young (MSU, NRAO) and G. B. Taylor (NRAO) report:
  We have analyzed additional VLA observations of the fast X-ray transient detected by BeppoSAX (GCN 333) taken at 1.4, 4.8 and 8.5 GHz beginning on May 20.93 UT. The positional accuracy of the radio source reported in GCN 334 has improved to (equinox 2000) R.A.=08 36 1.1 (+/-0.1s), decl.=+51 17 6.5 (+/-1") and it is now coincident with the center of the optical galaxy reported by Bloom et al. (GCN 335). There was no evidence of any variability between the two VLA observations. It now appears that this is an inverted spectrum radio source, present also at a weak level (3-sigma) in the VLA FIRST survey at 1.4 GHz.
• GCN notice #338
  J. P. Halpern (Columbia U.), R. Barr (MDM), & E. Costa (IAS/CNR, Rome) report on behalf of the MDM Observatory GRB follow-up team:
  We imaged the field of the BeppoSAX fast X-ray transient 1SAX J0835.9+5118 (Piro GCN 333, Gandolfi et al., IAUC 7174) in the B band starting on May 21.15 UT using the MDM 1.3m. Total exposure time was 50 minutes in seeing of 1.2 arcseconds. Astrometry was performed with respect to 24 stars from the USNO A2.0 catalogue yielding an rms dispersion of 0.32 arcseconds. A limiting magnitude of approximately B = 22.5 was reached, referenced to the magnitudes listed for two stars in USNO A2.0 catalogue, denoted here A and B:
  

  Star        RA(2000)     Dec(2000)     B mag
  --------------------------------------------
  A        08 35 53.669  +51 18 30.72    16.4
  B        08 36 04.485  +51 17 26.46    18.5
  Radio    08 36 01.11   +51 17 06.7     20.9
  --------------------------------------------
  

  Also listed is the location and preliminary magnitude of the object which is coincident with the revised radio source position of Frail et al. (GCN 337). Although referred to as a galaxy in that circular and in GCN 335, it appears point-like in our images, as was also noted by Castro-Tirado et al. (GCN 336) from their R-band images. No other objects of note appear in our images. R-band images were also obtained by observers on the MDM 2.4m. Results of these will be reported later.
• GCN notice #339
  A. Henden (USRA/USNO), F. Vrba, C. Luginbuhl, R. Stone, B. Canzian, J. Munn, S. Levine, H. Guetter (USNO) report:
  We monitored the field of the BeppoSAX fast X-ray transient 1SAX J0835.9+5118 (Piro GCN 333, Gandolfi et al., IAUC 7174) on the night of UT 990521 with the USNO 1.0-m telescope. The night was photometric during the observations, though cirrus arrived shortly thereafter. Enough standards were observed to calibrate the field in BVRI. The calibration file can be found on ftp://ftp.nofs.navy.mil/pub/outgoing/aah/grb/j0835.dat where 19 Landolt standards were used for the photometric calibration, and USNO-A2.0 was used for the astrometric calibration. Stars brighter than V=13.5 are saturated and are included only as field identifiers.
  A report on our deeper imagery will be posted later.
• GCN notice #340
  H. Pedersen, J. Hjorth, B. L. Jensen (Copenhagen), A. O. Jaunsen (Oslo), R. Falomo (Padova), I. Burud (Liege), M. I. Andersen, H. Korhonen, H. Schwarz (NOT), E. Costa, L. Piro (IAS/CNR, Rome), and, E. Pian (Ist.TeSRE, Bologna), report:
  We have used the ALFOSC instrument (field 6' x 6') at the NOT, La Palma, to acquire three 4-minute R-band exposures of the position reported from BeppoSAX (Gandolfi et al., IAU Circ. 7174).
  The exposures were taken 1999 May 20.898 - 20.910 UT. The seeing was 0.7".
  Comparison to the DSS-2 did not reveal any new object. Comparison between the three exposures internally did not reveal any variable object.
  One object drew attention during our inspection of the images: this is the SN-like configuration of an apparently unresolved object 1.6" east, 0.5" south of a galaxy (which, in turn, is well visible on the DSS). The object may well be due to a surface brightness concentration in the said galaxy, or the chance superposition of a galactic star.
  To test the visual impression, we have analyzed the images, using the deconvolution technique applied in GCN 320; unfortunately, the low s/n ratio does not allow any clear distinction between resolved and not resolved models.
  The position of the object of concern is R.A. 08 h 35 m 52.44 s, Decl. +51o 18' 46.9" (J2000); uncertainty 0.5". Its brightness is estimated at m(R) = 22.0 +- 0.5. No variability is indicated. For the galaxy, we find m(R) = 19.75 +- 0.3. The photometric measurements stem from a standard calibration of the instrument, for which m(R) = 16.71 for an object at R.A. 08 h 35 m 52.88 s, Decl. +51o 18' 06.2" (J2000).
  A 2' by 2' section of the sum-image, centered on the object is placed on http://www.astro.ku.dk/~holger/g/XRT990520/imag.fits. A gif-version will be provided later.
  Before any use of this digital data in publications, please consult with us.
• GCN notice #341
  C. Luginbuhl (USNO), A. Castro-Tirado (LAEFF-INTA Madrid and IAA-CSIC Granada), B. Canzian (USNO), E. Costa (IAS Frascati), J. Greiner (AIP Potsdam), H. Guetter (USNO), A. Henden (USRA/USNO), M. Kuemmel (MPIA, Heidelberg), S. Levine (USNO), J. Munn (USNO), S. Phleps (MPIA Heidelberg), E. Pian (ITESRE Bologna), T. Rauch (Univ. of Tuebingen), and F. Vrba (USNO)
  report that a detailed comparison of R-band imaging described in GCN 336 (May 20.82 UT, limiting magnitude R=22.5) and GCN 339 (May 21.18 UT, limiting magnitude R=20.2), as well as additional imaging at the Calar-Alto 2.2m telescope (May 21.82 UT, limiting magnitude R=21.5) shows no variable objects at the three sigma level to approximately R=21.5 within or near the 3-arcmin radius error circle of 1SAX J0835.9+5118 (GCN 333). Areas within about 2 arcmin of the error circle are included in these images. There is some further processing that can be done that may improve the detection limits on the May 21.82 Calar-Alto image.
  The object noted by Bloom et al. (GCN 335) continues to appear stellar, with R=21.21 +/- 0.06. The galaxy noted by Pedersen et al (GCN 340) is clearly extended in these data, but shows no detail indicative of a superposed star or SN, though our seeing was at best just 1.3 arcsec.
  All photometric measures reported here are calibrated with the sequence by Henden et al. (GCN 339).
• GCN notice #343
  J. P. Halpern (Columbia U.), J. Parker (SwRI), L. Allen (U. Michigan), and E. Costa (IAS/CNR, Rome) report on behalf of the MDM Observatory GRB follow-up team:
  We imaged the field of the BeppoSAX fast X-ray transient 1SAX J0835.9+5118 (Piro GCN 333; Gandolfi et al., IAUC 7174) in the R band on two consecutive nights using the MDM 2.4m telescope. Total exposure time was 50 minutes on May 21.19 UT and 40 minutes on May 22.19 UT. Seeing was 1.5-1.6 arcsec on the first night, and 1.8-2.2 arcsec on the second night. Limiting magnitudes for 5 sigma detection are R = 23.5 on the first night and R = 22.1 on the second night. No variable object is found to the limit of R = 22.1 within the 3' radius error circle. Below we list magnitudes of objects referred to in previous circulars: the radio source of Frail et al. (GCN 337), and the galaxy described by Pedersen et al. (GCN 340). We adopt here the photometric calibration of Henden et al. (GCN 339).
  

  Radio Source     B = 21.58 +/- 0.08 (May 21.15 1.3m)
                   R = 20.81 +/- 0.05 (May 21.19 2.4m)
                   R = 20.69 +/- 0.07 (May 22.19 2.4m)
  
  Pedersen Galaxy  B = 20.62 +/- 0.09 (May 21.15 1.3m)
                   R = 19.63 +/- 0.03 (May 21.19 2.4m)
                   R = 19.62 +/- 0.09 (May 22.19 2.4m)
  

  The B magnitude of the radio source differs from the value given in our GCN 338 in part because of a 0.5 magnitude difference between the USNO-A2.0 values and the new calibration of Henden et al. (GCN 339). However, the magnitude of the radio source given as R = 21.21 +/- 0.06 by Luginbuhl et al. (GCN 341) might indicate a significant discrepancy, assuming that the same calibration is being used.
  With regard to the morphology of the Pedersen et al. galaxy, although our images have inferior seeing, the object in question appears consistent in both magnitude and shape on our May 21 image, including a faint extension in the same place as their point-like source.
• GCN notice #344
  R. M. Kippen, R. D. Preece, T. Giblin (University of Alabama in Huntsville) and C. Kouveliotou (USRA) report on behalf of the BATSE team:
  BATSE detected an untriggered event with significant flux (20-200 keV) coincident in time with the soft x-ray transient 1SAX J0835.9+5118 (Piro et al. GCN 333; Gandolfi et al. IAUC 7174). The BATSE on-board trigger system was disabled at the time, but the event was recorded in continuous data mode with 1.024 second temporal resolution. The BATSE location of (equinox 2000) R.A. = 141.1, Dec. = 56.8, with a statistical uncertainty radius of 9 deg, is consistent with that of the BeppoSAX wide field camera. The emission lasted ~8 s, with one main pulse (~2 s duration) preceded by a much weaker episode. The peak flux was 0.46 (-/+ 0.05) ph/cm2/s (50-300 keV; 1.024 s), with a total fluence of 8 (-/+ 3) E-7 erg/cm2 (>20 keV). The spectrum is consistent with a single power-law model (index -2.1 -/+ 0.4) or with curvature models with Epeak about 80 (-/+ 15) keV. Approximately 10% of the GRBs measured by BATSE have similar spectral properties, whereas the spectrum is significantly harder than that of a typical SGR burst. If this event is a gamma-ray burst, it ranks at the lower ~30% of the BATSE flux/fluence distribution.
• GCN notice #345
  N. Masetti, E. Palazzi, E. Pian, F. Frontera (ITESRE, CNR, Bologna), D. Gardiol, S. Benetti, A. Zacchei, A. Magazzu' (TNG), M. Della Valle (Univ. of Padua), E. Costa, M. Feroci and L. Piro (IAS, CNR, Rome) report:
  We imaged the error box of the X-ray transient source SAX J0835.9+5118 (Piro, GCN #333) with TNG plus OIG CCD camera on 1999 May 20.890 - 20.920 UT (five V frames; seeing 1."2 - 1".5) and on 1999 May 21.934 - 21.955 UT (one R and two V frames; seeing 0".9). The exposure time was 300s for all images.
  Calibration of V and R frames was performed using the photometry by Henden et al. (GCN #339). A limiting magnitude of V ~ 22.5 was obtained in each frame of the first night, while the limits V ~ 23 and R ~ 23 were reached in the images acquired on the second night (3-sigma limits).
  Comparison among the V frames of May 20 and 21 shows no object which varied by more than 0.3 magnitudes (at a 3-sigma level) down to V ~ 22.
  The optical counterpart (Bloom et al., GCN #335) of the radio source detected by Frail et al. (GCN #334) appears star-like as earlier noted by other authors (GCN #336, #338, #341) with magnitudes V = 21.08 +- 0.05 and R = 20.68 +- 0.05. Some scatter of 0.1 mag around the mean V magnitude of the object is seen, thus probably indicating the presence of flickering or of some other optical variability.
  The irregular galaxy reported by Pedersen et al. (GCN #340) has V = 20.20 +- 0.05 and R = 19.60 +- 0.05 in our frames and does not show any variability in the V band between the two nights. We see in our images that the faint knot noted by Pedersen et al. southeast of the galaxy appears brighter in the R band than in V.
  The R magnitudes of both objects are in agreement with the values reported by Halpern et al. (GCN #343).
  
• GCN notice #348
  H. Pedersen, J. Hjorth, B. L. Jensen (Copenhagen), H. Korhonen, and M. I. Andersen (NOT) report:
  We have conducted a second (cf. GCN 340) series of imaging using the 2.56-m Nordic Optical Telescope. The StanCam instrument was used, giving a field of 3' x 3', hence significantly smaller than the 3' radius error circle (GCN 333).
  Three 5-min R-band images were acquired on May 25.927 UT. The seeing was 0.9" FWHM.
  We find no evidence for variability between the two observing epochs. Specifically, the sn-like configuration mentioned in GCN 340 does not show any variability in support of its association with the high energy event. The sum image from the two runs can be inspected at http://www.astro.ku.dk/~holger/g/XRT990520/news.html
  The new images do not cover the position of the radio source discovered by Frail et al. (GCN 334).
  For the optical counterpart to this latter source (cf. GCN 335, 336, 338, 341, 345) we have deduced the brightness at the epoch of our first observing session, May 20.90 UT, finding R = 20.84 +- 0.05.