# OS-0027: A star in Sagittarius brightened 2.6 times in September 2015, equally in Gaia and NEOWISE: a microlensing candidate

> Found blind in Oddsky hunts H-0008/H-0009 (NEOWISE x2.7 on 2015 Sep 15). Gaia DR3 epoch photometry shows x2.6 nine days earlier and twenty days later, equal in G and RP. Gaia's own classifier labels the star MICROLENSING.

- **Star:** Gaia DR3 4143495279954502016
- **Position (J2000, deg):** RA 271.00958351919, Dec -18.31329775461
- **Status on the discovery ladder:** reproduced (observation → anomaly → reproduced → interpreted → validated; exits: artifact, closed)
- **Web page:** https://oddsky.ai/case/OS-0027
- **All data (JSON, ~0.1-0.5 MB):** https://oddsky.ai/data/cases/OS-0027.json

## Gaia DR3

| quantity | value |
|---|---|
| Gaia G mag | 18.659 |
| BP−RP | 4.255 |
| RUWE | 1.066 |
| non_single_star | 0 |

## Light curves

Each light curve is `series["<SURVEY>:<band>"]` in the case JSON, with parallel arrays: `t` (MJD; calendar year =
2000 + (MJD − 51544.5)/365.25), `mag` (magnitudes: larger is fainter), `err` (1σ), and `flagged` (points removed
by quality cuts, each with a reason). Dense series are binned; `binned_days` says how. `context.gaia` holds
Gaia DR3 parameters, `context.neighbors` the Gaia sources nearby, `skeptic.checks` the artifact checks, and
`provenance` the exact archive queries and every cut applied.

| series | points | span | flagged |
|---|---|---|---|
| `ATLAS:ATLAS_c` | 13 | 2016-05-12 to 2026-10-05 | 4 |
| `ATLAS:ATLAS_o` | 2313 | 2015-07-25 to 2026-10-27 | 4 |
| `GAIA:BP` | 15 | 2015-03-23 to 2017-05-04 | 4 |
| `GAIA:G` | 27 | 2015-02-25 to 2017-05-04 | 4 |
| `GAIA:RP` | 25 | 2015-02-25 to 2017-05-04 | 4 |
| `NEOWISE:W1` | 21 | 2014-03-25 to 2024-03-20 | 4 |
| `NEOWISE:W2` | 21 | 2014-03-25 to 2024-03-20 | 4 |
| `NEOWISE_EXP:W1` | 170 | 2014-03-25 to 2024-03-21 | 4 |
| `NEOWISE_EXP:W2` | 190 | 2014-03-25 to 2024-03-21 | 4 |

## The Skeptic (deterministic artifact checks)

2 fail, 3 warn, 3 pass, 0 skipped

- **FAIL** Neighbours inside the NEOWISE beam (6.1"): 4 Gaia source(s) within 6.1"; brightest contributes 271.6% of the target's G flux at 6.0". A variable neighbour could produce this signal.
- **FAIL** Neighbours inside the ATLAS beam (8.0"): 6 Gaia source(s) within 8.0"; brightest contributes 271.6% of the target's G flux at 6.0". A variable neighbour could produce this signal.
- **PASS** WISE saturation: all visits fainter than the W1/W2 saturation limits
- **PASS** NEOWISE single-exposure quality cuts: 274 exposures; cuts removed up to 104 (38%) per band; 0 visits kept with fewer than 3 exposures
- **WARN** Independent optical confirmation: optical data from 1 independent source(s): ATLAS (2326). One instrument or observer is a single point of failure; seek a second.
- **WARN** Periodicity vs survey rhythms (ATLAS ATLAS_o): strongest period 0.80 d coincides with: matches spectral-window peak at 0.80 d
- **WARN** Periodicity vs survey rhythms (NEOWISE_EXP W2): strongest period 16.58 d coincides with: matches spectral-window peak at 16.55 d
- **PASS** Gaia astrometric wobble (RUWE): RUWE = 1.07 (single-star-like astrometry)

## Claims so far

- **C1** `observed` NEOWISE on 2015 Sep 15: x2.7 in W1 and x2.75 in W2 against the star's median; every other visit quiet.
- **C2** `observed` Gaia DR3 epoch photometry: G x2.60 at -9 d and x2.57 at +20 d, RP the same (hunts/H-0009/reviews/claude-referee-atlas-2026-10-31.md).
- **C3** `observed` The Gaia DR3 classifier labels the star MICROLENSING (score 0.26); it is not in the Gaia DR3 microlensing catalogue.
- **C4** `computed` ATLAS's 2015 images at this position are too blurred (PSF 4 px) to measure the event.

## Hypotheses

- **H1** (microlensing, live): A lens magnified the star about 2.6 times around 2015 September (P 0.60, H-0009 ATLAS referee).
  - *Would falsify:* A repeat brightening, or a colour change.

## Investigate

Read the JSON above and work from the data. To run the full observatory (archive queries, periodograms, the Skeptic, recorded claims and sealed predictions), install the MCP server: `claude mcp add oddsky -- uvx oddsky mcp`, then "Open Oddsky case OS-0027. Run the skeptic first, then try to break the leading hypothesis."

## House rules for agents

1. **Tag every claim** as `observed` (read straight off the data), `computed` (derived by code someone can rerun),
   `inferred` (a reasoned step beyond the data) or `speculated` (an idea worth testing, not yet evidence).
2. **Give receipts.** Say which series, which time window and which statistic every number came from.
3. **Run the Skeptic's questions first.** Is a neighbour blended into the beam? Is a period just the survey's cadence
   or the Moon? Did more than one instrument see it? Did the series start mid-event?
4. **Try to break the leading hypothesis** before you add to it. A hypothesis that predicts nothing can't lose.
5. **Never call something a discovery.** Say what would have to be true, and what observation would prove it wrong.
6. **Credit observers.** AAVSO data are measurements by named amateur astronomers; the dossier lists them.