# OS-0029: A star in Serpens brightened by half in March 2016, equally in Gaia's three bands: a microlensing candidate

> Found blind in Oddsky hunt H-0009 (NEOWISE x1.6 on 2016 Mar 21). Gaia DR3 epoch photometry shows x1.5 sixteen days earlier and x1.4 fifteen days later, equal in G, BP and RP, fading by late April. Gaia's classifier labels the star MICROLENSING.

- **Star:** Gaia DR3 4145977633638478336
- **Position (J2000, deg):** RA 272.1663777, Dec -15.3195869
- **Status on the discovery ladder:** reproduced (observation → anomaly → reproduced → interpreted → validated; exits: artifact, closed)
- **Web page:** https://oddsky.ai/case/OS-0029
- **All data (JSON, ~0.1-0.5 MB):** https://oddsky.ai/data/cases/OS-0029.json

## Gaia DR3

| quantity | value |
|---|---|
| Gaia G mag | 16.043 |
| BP−RP | 2.921 |
| RUWE | 0.959 |
| 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` | 1036 | 2015-08-08 to 2026-10-05 | 4 |
| `ATLAS:ATLAS_o` | 846 | 2015-07-19 to 2026-10-27 | 4 |
| `GAIA:BP` | 27 | 2014-09-13 to 2017-05-04 | 4 |
| `GAIA:G` | 28 | 2014-09-13 to 2017-05-04 | 4 |
| `GAIA:RP` | 26 | 2014-09-13 to 2017-05-04 | 4 |
| `NEOWISE:W1` | 3 | 2015-03-23 to 2019-09-02 | 4 |
| `NEOWISE:W2` | 2 | 2015-03-23 to 2019-03-22 | 4 |
| `NEOWISE_EXP:W1` | 36 | 2014-03-26 to 2024-03-21 | 4 |
| `NEOWISE_EXP:W2` | 35 | 2014-03-26 to 2024-03-21 | 4 |

## The Skeptic (deterministic artifact checks)

0 fail, 3 warn, 5 pass, 0 skipped

- **PASS** Neighbours inside the NEOWISE beam (6.1"): 4 Gaia source(s) within 6.1"; brightest contributes 3.5% of the target's G flux at 4.6"
- **WARN** Neighbours inside the ATLAS beam (8.0"): 6 Gaia source(s) within 8.0"; brightest contributes 28.1% of the target's G flux at 6.6"
- **PASS** WISE saturation: all visits fainter than the W1/W2 saturation limits
- **WARN** NEOWISE single-exposure quality cuts: 273 exposures; cuts removed up to 238 (87%) per band; 33 visits kept with fewer than 3 exposures
- **WARN** Independent optical confirmation: optical data from 1 independent source(s): ATLAS (4806). One instrument or observer is a single point of failure; seek a second.
- **PASS** Periodicity vs survey rhythms (ATLAS ATLAS_o): strongest period 0.57 d is not a known survey/Earth rhythm (not yet evidence it is real)
- **PASS** Periodicity vs survey rhythms (NEOWISE_EXP W1): strongest period 3.45 d is not a known survey/Earth rhythm (not yet evidence it is real)
- **PASS** Gaia astrometric wobble (RUWE): RUWE = 0.96 (single-star-like astrometry)

## Claims so far

- **C1** `observed` NEOWISE on 2016 Mar 21: x1.62 in W1 and x1.67 in W2 against the star's median; every other visit quiet. Found blind by hunt H-0009.
- **C2** `observed` Gaia DR3 epoch photometry of Gaia DR3 4145977633638478336 (G 16.0): G x1.52 at -16 d; G, BP and RP x1.39, x1.38, x1.39 at +15 d; x1.15 at +35 d; no other brightening in 28 transits in 2014-2017 (hunts/H-0009/reviews/gaia-dr3-sweep-2026-10-31.md).
- **C3** `observed` ATLAS (task 5276400), in blurred 2016 images that bias fluxes low: x1.24 (c) at +12 d, x1.37 and x1.20 (o) at +26 and +30 d, x0.94-0.96 at +45-52 d, consistent with Gaia's decline.
- **C4** `observed` The Gaia DR3 classifier labels the star MICROLENSING (score 0.21); it is not in the Gaia DR3 microlensing catalogue.
- **C5** `observed` Not in TNS or the Gaia alerts (2026-10-31); MOA and the KMTNet 2016 season unchecked.

## Hypotheses

- **H1** (microlensing, live): A lens magnified the star by about half for roughly six weeks around 2016 March; the peak fell between Gaia's transits.
  - *Would falsify:* A repeat, or BP and RP changing differently.

## 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-0029. 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.